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BROADBAND RAILWAY DIGITAL NETWORK

A Seminar Report on

Prepared by : Mavani Yogesh S.

Roll No. : 23

Class : B.E.IV (Electronics & Communication Engineering.)

Semester :8th Semester

Year : 2006-2007

Guided by : Mr. Dhairya B Bapodra

Department

of

Electronics & Communication Engineering.

Sarvajanik College of Engineering & TechnologyDr R.K. Desai Road,

Athwalines, Surat - 395001,

India.


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Sarvajanik College of

Engineering & TechnologyDr R.K. Desai Road,

Athwalines, Surat - 395001,India.

Department

of

Electronics& Communication Engineering.

CERTIFICATEThis is to certify that the Seminar report entitled

BROADBAND RAIL DIGITAL NETWORK is prepared &

presented by Mr.MAVANI YOGESH S. , Class Roll No. 23 of

final year (B.E.IV) Electronics & Communication Engineering

during year 2006-2007. His work is satisfactory.

Signature of Guide Head of DepartmentElectronics & Communication Engineering

Signature of Jury Members


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ACKNOWLEDGEMENT

I take this opportunity to express my sincere thanks and deep sense of gratitude to

my guide Mr. Dhairya B Bapodra for imparting me valuable guidance during the

preparation of this seminar. They helped me by solving many doubts and suggesting

many references.

I am also thankful to Prof. Mehul Raval Department In-charge (DIC) of

Electronics & Communication Engineering Department of Sarvajanik College Of

Engineering and Technology, Surat.

I would also like to offer my gratitude towards faculty members of Electronics &

Communication department, who helped me by giving valuable suggestions and

encouragement which not only helped me in preparing this presentation but also in having

a better insight in this field. Lastly, I express deep sense of gratitude towards my

colleagues and also those who directly or indirectly helped me while preparing this

seminar.

MAVANI YOGESH S.

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ABSTRACT

Broadband Railway Digital Network (BRDN) integrating the emerging broadband

wireless access technologies will satisfy the requirements of next generation Railway

Information System (RIS). This paper proposes an overall implementation scheme for

BRDN with discussion on its main components, such as the intranet on train, and

infrastructure of BRDN. The paper aims at enabling deployment of innovative, cost-

effective, and interoperable multi-vendor broadband wireless access system for RIS on

fleeting train.

The traditional railroad communication system (RCS) is not only in charge of the

traditional train scheduling, but also offers broadband WLAN services to passengers and

provides the network platform to the intelligent railroad information system. This imposes

a major challenge on the capability of the RCS in response to the increasing application

requirements, particularly, the one for ubiquitous Internet access. To take the advantage of

the rapid evolving mobile communication technology, this paper proposes an IEEE

802.20 based broadband railroad digital network, namely BRDN, for the next generationRCS. The paper further presents the scenario how BRDN operates and identifies the IP

mobility as the major technical issue to be solved for BRDN. The predictability of mobile

IP for the train-based applications will ease the difficulties in implementing BRDN. With

the availability of IP standard for the next generation Internet - IPv6, BRDN will

eventually become a reality.

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LIST OF FIGURES

FIGURE NO. TITLE OF FIGURE PAGE NO.

1.1 Network Technologies 1

1.2.1 Satellite Communication 2

1.2.2 Cellular Network 3

1.2.3 LAN Based Network 3

2.1 BRDN Configuration 4

2.2 The Operation Scenario Of BRDN 5

3.1 Wireless LAN Architecture Using An

Infrastructure BSS

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4.1 The Operation Of Mobile Ipv6 24

III


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LIST OF TABLES

TABLE NO TITLE OF TABLE PAGE NO

1. IEEE 802.20 Vs Other Mobile

Techniques

13.

IV


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INDEX

CHAPTER NO. TITEL PAGE NO.

Acknowledgement I

Abstract II

List of figures III

List of tables IV

1. INTRODUCTION 1-3

1.1 Broadband Wireless Technology 1

1.2 BRDN Connectivity 2

1.2.1 Satellite Communications 2

1.2.2 Cellular Network 3

1.2.3 LAN-based Network 3

2. THE FRAMEWORK OF BRDN 4-5

2.1 The Configuration 4

2.2 The Operation Scenario 5

3. WIRELESS LAN 6-133.1 History 6

3.2 Benefits 7

3.3 Disadvantages 7

3.4 Architecture 9

3.5 IEEE 802.11a 9

3.6 IEEE 802.11b 9

3.7 IEEE 802.11g 10

3.8 IEEE 802.11n 10

3.9 IEEE 802.11 LAYER 11

3.10 IEEE 802.20 The Key Technology

For BRDN

12

4. WIRELESS WAN 14-204.1 Characteristics of WAN

environments

14

4.2 High level architectural tradeoffs in

WWAN environments

16

4.3. The WTCP approach 17

5. MOBILE IPV6 21-255.1 Introduction 215.2 FEATURES OF IPv6

5.2 Mobile IPv6 - The Key Issue In

BRDN Implementation

23

6. SUMMARY 26

BIBLOGRAPHY 27

V


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1. INTRODUCTION

1.1 Broadband Wireless Technology

Wireless Broadband Networks are defined as communication without wire over

distance by the use of arbitrary codes and increased bandwidth. More specifically WirelessBroadband Connection refers to technologies that use point-to-point or point-to-multipoint

microwave, in various frequencies between 2.5 GHZ and 43 GHZ, to transmit Signals

between hub sites and end users. The technology can be used to provide voice, data &

even video information and requires line-of-sight between the hub side and the end user

receiver.

Great efforts have been done in deploying data communications network for RIS

in adopting the latest broadband wireless technologies, such as IEEE 802.11x . The IEEE

802.11 standard (nicknamed as Wi-Fi) came into use in 1997 for Wireless LAN. In 1999,

the IEEE Standards Board enhanced the 802.11 standard: 802.11b working at 2.4 GHz

with the data rate up to 11 Mbps, and 802.11a working at 5 GHz and supporting higher

data rates up to 54 Mbps. In June 2003 IEEE approved 802.11g, which is compatible with802.11b with the same data rate as 802.11a. 802.11a/b/g has become popular worldwide.

In October 2004, P802.16-REVd/D5 has been approved as IEEE Standard 802.16-2004, a

revision and combination standard of 802.16 and 802.16a. In December 2004, IEEE

802.16e Task Group released the last draft 802.16e5a which defines air interface for fixed

and mobile broadband wireless access .In another aspect, the mobility-oriented wireless

metropolitan area network standard, IEEE 802.20, has been initiated. In December 2002,

the IEEE Standards Board approved the establishment of IEEE 802.20 Working Group.

IEEE 802.20 promises to support various vehicular mobility classes up to 250 km/h in a

MAN environment and targets spectral efficiencies, sustained user data rates and numbers

of active users that are all significantly higher than achieved by existing mobile systems.

Fig 1.1 Network Technologies1


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1.2 BRDN Connectivity

There are three possible independent recommended modes for providing

connectivity to the train:

1. Satellite Communication

2. Existing Cellular Networks3. LAN base Network

1.2.1 Satellite Communications

Satellite communications must be provided for Backup or as a fall-up mechanism

if trackside infrastructure or cellular network is out of service, let us say, because of an

earthquake, or as happened in the case of Hurricane Katrina, when the only

communication channel available was Satellite. For such situations, the service provider

must provide and support full "two-way" satellite links with Two-way satellite antenna

installed on the roof of the train to connect with the In-train Wi-Fi Network. The needs for

Homeland Security requirements are to provide seamless access to mission-critical voice,

video and data communications for emergency purposes. Since Satellite offers relatively

lower network bandwidth, this could be used for remote connectivity during emergency;

however, if Satellite connectivity is the only means to providing Internet connectivity in

the train, it must be at higher bandwidths as per current industry trends for higher

commuter satisfaction. The Wi-Fi connection for user connectivity must be maintained

despite short interruptions such as tunnels and the system must allow users to maintain a

continuous link to a local server onboard the train which would be critical for such

situations when the train is passing through a tunnel. For such situations, the satellite

communication should be capable of being restored rapidly. Satellite link with the train, if

used for commercial Wireless Internet, must be capable of at least 4MBit/s downlink and

2MBit/s uplink, with 10 users using the system via Wi-Fi enabled laptops, downloading

large files, streaming videos, etc. Satellite connectivity with the train, if used forcommercial Wireless Internet, must be capable of providing service to up to as many as

100 concurrent users per train in a scaled manner or 500 concurrent users as the user base

increases over all the train sets for the entire corridor. For satellite communication, the

service provider must use power management to adjust the amount of transmit power to

compensate for adverse features like rain fade.

Fig.1.2.1 Satellite Communication2


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1.2.2 Cellular Network

Cellular-based wireless networks are designed to transmit more or less continuous

streams of data. While a circuit connection is established between two users, the exchange

of data from voice, audio or video sources is continuous, even if there is no information to

send because a user is not talking at the time. This is very wasteful of valuable circuit

capacity and digital cellular system providers learned to utilize the quiet times and idlechannels for other purposes. It seems like a natural fit for the growing data transmission

market except for one factor, the whole data transmission industry is moving towards

packet data transmission, which breaks up continuous data streams into a sequence of

discrete packets. In short, cellular networks had to learn how to send IP data packets.

Fig. 1.2.2 Cellular Network

1.2.3 LAN-based Network

LAN based networks has its roots in data transmission. The genesis is the

ubiquitous local area network (LAN), which has been carrying data between office

computers for decades. It is based on the Ethernet system, upon which the entire Internetis built using the Internet Protocol (IP) for transmitting packets of data. LANs are

typically represented as clouds simply because the actual route taken by the data packets

is not important. All users are effectively connected to all other users and are

distinguished by an address. LANs are primarily for data transmission, but as market

pressure tends to do, the network providers soon learned how to send digitized voice

using IP packets (voice over IP, or VoIP) and enterprising service providers are now

offering long distance phone services over the Internet.

Fig. 1.2.3 LAN Based Network3


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2. THE FRAMEWORK OF BRDN

In this section, we construct a framework for the BRDN underpinned by two

wireless network standards the IEEE 802.20 wireless WAN (WWAN) on ground and

the IEEE 802.11x WLAN on train.

2.1 The Configuration

Figure 2.1 illustrates a configuration for a mobile network on the train

implemented. A WLAN is deployed in a train with a typical configuration: The Ethernet

(IEEE 802.3) cable is wired through all carriages via MVB (Multi Vehicle Bus); in each

carriage, Wi-Fi access points are deployed with complete coverage of the area, and are

hooked up to the Ethernet; end users access this WLAN via their mobile devices, such as

notebook computer, PDA, mobile IP phone, etc. An IEEE 802.20 client interfaced to the

Ethernet via a router connects the WLAN to the base stations of IEEE 802.20 on the

ground, which jointly offer the transparent services on the physical layer and data link

layer between the Internet and the mobiles nodes. Because of the availability of NAT

(Network Address Translate) technology, the whole network on train may only need asingle IP address. A DHCP server can automatically assign each mobile node of thenetwork on the train a dynamic IP address.

Fig 2.1 BRDN Configuration

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The interconnection scheme of WWAN between the IEEE 802.20 base stations

has several choices: IEEE 802.20, IEEE 802.16x, existing cellular communication

systems infrastructure, and the cable, because these base stations are still. As we

discussed in Section 3, IEEE 802.20 is supposed to be the best choice for its high-speed

mobility support, with high data rate, spectral efficiency and low latency. The train-based

IEEE 802.20 mobile client as a whole is on moving when the train is in operation,

switching from one network to another over time (about 10-30 minutes at the regularmoving speed) The mobile IP problem - how the train-based client remains reachable

from the ground stations, while moving around in the Internet quickly, regardless of its

current point of attachment to the Internet.

According the above configuration, the proposed BRDN will adopt IEEE 802.20

and 802.11x standards to implement the function of physical layer and data link layer. The

physical layer provides the air interface of wireless network, and the data link layer offers

reliable data frame transmissions between neighboring hosts, by use of the bit

transmission function provided by physical layer.

2.2 The Operation Scenario

The working scenario of BRDN is shown in Figure 3. When a train departs from

location A, the #1 base stations provides the Internet connection to the train via #1s

network. When the train approaches location B, it will contact both #1 base station and #2

base station at the border because there is a coverage overlapping between the two base

stations. After the IEEE 802.20 client on train switches the connection from #1 base

station to #2 base stations, #2s network starts to provide the path to the Internet, and the

network connection handover is complete. The network handovers happens in the same

way when the train arrives at location C and location D.

Fig. 2.2 The Operation Scenario Of BRDN

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3. WIRELESS LAN

A wireless LAN orWLAN is a wireless local area network, which is the linking

of two or more computers without using wires. WLAN utilizes spread-spectrumtechnology based on radio waves to enable communication between devices in a limited

area, also known as the basic service set. This gives users the mobility to move around

within a broad coverage area and still be connected to the network.

For the home user, wireless has become popular due to ease of installation, and

location freedom with the gaining popularity of laptops. For the business, public

businesses such as coffee shops or malls have begun to offer wireless access to their

customers; some are even provided as a free service. Large wireless network projects are

being put up in many major cities. Google is providing a free service to Mountain View,

California and has entered a bid to do the same for San Francisco.

3.1 History

In 1971, researchers at the University of Hawaii developed the worlds first

WLAN, or wireless local area network, named ALOHAnet. The bi-directional star

topology of the system included seven computers deployed over four islands to

communicate with the central computer on the Oahu Island without using phone lines.

Originally WLAN hardware was so expensive that it was only used as an

alternative to cabled LAN in places where cabling was difficult or impossible. Early

development included industry-specific solutions and proprietary protocols, but at the end

of the 1990s these were replaced by standards, primarily the various versions of IEEE

802.11 (Wi-Fi). An alternative ATM-like 5 GHz standardized technology, HIPERLAN,has so far not succeeded in the market, and with the release of the faster 54 Mbit/s

802.11a (5 GHz) and 802.11g (2.4 GHz) standards, almost certainly never will.

In November 2006, the Australian Commonwealth Scientific and Industrial

Research Organisation (CSIRO) won a legal battle in the US federal court of Texas

against Buffalo Technology which found the US manufacturer had failed to pay royalties

on a US WLAN patent CSIRO had filed in 1996. CSIRO are currently engaged in legal

cases with computer companies including Microsoft, Intel, Dell, Hewlett-Packard and

Netgear which argue that the patent is invalid and should negate any royalties paid to

CSIRO for WLAN-based products.

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3.2 Benefits

The popularity of wireless LANs is a testament primarily to their convenience,

cost efficiency, and ease of integration with other networks and network components. The

majority of computers sold to consumers today come pre-equipped with all necessary

wireless LAN technology.

The benefits of wireless LANs include:

Convenience: The wireless nature of such networks allows users to accessnetwork resources from nearly any convenient location within their primary

networking environment (a home or office). With the increasing saturation of

laptop-style computers, this is particularly relevant.

Mobility: With the emergence of public wireless networks, users can access theinternet even outside their normal work environment. Most chain coffee shops, forexample, offer their customers a wireless connection to the internet at little or no

cost.

Productivity: Users connected to a wireless network can maintain a nearlyconstant affiliation with their desired network as they move from place to place.

For a business, this implies that an employee can potentially be more productive

as his or her work can be accomplished from any convenient location.

Deployment: Initial setup of an infrastructure-based wireless network requireslittle more than a single access point. Wired networks, on the other hand, have the

additional cost and complexity of actual physical cables being run to numerous

locations (which can even be impossible for hard-to-reach locations within abuilding).

Expandability: Wireless networks can serve a suddenly-increased number ofclients with the existing equipment. In a wired network, additional clients would

require additional wiring.

Cost: Wireless networking hardware is at worst a modest increase from wiredcounterparts. This potentially increased cost is almost always more than

outweighed by the savings in cost and labor associated to running physical cables.

3.3 Disadvantages

Wireless LAN technology, while replete with the conveniences and advantages

described above has its share of downfalls. For a given networking situation, wireless

LANs may not be desirable for a number of reasons. Most of these have to do with the

inherent limitations of the technology.

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Security: Wireless LAN transceivers are designed to serve computers throughout

a structure with uninterrupted service using radio frequencies. Furthermore, because of

space and cost, the "antennas" typically present on wireless networking cards in the end

computers are generally nothing more than the most naive of reception devices. In order

to properly receive signals using such limited antennas throughout even a modest area,

the wireless LAN transceiver utilizes a fairly considerable amount of power. What this

means is that not only can the wireless packets be intercepted by a nearby adversary'spoorly-equipped computer, but more importantly, a user willing to spend a small amount

of money on a good quality antenna can pick up packets at a remarkable distance; perhaps

hundreds of times the radius as the typical user. In fact, there are even computer users

dedicated to locating and sometimes even hacking into wireless networks, known as

wardrivers. On a wired network, any adversary would first have to overcome the physical

limitation of tapping into the actual wires, but this is not an issue with wireless packets.

To combat this consideration, wireless networks may choose to utilize some of the

various encryption technologies available. Some of the more commonly utilized

encryption methods, however, are known to have weaknesses that a dedicated adversary

can compromise.

Range: The typical range of a common 802.11g network with standard equipmentis on the order of tens of meters. While sufficient for a typical home, it will be

insufficient in a larger structure. To obtain additional range, repeaters or additional

access points will have to be purchased. Costs for these items can add up quickly.

Other technologies are in the development phase, however, which feature

increased range, hoping to render this disadvantage irrelevant.

Reliability: Like any radio frequency transmission, wireless networking signalsare subject to a wide variety of interference, as well as complex propagation

effects (such as multipath, or especially in this case Rician fading) that are beyond

the control of the network administrator. In the case of typical networks,

modulation is achieved by complicated forms of phase-shift keying (PSK) or

quadrature amplitude modulation (QAM), making interference and propagation

effects all the more disturbing. As a result, important network resources such as

servers are rarely connected wirelessly.

Speed: The speed on most wireless networks (typically 1-54 Mbps) is far slowerthan even the slowest common wired networks (100Mbps up to several Gbps).

There are also performance issues caused by TCP and its built-in congestion

avoidance. For most users, however, this observation is irrelevant since the speed

bottleneck is not in the wireless routing but rather in the outside network

connectivity itself. For example, the maximum ADSL throughput (usually 8Mbps

or less) offered by telecommunications companies to general-purpose customers is

already far slower than the slowest wireless network to which it is typicallyconnected. That is to say, in most environments, a wireless network running at its

slowest speed is still faster than the internet connection serving it in the first place.

However, in specialized environments, the throughput of a wired network might

be necessary.

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3.4 Architecture

FIG 3.1Wireless LAN Architecture Using An Infrastructure BSS

You can connect through a cell phone card via GSM network, satellite hardware

from your satellite company, or the most common a 802.11 router and a either a network

card for AGP slot, PCI/PCI express slot on a desktop, or PCMCIA card for a laptop.

Some laptops already come prepared with built-in wireless.

3.5 IEEE 802.11a

The 802.11a amendment to the original standard was ratified in 1999. The

802.11a standard uses the same core protocol as the original standard, operates in 5 GHz

band, with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable

throughput in the mid-20 Mbit/s. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6

Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4

to point to point. It is not interoperable with 802.11b, except if using equipment that

implements both standards. Since the 2.4 GHz band is heavily used, using the 5 GHz

band gives 802.11a the advantage of less interference. However, this high carrier

frequency also brings disadvantages. It restricts the use of 802.11a to almost line of sight,

necessitating the use of more access points; it also means that 802.11a cannot penetrate as

far as 802.11b since it is absorbed more readily, other things.

3.6 IEEE 802.11b

The 802.11b amendment to the original standard was ratified in 1999. 802.11b

has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access

method defined in the original standard. Due to the CSMA/CA protocol overhead, in

practice the maximum 802.11b throughput that an application can achieve is about 5.9

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Mbit/s using TCP and 7.1 Mbit/s using UDP. 802.11b products appeared on the

market very quickly, since 802.11b is a direct extension of the DSSS (Direct-sequence

spread spectrum) modulation technique defined in the original standard. The dramatic

increase in throughput of 802.11b (compared to the original standard) along with

substantial price reductions led to the rapid acceptance of 802.11b as the definitive

wireless LAN technology. 802.11b is usually used in a point-to-multipoint configuration,

wherein an access point communicates via an Omni-directional antenna with one or moreclients that are located in a coverage area around the access point. Typical indoor range is

30 m (100 ft) at 11 Mbit/s and 90 m (300 ft) at 1 Mbit/s. With high-gain external

antennas, the protocol can also be used in fixed point-to-point arrangements, typically at

ranges up to 8 kilometers (5 miles) although some report success at ranges up to 80120

km (5075 miles) where line of sight can be established. This is usually done in place of

costly leased lines or very cumbersome microwave communications equipment. 802.11b

cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1 Mbit/s if signal

quality becomes an issue. Since the lower data rates use less complex and more redundant

methods of encoding the data, they are less susceptible to corruption due to interference.

3.7 IEEE 802.11g

In June 2003, a third modulation standard was ratified: 802.11g. This works in

the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or

about 24.7 Mbit/s net throughputs (like 802.11a). 802.11g hardware is compatible with

802.11b hardware. Details of making b and g work well together occupied much of the

lingering technical process. In older networks, however, the presence of an 802.11b

participant significantly reduces the speed of an 802.11g network. Even though 802.11g

operates in the same frequency band as 802.11b, it can achieve higher data rates because

of its similarities to 802.11a. The maximum range of 802.11g devices is slightly greater

than that of 802.11b devices, but the range in which a client can achieve the full 54 Mbit/s

data rate is much shorter than that of 802.11b. Despite its major acceptance, 802.11g

suffers from the same interference as 802.11b in the already crowded 2.4 GHz range.Devices operating in this range include microwave ovens, Bluetooth devices, and cordless

telephones. In January 2004, IEEE announced that it had formed a new 802.11

amendment to the 802.11 standard for wireless local-area networks. The real data

throughput is 100Mbit/s (which require an even higher raw data rate at the physical

layer), and is up to 50 times faster than 802.11b, and up to 10 times faster than 802.11a or

802.11g.

3.8 IEEE 802.11n

802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input

multiple-output). MIMO uses multiple transmitter and receiver antennas to allow for

increased data throughput through spatial multiplexing and increased range by exploitingthe spatial diversity, through coding. On 19 January 2007, the IEEE 802.11 Working

Group unanimously approved 802.11n to issue a new Draft 2.0 of the proposed standard.

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3.9 IEEE 802.11 LAYER

Moving data through a wireless network involves three separate elements: the

radio signals, the data format, and the network structure. Each of these elements is

independent of the other two, so its necessary to define all three when you invent a new

network. In terms of familiar OSI (Open System Interconnection) reference model, the

radio signal operates at the physical layer, and the data format controls several of thehigher layers. The network structure includes the interface adapters and base stations that

send and receive the radio signals. In wireless network, the network adapters in each

computer convert digital data to radio signals, which they transmit to other devices on the

network, and they convert incoming radio signals from other network elements back to

digital data.

The IEEE 802.11b standard, as its older version IEEE 802.11 standard, places

specification on the parameters of both:

1. Physical (PHY) Layers

2. Data-Link Layers (Medium Access Control (MAC))

Operation of Physical LayerIn an 802.11 network, the radio transmitter adds a 144-bit preamble to each packet,

including 128 bits that the receiver uses to synchronize the receiver with the transmitter

and a 16 bit start-of-frame field. This is followed by a 48-bit header that contains

information about the data transfer speed, the length of the data contained in the packet,

and an error checking sequence. This header is called the PHY preamble because it

controls the physical layer of the communications link. Because the header specifies the

speed of the data that follow it, the preamble and the header are always transmitted at 1

Mbps. Therefore, even if a network link is operating at the full 11 Mbps, the effective

data transfer speed is considerably slower. That 144-bit preamble is a holdover from the

older and slower DSSS systems, and it has stayed in the specification to ensure that802.11b devices will still be compatible with the older standards, but it really doesnt

accomplish anything useful. So theres an optional alternative that uses a shorter, 72-bit

preamble. In a short preamble, the synchronization field has 56 bits combined with the

same 16-bit start-of-frame field used in long preambles. The 72-bit preamble is not

compatible with old 802.11 hardware, but that doesnt matter as long as all the nodes in a

network can recognize the short preamble format. In all other respects, a short preamble

works just as well as long one. It takes the network a maximum of 192 milliseconds tohandle a long preamble, compared to 96 milliseconds for a short preamble. In other words,

the short preamble cuts the overhead on each packet in half. This makes a significantdifference to the actual data throughput, especially for things like streaming audio and video

and voice-over-internet services.

MAC LayerThe MAC layer controls the traffic that moves through the radio network. It

prevents data collisions and conflicts by using a set of rules called Carrier Sense Multiple

Access with Collision Avoidance (CSMA/CA), and it supports the security functions

specified in the 802.11b standard. When the network includes more than one access point,

the MAC layer associates each network client with the access point that provides the best

signal quality. When more than one node in the network tries to transmit data at


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the same time, CSMA/CA instructs all but one of the conflicting nodes to back off and try

again later, and it allows the surviving node to send its packet. CSMA/CA works like this:

when a network node is ready to send a packet, it listens for other signals first. If it

doesnt hear anything, it waits for a random (but short) period of time and then listens

again. If it still doesnt sense a signal, it transmits a packet. The device that receives the

packet evaluates it, and if its intact, the receiving mode returns an acknowledgement. But

if the sending node does not receive the acknowledgment, it assumes that there has been acollision with another packet, so it waits for another random interval and then tries again.

CSMA/CA also has an optional feature that sets an access point (the bridge between

wireless LAN and the backbone network) as a point coordinator that can grant priority to

a network node that is trying to send time-critical data types, such as voice or streaming

media.

The MAC layer can support two kind of authentication to confirm that a network

device is authorized to join the network: open authentication and shared key

authentication. When you configure your network, all the nodes in the network must use

the same kind of authentication.

The network supports all of these housekeeping functions in the MAC layer byexchanging (or trying to exchange) a series of control frames before it allows the higher

layer to send data. It also sets several options on the network adapter.

3.10 IEEE 802.20 The Key Technology For BRDN

Since July 1999, the IEEE 802.16 Working Group on Broadband Wireless Access

has been openly developing voluntary consensus standards for Wireless Metropolitan

Area Networks with global applicability. Addressing the demand for broadband access to

buildings, IEEE 802.16 provides solutions that are more economical than wired-line

alternatives. The standards set the stage for a revolution in reliable, high-speed network

access in the last mile of Internet by homes and enterprises. On December 11th, 2002,the IEEE Standards Board approved the establishment of IEEE 802.20 Mobile Broadband

Wireless Access (MBWA) Working Group.It described the scope of IEEE 802.20 as:

Specification of physical and medium access control layers of an air interface for

interoperable mobile broadband wireless access systems, operating in licensed bands

below 3.5 GHz, optimized for IP-data transport, with peak data rates per user in excess of

1 Mbps. It supports various vehicular mobility classes up to 250 Km/h in a MAN

environment and targets spectral efficiencies, sustained user data rates and numbers of

active users that are all significantly higher than achieved by existing mobile systems.

According to the above scope, the basic features of IEEE 802.20 include compatibility,

coexistence, distinct identity, technology feasibility, and economic feasibility . According

to the MBWA announcement, IEEE 802.20 is aimed at mobile communication, and its

data rate can reach more than 2Mbps in high speed mobile application. IEEE 802.20 isthe first real broadband wireless network standard that dedicatedly supports the mobility

of network. A comparison between IEEE 802.20 and others mobile techniques for

traditional RCS are shown in Table 1.

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Table: IEEE 802.20 Vs Other Mobile Techniques Used By TraditionalRCS

13

Characteristic GSM-R TETRA version2 GT800(3G) IEEE 802.20

Data rate 2.4-28.8Kbps 96-384Kbps 2Mbps,2Mbps

at the speed of

250mk/h

Latency About 1000ms About 500ms About 250ms About 30ms

Spectralefficiency

200KHz/8ch. 25KHz/4ch. About0.2b/s/Hz/cell

>1b/s/Hz/cell

Cell radius 5-10 Km 10-15 Km 2-5 Km >15Km

Spectrum Licensed bands876-880

921-915MHz

Licensed bands806-821

851-866MHz

Licensed bandsbelow

2.7GHz

Licensed bandsbelow

3.5GHz

Switching

method

Circuit Circuit Circuit/Packet Packet


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4. WIRELESS WAN

4.1 Characteristics of WAN environments

At a high level, most WWAN environments exhibit one or more of the following

characteristics: the bandwidth is very low and varying, the latency is very high andvarying, blackouts exceeding 10 s occur occasionally, and for unreliable

WWAN networks, non-congestion related packet loss may be significant when the user is

traveling at moderate to high speeds.

We have used CDPD as the evaluation platform for WTCP though our design is

not specific to any particular data network. CDPD is a packet data network that overlays

the AMPS cellular telephone infrastructure. A CDPD full duplex channel is a pair of

unidirectional channels; each with a raw capacity of 19.2 Kbps. Up to maximum of 30

users can share a pair of uplink/downlink channels. A set of channels may be dedicated

for CDPD transmission, or CDPD users may be dynamically assigned to channels that are

preferentially shared by cellular phone calls in the latter case, users are more likely to

see short-term blackouts. CDPD transmits compressed and encrypted data, and adds48.2% ReedSolomon coding overhead for forward error correction. Two characteristics

of CDPD are germane to our discussion: the effective throughput of a CDPD channel

typically does not exceed 12 Kbps, and the majority (_75%) of the end-to-end latency is

incurred in the CDPD part of the network between the mobile switching station and the

mobile host. We will revisit the impact of the latter point when we discuss wireless

transport protocols that rely on TCP-aware smarts at the base station .

WWAN wireless networks in general and CDPD networks in particular, typically

exhibit the following six characteristics.

1. Non-congestion related packet loss. Even though CDPD adds 134 bits of Reed

Solomon error-correcting code to every 278 bit block of data, we have measured nocongestion related error ranging from 0 to 10% depending on the speed of mobility

(measured over a range of 055 mph), location of the user vis--vis the base station, and

co-channel interference. TCP assumes that all packet losses result from congestion. A 5%

non-congestion related packet loss can, thus, significantly degrade the performance of

TCP.

2. Very low bandwidth. As we mentioned above, between 1 to 30 users may share a

single CDPD channel of raw capacity 19.2 Kbps. For RAM, the channel is 8 Kbps, while

for Ardis, the channel may be 4.8 Kbps or 19.2 Kbps. Due to the extremely low

bandwidth, the delay-bandwidth product of a connection is small (typically 2 or 3

packets). This can affect the congestion control and fast retransmit mechanisms of TCP

adversely. TCP sometimes observes artificially larger congestion windows as a result of

deep buffering in the CDPD network. While this allows a connection to pump in more

packets into the network, it artificially increases the round trip time and significantly

affects TCP performance in case of a timeout.

3. Large round trip time and variance in round trip time. In CDPD, we have observed

typical round trip times between 800 ms to 4 s. A large fraction of this time is due to

transmission on the wireless link (e.g., transmitting a 512 byte packet at 12K bps takes


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300 ms), and over 75% of the latency is typically incurred in the segments of the

connection lying in between the mobile switching station and the mobile host. 3(a) we

observe the variation in round trip time for UDP handshakes that progress in bursts of 8

from 1.8 s to 6 s for successful handshakes.

This plot shows the impact of large transmission delay on rtt when the sender

bursts packets. TCP sets the retransmission timeout to be the sum of the average roundtrip time and four times the mean deviation of the round trip time This may result in very

large RTOs (e.g., 32 s) in CDPD because of two reasons:

(a) rtt and _(rtt) are inherently large, and

(b) ACKs from the receiver get bunched (see below), and since ACKs clock data packets

in TCP, data packets are sent out in bursts, which further increase the mean and deviation

of rtt. Thus, timeouts affect TCP performance very severely on CDPD.

4. Asymmetric channel bunching of ACKs. CDPD uses DSMA/CD for contention

resolution in the channel. Contentions among mobile users for the uplink channel are

resolved by binary exponential backoff. Consequently, CDPD suffers from the well

known capture syndrome of binary exponential backoff, in which a highly loaded

shared medium ends up bursting the queued packet transmissions of each contending hostin turn. RAM also suffers from the same problem For the common case of downlink data

transmission, ACKs from the mobile to the backbone host get bunched. This further

skews the round trip time computation, and also causes the sender to burst out packets as

mentioned above.

5. Occasional blackouts. Prolonged fades, sudden degradation in signal quality such as

traveling through a tunnel or between overlapping base stations, and temporary lack of

available channels (when cellular phone calls are occupying the channels) can cause

blackouts lasting 10 s or more, and results in the back-to-back loss of a sequence of

packets. Traveling at 55 mph, we observed several blackouts ranging from 10 s to 10 min

during the course of a day.

6. Inter-packet delays as a congestion metric. We have observed that sharp increases in

the short-term average interpacket delay observed by the receiver almost always precede

congestion-related packet loss in the CDPD network. Specifically, an increase in the

average interpacket delay perceived by the receiver is an indication of increased

contention for the wireless link and is a precursor to loss unless connections throttle back

their sending rate. The sender sends packets with a constant interpacket separation of 1 s.

Clearly, interpacket separation is a useful metric to pace the progress of the connection

and can be used to perform rate control.

In wireline networks, the use of both delays and interpacket separation as a metric

for predicting congestion has not been well accepted because of the large variation indelays experienced by packets over the Internet. However, this approach works well in

our target environment because of the extremely low bandwidth of WWANs, wherein the

transmission time over the wireless link predominates. TCP-Vegas uses a variant of this

approach by monitoring round trip times at the sender instead of interpacket delays at the

receiver. Unfortunately, using TCP-Vegas as-is will not work as well in WWAN

environments because of asymmetric channels and the effect of bursting packets on the

computed rtt (points 2 and 4 above).

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4.2 High level architectural tradeoffs in WWAN environments

Before going into the details of the WTCP design, we need to step back and

discuss the tradeoffs between using end-to end mechanisms for wireless TCP versus using

smart mechanisms in the network in order to assist wireless-unaware TCP end-points.

Related work on wireless optimizations for TCP has typically argued against end-to-end

mechanisms on the grounds that it is impractical to change the protocol stacks of allstationary hosts merely to accommodate mobile hosts.

Thus, most of the previous work has focused on making the lossy nature of the

wireless link transparent to the stationary end host by introducing smarts at the base

station via one of three mechanisms: reliable link layers, TCP-aware snoop mechanisms

, or splitting the connection into two (wireline and wireless) distinct components.

Link layer retransmission works well when the latency over the wireless link is

small compared to the coarse grained TCP timer. In the ideal case, the link layer

retransmissions are not expected to significantly interfere with the end to- end rtt

computations or congestion control mechanisms except to eliminate random channel

loss. In WWAN networks, it is the transmission time over the wireless network that

constitutes the bulk of the observed end-to-end latency. Consequently, providing only a

reliable link layer abstraction at a packet-level time scale and keeping TCP unchanged at

the end hosts simply will not work because they will interfere with the reliability and

congestion control mechanisms of TCP. If the wireless data network provides fine

grained link level retransmissions, e.g., RLP used in HDR , then link layer mechanisms

can effectively mask channel error from higher layers.

The Snoop protocol instantiates TCP-aware smarts at the base station (or mobile

switching station) in order to eliminate the problem of false fast retransmits or slow starts

due to random packet loss over the wireless channel. This approach assumes that the

transmission time over the wireless link is significantly smaller than the coarse-grainedTCP timer and round trip time. Moreover, Snoop works well only when the bandwidth-

delay product of the wireless link is at least 3 packets long. However in WWAN

environments, Snoop does not work well because of two reasons:

(a) it exacerbates the problem of large and varying round trip times by

suppressing duplicate ACKs, and

(b) Duplicate retransmissions may be initiated by both the Snoop agent and the

end host (which may observe a timeout) because of comparable timeout values at the two

entities. In fact, we have observed that snooping may possibly degrade the performance

of TCP when the latency over the wireless link dominates the round trip time. In

summary, we believe that Snoop works well in the environment for which it was

designed, but it does not work well in the WWAN environment.

Indirect TCP protocols break the TCP connection at the base station, and maintain

two separate connections one over the wireline network and one over the wireless

network. I-TCP violates the fundamental end-to-end guarantees of TCP by splitting the

connection. Note that the connection split must happen at the base station (or mobile

switching station) serving the mobile host, and the connection state must be moved across

base stations upon handoff. The wireless component of I-TCP is quite simplistic and does

not address issues such as

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(a) non-conformancewith end-to-end semantics,

(b) overhead of moving state between base stations,

(c) deployability constraints due to mandatory changes in base stations, and

(d) design of the transport protocol for the wireless link.

As a practical matter, the I-TCP architecture may not be feasible forWWANs

because it requires significant infrastructure support and maintenance of connection statefrom the WWAN network, which is autonomously managed and may not even

understand TCP/IP internally (e.g., RAM). It is important to note that I-TCP calls for

splitting each transport connection transparently to the TCP end-points. It is, thus, quite

distinct from the WWAN deployment scenario of a mobile host connecting to a dedicated

stationary proxy in the backbone.

We believe that previous approaches that seek to hide the problems of WWAN

networks from TCP at the end host by adding TCP-aware smarts in the mobile switching

station are not applicable to WWAN networks for two reasons:

(a) such approaches require the base station to maintain significant state,

understand TCP/IP, and are often tuned to specific flavors of TCP, and

(b) the fact that the latency between the mobile switching station and the mobilehost is the dominant component of a large and varying round trip time makes such

approaches less effective. The bottomline is that for WWAN environments, both

endpoints must cooperatively address the issues unique to the environment. Moreover, it

is desirable to eliminate network-level smarts because the base stations are owned by an

autonomous entity that may not even be running IP internally. We believe that the key

issues that need to be addressed the non-congestion related packet loss, large and highly

varying latency, asymmetry in data/ACK channel behavior can be effectively solved

with the end-to-end mechanisms proposed in this paper. Of course, the penalty for using

the end-to-end mechanism is that the remote end-point in the backbone must also change.

Luckily, the nature of the WWAN environment and the current deployment pattern

already supports the common case of WWAN users typically connecting through a

dedicated proxy server on the backbone.

4.3. The WTCP approach

Any reliable transport protocol must provide the following functions:

(a) Connection management,

(b) Congestion control,

(c) Flow control, and

(d) Reliability.

WTCP reuses the standard TCP mechanisms for flow control and connection

management. We now focus on the key design choices in WTCP for congestion control

and reliability.

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1. Congestion control

The key aspects of congestion control in WTCP are that it is rate based, uses

interpacket delay as the primary mechanism to determine rate adaptation, performs the

rate adaptation computations at the receiver, predicts the cause of packet loss and reacts

accordingly, and varies the granularity of rate increase/decrease depending on the type of

congestion observed. Additionally, WTCP also tailors its startup behavior to work wellfor short-lived flows. We describe the design aspects ofWTCP congestion control in more

detail below.

1. Rate-based transmission control. As for the common case of bulk data transfer from the

backbone host to the mobile host, ACKs are often bunched together on the return path to

the sender because of the nature of channel arbitration, e.g., DSMA/CA used in CDPD.

With the window-based self-clocking mechanism of TCP, this results in the sender

bursting back-toback data packets, which skews round trip time computations, causes

more bursty queuing at the base station, and consequently more packet drops. WTCP

alleviates these problems by using a rate based scheme that does not use ACKs for self-

clocking. Adopting a rate-based approach does involve explicit clocking and shaping the

traffic according to the current transmission rate of the connection; however, the rates aretypically small enough that coarse grain timers (O(100 ms)) are sufficient to perform the

clocking effectively.

2. Inter-packet delay as the main mechanism for transmission control. We have observed

that monitoring the average interpacket delay at the receiver provides a fairly accurate

measure of the available channel rate for low bandwidth channels. Specifically, the ratio

of the average inter-packet separation at the receiver and the average inter-packet

separation at the sender provides a responsive metric to determine if the transmission rate

needs to be increased, or decreased. Thus, when the network is uncongested or has

incipient congestion, reacting tochanges in the interpacket delay ratio serves to keep the

network uncongested, and significantly reduces the number of congestion-related packet

losses. WTCP, thus, uses this mechanism as the primary transmission rate control

mechanism, and essentially uses incipient congestion detection without waiting to lose

packets before throttling down the sending rate.

3. Distinguishing the cause of packet loss and adjusting transmission rate accordingly.

While inter packet delays are the main mechanism for dealing with incipient congestion,

if the network suddenly moves from uncongested to congested state (e.g., due to a sudden

influx of new

Connections or sudden decrease in available resources), then packets are dropped

due to congestion. Our algorithm must detect such losses and throttle the sending rate

aggressively. In other words, when the receiver observes packet losses, it must predict thecause of the losses and react appropriately. If the loss is predicted to be due to congestion,

then the sending rate is throttled down. We use a heuristic based on the average per-

packet separation to distinguish congestion losses from random losses. In this heuristic,

the receiver initially predicts that all losses are non-congestion losses.

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4. Performing transmission control computations at the receiver.

In WTCP, the receiver performs the rate control mechanisms described above, and

computes the new transmission rate of the sender. With each data packet, the sender

transmits its current interpacket separation. Based on local state and the state in thepacket, the receiver has all the information it needs to update the transmission rate. This is

done at regular intervals, which we refer to as update epochs. An update epoch begins

when period. At the end of the epoch, the receiver performs the rate control computations

and sends the rate update back to the sender in its acknowledgment. Having the receiver

perform the rate computations eliminates the effect of delay variations and losses in the

ACK path. Even if ACKs get bunched, delayed, or lost, the transmission rate is not

altered. WTCP can thus deal with asymmetric channels effectively.

5. Variable granularity rate adjustment. TCP uses the well known linear-increase

multiplicative-decrease policy for adjusting its congestion window. While LIMD is stable

and asymptotically converges to fair channel allocation, the efficiency of the LIMD

algorithmis a function of how severely the decrease is performed. TCP reduces itscongestion window by half upon observing a packet loss. In

WTCP, we seek to detect incipient congestion and react to it early on in the common

case. The goal ofWTCP is to decrease the transmission rate multiplicatively in order to

ensure fairness, less aggressively when reacting to incipient congestion in order to

improve efficiency, and more aggressively when reacting to real congestion in order to

reduce packet loss and alleviate congestion quickly. In order to achieve these goals,

WTCP maintains a history of transmission increase/decrease in the recent past. If the

receiver is required to perform transmission decrease multiple times in quick succession,

it starts to decrease its transmission rate more aggressively. If the receiver observes a

congestion based packet loss, it halves its rate.

As a result of this approach, incipient congestion is handled by a gentle decrease of the

transmission rate, but severe congestion is handled by an aggressive decrease in

transmission rate.

6. Startup behavior.

Since round trips are large in WWAN environments, and since some data transmissions

may be short-lived, WTCP attempts to compute the appropriate transmission rate for a

connection immediately upon startup rather than going through slow start. WTCP uses

the packet-pair approach, wherein it sends two back-to-back packets of maximum

segment size (MSS) and computes their interpacket delay during connection

establishment. This serves as an approximate estimate for the sending rate. Though thepacket-pair approach is known not to work too well in wire line environments, we have

used it as a first-cut approach. We will investigate this approach in the near future.

7. Blackout detection.

Blackouts occur when the connection experiences back-to-back losses for extended

periods of time due to poor channel conditions or lack of available channels. The

reliability mechanism of WTCP elicits an acknowledgment (positive or negative) from


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the receiver for every packet that has been sent, before the sender decides to resend it.

Thus, if a packet has not been acknowledged (positively or negatively) for a threshold

period of time, the sender enters the blackout phase and starts probing the receiver in

order to elicit an acknowledgment. Upon a successful packet handshake after entering the

blackout phase, the sender reverts to the old transmission rate that it was using before the

onset of the blackout phase. This ensures that the packet losses during the blackout phase

do not affect the transmission rate.

2. Reliability

The key aspects of reliability in WTCP are that it uses selective

acknowledgments, it does not use retransmit timeouts, and that it tunes the frequency of

sending acknowledgments to the dynamic network conditions. We describe these aspects

below:

1. Selective acknowledgments. As noted in related work, selective acknowledgments are

very useful in TCP . WTCP uses selective acknowledgments for ensuring reliability.

The receiver periodically sends ACKs at a frequency tuned by the sender (see below),

containing the cumulative and selective ACK. By inspecting the ACK, the sender candetect a hole in the receivers sequence of received packets. By comparing the state

contained in the ACK with local state stored with the last (re)transmission for each

unacknowledged packet, the sender can determine if this last (re)transmission was lost, or

could still be in transit. Thus, selective acknowledgment allows the sender to retransmit

only lost packets.

2. No retransmit timeouts.

It is exceedingly difficult to maintain a reliable estimate of the retransmit timeout.

In fact, many of the performance problems observed in various TCP flavors are caused by

erroneous RTO estimation. WTCP does not use RTOs. Instead, it modifies the SACK

algorithm in order to achieve reliable transmission without RTOs. This mechanism is

described in section 4, and is a very important aspect of WTCP.

3. Controlling ACK frequency.

ACKs carry both reliability and transmission control information, and the sender

must receive ACKs periodically in order to react to the new transmission rate, and

perform flow control. The sender tunes the desired ACK frequency (and notifies the

receiver in the data packet) such that it expects to receive at least one ACK in a threshold

period of time If the sender has one or more packets pending acknowledgment for more

than a threshold period of time, it goes into the blackout mode. The tuning of the ACK

frequency is governed by several factors: (a) observed ACK loss at the sender, (b) half-duplex or full-duplex nature of the WWAN channel, and (c) average and deviation in the

inter-ACK separation observed at the sender. Note that a receiver may also voluntarily

generate a SACK immediately upon observing a hole in the packet sequence. The effects

of these factors on controlling the acknowledgment frequency are part of ongoing work.

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5. MOBILE IPV6

5.1 Introduction

Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched

internet works. It is designated as the successor of IPv4, the current version of the Internet

Protocol, for general use on the Internet.

The main improvement brought by IPv6 (Internet Protocol version 6) is the increase

in the number of addresses available for networked devices, allowing, for example, each

cell phone and mobile electronic device to have its own address. IPv4 supports 232

(about

4.3 billion) addresses, which is inadequate for giving even one address to every living

person, let alone supporting embedded and portable devices. IPv6, however, supports 2128

addresses; this is approximately 51028

addresses for each of the roughly 6.5 billion

people alive today.

By the early 1990s, it was clear that the change to a classful network introduced a

decade earlier was not enough to prevent the IPv4 address exhaustion and that furtherchanges to IPv4 were needed. By the winter of 1992, several proposed systems were

being circulated and by the fall of 1993, the IETF announced a call for white papers (RFC

1550) and the creation of the "IPng Area" of working groups.

IPng was adopted by the Internet Engineering Task Force on July 25, 1994 with the

formation of several "IP Next Generation" (IPng) working groups[1]

. By 1996, a series of

RFCs were released defining IPv6, starting with RFC 2460. (Incidentally, IPv5 was not a

successor to IPv4, but an experimental flow-oriented streaming protocol intended to

support video and audio.)

It is expected that IPv4 will be supported alongside IPv6 for the foreseeable

future. However, IPv4-only clients/servers will not be able to communicate directly withIPv6 clients/servers, and will require service-specific intermediate servers or NAT-PT

protocol-translation servers.

5.2 FEATURES OF IPv6

To a great extent, IPv6 is a conservative extension of IPv4. Most transport- and

application-layer protocols need little or no change to work over IPv6; exceptions are

applications protocols that embed network-layer addresses (such as FTP or NTPv3).

Applications, however, usually need small changes and a recompile in order to run

over IPv6.

Larger address spaceThe main feature of IPv6 that is driving adoption today is the larger address space:

addresses in IPv6 are 128 bits long versus 32 bits in IPv4.

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The larger address space avoids the potential exhaustion of the IPv4 address space

without the need for NAT and other devices that break the end-to-end nature of Internet

traffic. The drawback of the large address size is that IPv6 is less efficient in bandwidth

usage, and this may hurt regions where bandwidth is limited.

For corporate networks however, this will simplify the already complex method of

sub netting.

Stateless auto configuration of hostsIPv6 hosts can be configured automatically when connected to a routed IPv6

network. When first connected to a network, a host sends a link-local multicast

(broadcast) request for its configuration parameters; if configured suitably, routers

respond to such a request with a router advertisementpacket that contains network-layer

configuration parameters.

If IPv6 auto configuration is not suitable, a host can use stateful auto configuration

(DHCPv6) or be configured manually. Stateless auto configuration is only suitable for

hosts; routers must be configured manually or by other means.

MulticastMulticast is part of the base protocol suite in IPv6. This is in opposition to IPv4,

where multicast is optional.

Most environments do not currently have their network infrastructures configured to

route multicast; that is the link-scoped aspect of multicast will work but the site-scope,

organization-scope and global-scope multicast will not be routed.

IPv6 does not have a link-local broadcast facility; the same effect can be achieved

by multicasting to the all-hosts group (FF02::1).

The m6bone is catering for deployment of a global IPv6 Multicast network.

Jumbo gramsIn IPv4, packets are limited to 64 KiB of payload. When used between capable

communication partners, IPv6 has support for packets over this limit, referred to as jumbo

grams. The use of jumbo grams improves performance over high-throughput networks.

Faster routingBy using a simpler and more systematic header structure, IPv6 was supposed to

improve the performance of routing. Recent advances in router technology, however, may

have made this improvement obsolete

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Network-layer securityIPsec, the protocol for IP network-layer encryption and authentication, is an integral

part of the base protocol suite in IPv6; this is unlike IPv4, where it is optional (but usually

implemented). IPsec, however, is not widely deployed except for securing traffic between

IPv6 BGP routers.

MobilityUnlike mobile IPv4, Mobile IPv6 (MIPv6) avoids triangular routing and is therefore

as efficient as normal IPv6. This advantage is mostly hypothetical, as neither MIP nor

MIPv6 are widely deployed today.

5.2 Mobile IPv6 - The Key Issue in BRDN Implementation

Broadband wireless network has generally raised a number of research issues

aiming at the requirements from the mobile applications. Although many have proposed

various resolutions for these issues, for example, mobile IP telephony, more effort has to

be input to find the better ones that will eventually lead to commercialization. In the

context of railroad based mobile applications, the same problems exist as in other mobile

IP applications, but there is a possibility that the problems in railroad based mobile

applications can be solved by alternative approaches with better outcomes because of the

specialty of the application environment.

This section briefly covers two most outstanding topics:

1) IP mobility in BRDN, and2) VoIP for BRDN.

Mobile IP introduced a solution of bi-address, with a long-term IP address on ahome network (HN) and a "care-of address" (COA) when away from its home network.

This home address (HA) is administered in the same way as a "permanent" IP address is

provided to a stationary host. The COA is associated with the mobile node (MN) and

reflects the mobile node's current point of attachment Every time when MN handover to a

foreign network, mobile IP requires it to bind its COA to Home Agent (HA). If MN is

moved far away to HA, the latency caused by handover will be long, and will lose the

packets sent to MN, and the network performance will decline seriously. This Seamless

Handover question has attracted wide attentions.

The IEEE 802.20 mobile client on train is always on moving along with train,

switched from one network to another at a fixed trajectory, particularly, the period of

handover can be predicted by the train schedule . Based on these premises, BRDN adoptsa Predictive Pre-Handover (PPH) algorithm. PPH is based on the highly predictive of

trains moving from one network to the neighboring network. Not only the arriving time,

but also the ID and routers address of the next network are predictable and storage. The

flow of PPH algorithm illustrated as below:

1) Initialization: Create the entry for each network handover and get thepredictive handover table according to the trains schedule.

2) Handover predict: BRDN predicts the next networks ID, routers address and


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arrive time (tp) according to the handover table.

3) BRDN creates the frequency of detection and detects the foreign networkactively.

4) Pre-registering: BRDN implements the preparations for registering, such asauthorization and authentication.

5) Pre-transmitting and storage: According to the result of step 3 and tp,BRDN copies the communication data to the router of the network whichabout to handover.

6) BRDN triggers the handover process when it detected that the quality of thenew networks signal is better than that of the former.

7) If there has any data missed, auto-transmit from the routers storage, and BRDN amends the predictive handover table according the result of this

handover.

IPv6 is presently the IP standard for the next generation Internet, which can

overcome the many known weaknesses of IPv4. With its 128 bits IP address IPv6 has

realized the Stateless Address Auto-configuration (SAAC). Applied to the BRDN, IPv6 is

able to configure each communication node on a train automatically with a unique global

IP address freely. That implies that besides current IPv4 based dynamic IP assigning, IPv6has the potential to provide a better solution. The operation for mobile IPv6 is shown in

Figure 5.1.

Fig. 5.1 the Operation of Mobile Ipv6

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In a hybrid scheme, the WLAN on trains may still use mobile IPv4, which has

been well explored recently, while the main connection to the Internet will be IPv6-based

in order to support better IP mobility by taking the advantage of IPv6. A gateway between

the WLAN and the IEEE 802.20 based WWAN will take care of the IP address

translation. Voice communication is a fundamental service demanded for RCS. In particularly, unless RBDN realizes the priority management, urgent call support,

broadcast, multicast and quick establishment for call question, the RBDN based on

IEEE802.20 can implement in rail-transit. VoIP application based.

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6. SUMMARY

The next generation RCS is a necessity for the advanced mobile network

infrastructure. This paper presents current status of MCT and RCS, and introduces the

latest mobile broadband wireless access technology IEEE 802.20.

This paper proposes the architecture and implementation of BRDN, which will be

built upon IEEE 802.11x, IEEE 802.16x and next generation Internet technologies. The

further research work will be focused on the implementation of mobile IPv6 in

conjunction with the DSS scheme at data link layer.

We further compared IEEE 802.20s technology advantage with other RCS

solutions, and identified that constructing the BRDN based on IEEE 802.20 standard is

not only feasible, but also urgent. This paper further brought forward a configuration for

BRDN, with the considerations in the required features of next generation RCS and the

technology advantages of IEEE 802.20. We also discussed the mobile IP problem for

BRDN the key issue in implementing the next generation RCS, covering the predictive

mobility management for mobile IP and VoIP over BRDN.

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BIBLIOGRAPHY

WEBSITES

http://www.mintc.fi/oliver/upl970-Julkaisuja%2084_2005.pdf

http://www.comp.brad.ac.uk/het-net/HET-NETs04/CameraPapers/P27.pdf

http://www.calccit.org/projects/3b.WiFi_Technical%20Recommendations.pdf

http://www.telenor.com/telektronikk/Bryhni_2004-11-24-perspektiver-tradlos.pdf

http://delivery.acm.org/10.1145/510000/506915/p301-sinha.pdf?key1=506915&key2=4535512711&coll=GUIDE&dl=GUIDE&CFID=15151515&CFTOK

EN=6184618

http://129.118.51.86/zlin/pdf/CSI-98.pdf

http://www.en.wikipedia.org/IPv6

http://www.en.wikipedia.org/WLAN

BOOK

Andrew S Tanenbaum, Computer Networks, Prentice Hall India, 3rd edition 2002

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Documents

Wireless Broadband Railway Digital Network_23