Abstract 1 (i-Baron Technology Limited, Hong Kong )

Interim report (ELEC4890A) Chung Kei IP (2111640)

1. Introduction Recently, wireless communications technology is getting more and more popular in the global

market. The potential users keep increasing in terms of personal, residential, business and

engineering usage purposes. There are several wireless protocols available, which perform

wireless communications including Internet connectivity. The Institute of Electrical and

Electronics Engineers (IEEE) specifies those standards, which are recognized universally. For

instance, IEEE802.15 is a standard for Wireless Personal Area Network (WPAN), also know as

Bluetooth, which is implemented and used within a very short distance (less than 10 meters)

for mobile phone or personal digital assistant (PDA) users, for example. IEEE802.11 is a

standard for Wireless Local Area Network (WLAN), which provides the wireless Internet

connectivity so called Broadband Wireless Access (BWA) with a local area such as a home or a

small office network. IEEE802.16 is a standard for Wireless Metropolitan Area Network (WMAN),

which is able to provide Internet connectivity in much longer distance (in miles) compare with

IEEE802.11 (in terms of meters). Due to the advantages and conveniences IEEE802.16 standard

can bring to the public, it is expected that the wired local area network (LAN) will be mostly

replaced by wireless technology in the future. Finally, IEEE802.20 is a standard for Wireless

Wide Area Network (WWAN), which works at even longer distance then IEEE802.16. Table 1

shows the summary of all IEEE wireless standards.

IEEE Standard 802.11 802.15 802.16 802.20

Operation WLAN WPAN WMAN WWAN

Table 1 – Summary of IEEE wireless standards

IEEE802.15 and IEEE802.11 standards have already had many commercial products available in

the market, and they are working admirably. IEEE802.16 was only developed and recognized in

2001, and so many processes/tasks are still not being done and many are still in testing stages.

In addition, engineers are working on the IEEE802.20, it is still under construction. The purpose

of this report is to examine more on the IEEE802.16 standard. The IEEE802.16 standard is also

called WiMax (Worldwide Interoperability for Microwave Access) standard. WiMax is an

air-interface for fixed broadband wireless access system. It is also know as the IEEE

WirelessMAN air interface. WiMax based technology can be used to transmit signals/data as far

as 30 miles (50 kms), and it offers a solution to the “last-mile” problem by connecting every

signal individual homes’ and business offices’ communications regardless as their locations.

More on this aspect is to be mentioned later in this report. Several topics are covered within

this document. It will include descriptions about the WiMax standard, and then go into deep

details in its MAC layer protocols, descriptions, functionalities and its implementations. Finally,

we describe how this standard can be implemented in OPNET simulation package.

University of Newcastle, Australia 1


2. Academic staffs involve in the project

Dr. Jamil Khan: Direct supervisor of the project. He is also responsible for my coding,

documentations, technical supports and managing the project progresses.

Mr. XinZhi Tan: Assist in OPNET programming, and other technical supports.

3. Project specification and plan

3.1 Scope

As mentioned previously, the scope of this project is to design and develop a simulation model

using OPNET simulation package for the IEEE802.16 WiMax standard. This model should

concentrate in the MAC layer protocols and its functionalities, and it should obtain and analysis

traffic statistics from different scenarios for the use of future development. Figure 1 shows a

conceptual view of IEEE802.16 deployment. As in the figure, a BS services multiple SSs. Each SS

may locate with different distances and they communicate with the BS with different data

rates and packet formats etc.

Figure 1 - IEEE 802.16 Point-to-Multipoint fixed Deployment (Source: Nokia network)

3.2 Equipment

As mentioned previously, the majority of the project relies on the OPNET software simulation

package. Hence, there is no hardware involved, what is really needed is only a computer with

that software installed. Due to the licensing difficulties, the OPNET simulation package is

available in the EE lab in the engineering faculty at the University of Newcastle, Australia. For

more details, please visit the OPNET official website, http://www.OPNET.com.



3.3 What scenario of the WiMax will be investigated?

In this project, we will, hopefully, develop a complete model for the WiMax system. A base

station services multiple SSs, and they are able to generate different types of traffic.

Therefore, the model that will be simulating will be based on this scenario. As will be

mentioned later in this document (section 6 – Implementation in OPNET), a conceptual model

is developed as a future reference for developing this particular model. However, this may

subject to change as the project goes alone.

3.4 Project plan

Various goals and requirements were set for the project:

o Be able to use and understand OPNET programming language, and all its necessary

functionalities within the software.

o Be able to use OPNET to simulate a single source-sink model with transmitters and

receivers that connect to each component in two nodes scenario, where the sources

generate packets and the sinks discard the incoming packets. Figure 2 shows the

implementation of this scenario.

Figure 2 – Simple source-sink model with transmitters and receivers

o Change the central link to a wireless link using different transmitters and receivers.

o Be able to use OPNET to generate a BS to communicate with a single SS by sending

data packets in the downlink direction (BS to SS), and obtain statistics. Then

implement in the same way as in step 2 and 3, but for both uplink (SS to BS) and

downlink directions.

o Modify the code from step 4 and generate a BS, which communicates with multiple

SSs by sending data packets in the downlink direction, and obtain statistics. Then

implement in the same way, but for both uplink and downlink directions.

o Be able to user OPNET to design and implement the model with MAC layer

functionalities (network, node and process models) together with all the necessary

frames, subframes and packets formats as well as frequency division duplexing (FDD).

o Use the MAC layer that is developed in step 6 and implement it into a BS and a single

SS that generate fixed length data packets, and obtain statistics. Then implement in

the same way, but with multiple SSs.

o Use the MAC layer that is already developed in step 7 and implement into a BS and

multiple SSs, which each SS generate different data traffic. Then obtain statistics.



4. Overview of WiMax The following topics were researched before the project design was started. These were

researched so that a better understanding of this particular technology. This section describes

the WiMax standard in general. It firstly states what WiMax is and why we need to adopt this

technology. Furthermore, it describes different standards that are available for the WiMax

standard.

4.1 What is WiMax?

According to Song (2004), “WiMax is an industrial trade organization formed by leading

communications component and equipment companies to promote and certify compatibility

and interoperability of broadband wireless access equipment that conform to the IEEE 802.16

and ETSI HIPERMAN standards. The 802.16 standard is a wireless metropolitan area network

(MAN) technology that will provide a wireless alternative to cable, DSL and T1/E1 for last mile

broadband access. It will also be used as complimentary technology to connect 802.11 hot

spots to the Internet.”

In the other word, the WiMax technology can support a base station (BS), which communicates

with multiple subscriber stations (SS - end customers), which employ a point-to-multipoint

(PMP) architecture in its downlink and employ point-to-point in its uplink direction.

Alternatively, it can communicate with a wireless access router, which is connected to a wired

LAN, for example an Ethernet LAN. Such a case we say it connects to a hot spots to the Internet.

Figure 3 shows an example of two of the most basic scenarios that we should focus and analysis

in this project. The first case (top) is a BS communicates with multiple SSs, and the second

case (bottom) is a BS communicates with an Ethernet LAN (for example). In addition, we can

also analysis other traffic sources or scenarios, such as ATM network etc.

Figure 3 - Two different WiMax Scenarios



4.2 Why needs WiMax?

Currently, there are cable and digital subscriber line (DSL) broadband access services available

in the marketplace. It is extremely popular for many potential Internet users (both business

and residential). In addition, dial-up (using modem) Internet services will almost be replaced

with the cable and DSL broadband access service, because it brings a much faster and stable

Internet connections (i.e. faster data transfer rate for uploading and downloading and better

connection stability).

Despite of all the advantages that were mentioned above, it does however have many practical

limitations. First of all, in order to receive such kind of services, cables must firstly be installed

between the users’ Internet Service Providers’ (ISP) central office switches and the end users

(i.e. the sockets on the walls). In fact, it is extremely costly for such kind of tasks; it is an

all-consuming and expensive process (including installations and maintenances). In fact,

according to Song (2004), “…DSL can only reach about 18,000 feet (three miles) from the

central office switch…”. Therefore, there are many potential users who are in rural and

suburban areas are not able to get the fast DSL connectivity.

By adopting WiMax technology, the so called “last-miles” problem can be solved. WiMax can

solve the problems because it does not need any cable to connect between the central office

switches and subscriber stations (i.e. it can be done wirelessly). Furthermore, it supports a

much long distance between BS and SSs compare with WLAN technology (in terms of miles). In

fact, the main equipment the ISPs need is a BS, and the SS need a ready-to-use WiMax wireless

access card. WiMax technology is the major milestone in broadband wireless Internet access. It

has been improved a lot in terms of its data rates and service coverage distances due to the

nature of the standard. Now, there are only Bluetooth (IEEE802.15, WPAN) and WLAN

(IEEE802.11) technologies available for public use in the market. It is expected that most of

the technology will be replaced by wireless in the future decays. Therefore, a successfully

implementation of WiMax will bring the future technology closer to the real world.

4.3 Globally recognizable standard and different 802.16 standards

The Institute of Electrical and Electronics Engineers (IEEE) already specify a globally

recognizable standard – IEEE802.16. Although 802.16 is the fixed standard, it performs

different operations (i.e. they support different distance and different data rate), and so they

have different “sub-standards”. Customers are able to choose the “sub-standard” so to fit

their requirements as close as possible. Table 2 shows different types of 802.16 standards as

well as their descriptions. In fact, the one that support a longer distance generally has a slower

data rate. On the other hand, a much faster data rate can be achieved in a shorter distance

between the BS and SSs.



Standard IEEE802.16 IEEE802.16c IEEE802.16a IEEE802.16d IEEE802.16e

Time Invented Dec 2001 Dec 2002 Jan 2003 Dec 2003 Dec 2003

Descriptions - 10-66GHz

- Line of sight

(LOS) only

- Up to 134Mbps

Improvement

from IEEE802.16

- 2-11HGz

- Non Line of

sight (NLOS)

- Up to 70Mbps

- Cover 31miles

- 802.16a

modifications

and interoperability

- Support three

physical layer

- Nomadlc

mobility SNS

802.11/16

- Support for

mobility

Table 2 – WMAN evolution [9]

As mentioned above, several sub-standards have been invented for WiMax technology (i.e.

IEEE802.16, IEEE802.16c, IEEE802.16a, IEEE802.16d and IEEE802.16e). Specifically, the

IEEE802.16a standard has got a big change from its original standard (IEEE802.16). In its

original release the 802.16 standard, which was released in December 2001, addressed

applications in licensed bands in the 10 to 66GHz frequency range, it provides a data rate of up

to 134Mbps but it has a short wavelength and it requires a LOS with the base station. [9] LOS

basically means that the signal can reach from the transmitter to the receiver without any

reflection, refraction and diffraction. It is defined in terms of the clearing of Fresnel Zone (FZ).

On the other hand, the 802.16a standard is designed for the frequency between 2-11GHz,

which is able to provide broadband wireless connectivity to fixed, portable and nomadic

device. It uses multipoint-to-multipoint (mesh) topology and it fills the gap between Wireless

LANs and wide area networks. It also can be applied as complimentary technology to connect

802.11 hot spots to the Internet (i.e. connect to an access router which connects to a wired

LAN). Furthermore, it support QoS (to be mentioned in the next section) and it covers up to

50Km of service area, which allow users to get broadband connectivity without the need of

direct LOS with the base station (also called non line of sight - NLOS), and it also provides up to

70Mbps of data rate per BS. That speed (70Mbps) is enough to support hundreds of businesses

users who adopt T1 type or E1 type connectivity and thousands of homes users with DSL type

connectivity with a single BS. [7]

To be able to support mobility is the main feature for IEEE802.16e, which is defined in

IEEE802.16e amendment for medium access control (MAC) layer. Figure 4 shows the mobility

scenarios for IEEE802.16e. We can see from the figure that mobiles users (laptops)

communicate with the access points (AP) then to the rest of the network. In order to

successfully implement mobility issue with an acceptable quality of service (QoS), it requires

very low or zero packet loss and low latency handovers that are acceptable to real-time

applications such as voice over Internet protocol (VoIP). [9]



Figure 4 – Mobility scenarios for IEEE802.16e [10]

5. IEEE802.16 MAC layer descriptions and requirements 5.1 MAC layer descriptions

The 802.16 MAC protocol was designed for point-to-multipoint (PMP) broadband wireless

access, together with a controlling BS, which communicates with SSs not directly to each other

but to various public networks that is linked to the BS. The main focus of the MAC layer in the

WiMax standard is to manage the resources in the air-interface link to operate in an efficient

manner. In addition, to be able to support variety of service, 802.16 needs to accommodate

busty and continuous data traffic, while still keeping the required quality of service (QoS). The

802.16 MAC uses a variable length PDU. Multiple MAC PDUs can be concatenated into a single

burst to save physical overhead. The MAC uses a self-correcting bandwidth request/grant

(please see section 5.4 for details) scheme that eliminates the overhead and delay of

acknowledgements, while simultaneously allowing better QoS handling than traditional

acknowledge schemes. [15]

The 802.16 MAC layer is connection oriented, regardless of the upper layer protocols, that is,

no matter the upper layer is connection-oriented or connectionless. Therefore, all traffic

including inherently connectionless traffic is mapped into a connection. Every service flow is

mapped to a connection and the connection is associated with a level of QoS. Connections are

unidirectional and they are identified using a 16-bits connection identifier (CID). Moreover, the

connections in the downlink direction (BS to SS) are either unicast or multicast with TDMA

technique, while uplink (SS to BS) connections are always unicast with TDM technique.

Furthermore, the MAC is designed for very high bit rates, which can operate up to 268Mbps in

both directions, while delivering ATM compatible QoS. In addition, each SS comes with a

unique 48-bit MAC address, and it serves as an equipment identifier.



The MAC layer consists of three sublayers, which defines the access mechanisms and packet

formats; they are service specific convergence sublayer (CS), MAC common part sublayer (MAC

CPS) and MAC privacy sublayer (MAC PS). Figure 5 shows the architecture of the MAC layer with

its sublayers. Apparently, different sublayers perform different operations. The MAC CPS layer

mainly interfaces with higher layer protocols, such as IPv4, IPv6 or ATM. The MAC PS basically

does authentication and data encryption. [5, 17]

Figure 5 – MAC layer with its sublayers architecture [17]

The CS sublayer provides any transformation or mapping of external network data, and it

receives through the CS service access point (SAP). In addition, CS also performs the following

functions,

o Accept protocol data units (PDUs) from its higher layer

o Classify higher layer PDUs, and support multiple PDUs in single transmission (for both

uplink and downlink)

o Classify external network service data units (SDUs). In addition, it also fragment a

larger SDU into multiple PDUs, this process can be done by SS or BS.

o Associate SDUs to the proper MAC service flow identifier (SFID) and connection

identifier (CID)

o Process the higher layer PDUs according to the specific classification

o Deliver CS PDUs to the appropriate MAC service access point (SAP)

o Receiving CS PDUs from peer entity

o Payload header suppression (PHS), perform CRC if requested.



In addition, CS also performs several tasks like scheduling, bandwidth request and allocation,

ranging and connection establishment maintenance and control. The MAC protocol supports

either full duplex or half duplex subscriber stations. For full duplex subscriber station, SS can

communicate to BS simultaneously. While for half duplex subscriber stations, SS can

communicate to BS only in one direction in any given time. [5]

5.2 Setting up a connection

There are several steps SSs need to perform before joining into a particular WiMax network

with the BS. And it is described in the following. This process is similar to the initialization

process in a cellular phone network. Figure 6 shows the initialisation process in a flow chart for

a clear and more understandable idea of this process.

1. A new SS firstly scans for a downlink channel and establishes synchronization with the

BS. This channel can be obtained from the memory (if this SS has visited a network

previously). Alternatively, it can scan for a new channel among all possible downlink

channels. If it finds a channel that is available, then it synchronizes and attempts to

obtain the channel control parameters.

2. Obtain transmit parameters. SS searches for an uplink channel descriptor message

from the BS, which retrieves the transmission parameters from its uplink channel.

3. According to the messages and parameters from the BS, the SS automatically adjust its

local parameters, and start initial ranging.

4. The SS negotiates and exchange basic capabilities with BS. The SS sends a request

message for information, and it receives an ACK if the request is approved.

5. Perform authentication and registration for SS. If the above process is succeeded, the

BS authorizes the connection for SS. Then it registers the SS in the network. BS sends

additional management messages and the SS becomes managed by the SS. Then, BS

assigns an IP address for SS by the means of DHCP.

6. After successful registration, the BS and SS have the same timing information. Finally

the link is established, the BS sends additional configuration information and transfer

operational parameters between BS and SS. Finally, the connection is set up.

During the initialisation of an SS, there are three CIDs that are established in both directions.

The Basic Connection is used for short time critical messages. The Primary Management

Connection is used to exchange longer and more delay tolerant messages. The Secondary

Management Connection is deployed for higher layer management messages and SS

configuration data. The message on the Secondary Management connection is piggybacked in

IP packets. [18]



Figure 6 – Network entry process [18]

5.3 Bandwidth allocation and request/grants mechanisms

As mentioned previously, during network entry and initialization SS is assigned up to three CIDs

sending and receiving control messages. These are used to allow differentiated levels if QoS to

be applied to different connections carrying traffic. The following two topics describe how a SS

can request a bandwidth (BW) allocation, and BS grants its request.

5.3.1 Request

Request mechanism basically means that subscriber stations indicate to the BS when they

require uplink bandwidth allocation. This request message may come as a stand-alone

bandwidth request header or it may be piggybacked in another incoming message. In addition,

all requests for bandwidth are being made in terms of number of bytes, which is needed to

carry the MAC header and its payload, but not in the PHY overhead, since the uplink burst

profile can be changed dynamically. The request messages can be sent during any uplink

allocation, however except for the time SSs initialize with the BS.

There are two kinds of bandwidth request, incremental request and aggregate request. Firstly,

when the BS receives an incremental BW request, it adds the quantity of bandwidth to its

current perception of the BW needs of the connection. On the other hand, when the BS

receives an aggregate request, it replaces its perception of the BW needs of the connection

with the quantity of BW that is requested. We can examine the TYPE field of the message

header to see if the request is either incremental or aggregate. However, the piggybacked BW

request is always incremental because it does not have a TYPE field. [5]



5.3.2 Grants

Grant mechanism basically means that the BS approves the BW request from a SS. For

subscriber station, BW requests individual connections, while each BW grant is addressed to

the SS’s basic CID, but not to individual CIDs. Grants are either per Connection (GPC) or per

Subscriber Station (GPSS). Moreover, grants are carried in the up-link MAP (UL-MAP) messages.

Figure 7 shows a flow chart of request/grant mechanism.

Figure 7 – SS request/grant mechanism

5.4 Duplex scheme – FDD model in standard

There are two duplexing schemes specified in the standard – Time Division Duplexing (TDD) and

Frequency Division Duplexing (FDD). FDD duplexing scheme will mainly be considered and

adopted within this project. In the FDD mode, upstream and downstream transmission

channels are located on separate frequencies (or RF channels) and the downlink data can be

transmitted in bursts, where the performance requirements of the transmitter and receiver

determine the frequencies separation. It is static asymmetry and it is done by creating a



frequency channel with two separate operating frequencies, whereas one channel is for

transmission and the other is for reception. In addition, there is seamless support for half

duplex SSs. This system makes possible for the transmission and reception work simultaneously.

In other words, this mechanism allows SSs to transmit and receive data at the same time. The

transmission in the downlink direction is done in TDM fashion, with the re-synchronization

preambles, which is able to improve the statistical multiplexing in a deployment with

half-duplex FDD terminals. On the other hand, the uplink operates in TDMA fashion. [5]

A fixed duration frame is used for both uplink and downlink transmission. It allows

simultaneous use of both full-duplex SSs and optionally half duplex SSs. For full-duplex SS, it is

capable of continuously listening to the downlink channel at any time. However, on the other

hand, if half duplex SSs are used, the BW controller will not allocate uplink bandwidth for a

half-duplex SS at the same time, when that particular expects to receive some data on the

downlink channel at the same time. That is, SS can only listen to the downlink channel only

when it is not transmitting in its uplink channel.

5.5 MAC frames and subframes

5.5.1 Frame structure

The frame structure will be dependent on the selection of the access and duplexing techniques,

and it is related with minimal overhead requirements, timing accuracy requirements and

carrier recovery complexity. The typical value for the downstream frame size is about 2-3m sec.

In addition, in IEEE802.16, a frame PHY with a frame duration of 1 ms is employed, which

provides a good compromise between delay and statistical multiplexing.

As mentioned previously, there are two communication paths available, that is, uplink (SS to BS)

and downlink (SS to BS). In the downlink communication path, because the data packets from

the BS are broadcast to all subscriber stations, and the desired SS only picks up the one with

the correct destination address. The downlink MAP message (DL-MAP) uses to specific the

downlink data burst profiles, like DIUC, in each time period in the current downlink subframe

(see 5.6,2), which is suitable to be used in both TDD and FDD system. While in the uplink

communication path, multiple SSs share the channel in a TDMA fashion. The Uplink MAP

message (UL-MAP) provides the channel access assignment to the SSs. This message is being

transmitted from the BS at the beginning of the frame, which define the uplink channel access

together with the uplink data burst profiles in the uplink subframe (see 5.6.3). Figure 8 shows

the format of the frame structure. As can be seen below, the frame is divided into downlink

(DL) subframe and uplink (UL) subframe. Each type of the subframe consists of different fields.



Figure 8 – Frame structure [18]

5.5.2 Downlink subframe

The WiMax standard specifies different downlink frames for both FDD and TDD system, which

described previously. We only consider FDD system here. The FDD downlink subframe has both

downlink MAP (DL-MAP) message and uplink MAP (UL-MAP) message, which are the two

preambles it starts with. These two preambles are used in physical layer transition and

synchronization. The DL-MAP message defines the downlink transmission by giving the

downlink Interval Usage Codes (IUC) together with the starting instants for each interval. In

addition, it specifies the bursts’ start times on the downlink on both TDM and TDMA system.

Figure 9 shows the structures of the downlink subframe for FDD system.

Figure 9 – FDD downlink subframe [17]



Notice that in the figure above the downlink subframe is divided into two portions, that is, TDM

and TDMA portions, where TDM portions comes first and then follow by the TDMA portions.

Each TDM portion consists of data, which is being transmitted to either full duplex, half duplex

of subscriber stations (SSs). These data from SSs is scheduled to transmit later in the frame.

Unlink TDM system where it does not contain any gap or preambles, TDMA portions are

separated by gaps and preambles. In addition, the TDMA portion uses to transmit data to half

duplex SSs, which have been scheduled to transmit at the beginning of the frame.

5.5.3 Uplink subframe

The uplink subframe is used by the SSs, which transmits data to the BS. It gives the stating time

measured at the BS of each transmission from an SS together with the uplink IUC for each burst.

It supports three periods; initial maintenance period, request contention opportunities period

and scheduled data grants period. The base stations are freely to specify all three periods in

any length and order. In addition, it (i.e. the BS) can group initial maintenance period and

request contention opportunities period together and leave the last period (that’s, scheduled

data grants period) for data transmission. Figure 10 shows the structure of the uplink subframe.

The BS uses the request from initial maintenance period to determine the delay in the network

and the requested power or any downlink burst profile changes. During this period, some new

SSs may join into the network. Although multiple SSs can access to the channel at the same

time, collision may occur, which may interfere the other SSs. In the request contention

opportunities period, bandwidth is requested by SSs, which depends on the multicast and

broadcast pools by the BS. Similar to the first period, multiple SSs can access to the channel,

but collision may occur. Finally in the scheduled data grants period, the SS send out data

packets according to the grants that are allocated by the BS. Transition gap separates. In

addition, each period starts with a preamble to allow the new SS to synchronize. [5]

Figure 10 – Uplink subframe [17]



5.6 Fragmentation and packing of packets

5.6.1 Fragmentation

According to the specification of the standard [5], fragmentation is the process by which a MAC

SDU is divided into one or more MAC PDUs. This process allows efficient use of available BW

relative to the QoS requirements of a connection’s service flow. Each connection can be in only

a single fragmentation state at any time. In order to perform the fragmentation process,

sub-header is needed to be added as a part of the packet. There is a fragmentation control

(FC), which is 2-bit long. The FC sub-header contains several different fields, such as

unfragmented, first and last fragment, and continue fragment (see Table 3). Furthermore,

there is another field in the sub-header called fragmentation sequence number (FSN), which is

used to indicate missing continuing fragments and continuous counter across SDUs.

Fragment Fragmentation Control (FC)

First fragment 10

Continuing fragment 11

Last fragment 01

Unfragmented 00

Table 3 – Fragmentation rules

Each connection can only be a single fragmentation state at any one time. In addition, there is

a 3 bit Fragmentation Sequence Number (FSN), which is required to detect any missing

continuing fragments. Fragmentation is initialised for the downlink connections for a BS and

for the uplink connections for SS.

5.6.2 Packing

Packing is the opposite operation of fragmentation. Packing refers to combining multiple MAC

SDUs back into a single MAC PDU. On connections with variable length MAC SDUs, the packed

PDU consists of a sub-header for each packed SDU. On connections with fixed length MAC SDUs,

on the other hand, there does not need any packing sub-header. If packing and fragmentation

can be combined, it can save up to 10% of the system bandwidth.

6. Implementation in OPNET 6.1 What is OPNET?

OPNET is a powerful and professional network simulation software, which is mainly based on

C/C++ programming language. It is object oriented, and it operates according to different

states in state machines (under process model). OPNET is able to create any kinds of networks,

and it is able to get different types of statistics in order to examine the performance of a

particular network. More details will be mentioned later in this document.



6.2 How OPNET will be used to evaluate the WiMax model?

6.2.1 Basic model

Figure 11 shows the idea of a basic OPNET model. Basically a base station and multiple

subscriber stations are all being connected to two common buses, one for transmission

direction and one for reception direction. In OPNET, the hexagon shape components in the

figure below are built in the network model. A node model is built inside each node from the

network model, and a process model is built inside each node model, where the process model

is a state machine that indicates the operation in a state-by-state manner.

Figure 11 – Basic OPNET model (network model)

Figure 12 shows a generic IEEE802.16 model. The scope of this project is to concentrate in it

MAC layer functionalities. Therefore, we will mainly consider in the MAC layer entity level.

Notice that this is a transmitter and a receiver connected to the MAC layer node model. These

two components use to communicate with the other nodes within the network.

Figure 12 – Generic IEEE802.16 model

Figure 13 shows how node model of the MAC layer can be implemented. Notice that there are



two transmitter and receiver pairs connects to the MAC layer node model, the top pair

connects to it convergence sublayer, and the bottom pair connects to it physical layer. This is

the main model that needs to build and it is built inside the hexagon shape components

(network model) as in Figure 11.

Figure 13 – MAC layer implementation (node model)

6.2.2 Conceptual model of WiMax system

Figure 14 shows the conceptual model of the WiMax system that will be simulating. Basically a

single base station and multiple subscriber stations are all connected to two common buses,

where one bus is for reception and the other one is for transmission. Notice that the node

model as in Figure 13 is built in each of the node (the hexagon shape component) in Figure 14,

which implements the MAC layer and its functionalities. Each node is connected to a router,

which routes the traffic from a source to a destination. The destination is where the other end

of the router is connected to. As expected, different type of networks will be examined, such

as Ethernet (10 Base-T and 100 Base-T), ATM network and token ring network etc. Form the

model in Figure 14, it allows us to examine different statistics from different networks. Since it

is currently the development stage of the project, so there are many issues that will still need

to be confirmed and investigated. Therefore, this model may subject to change and/or modify

in the near future.

According to the IEEE802.16 WiMax standard specification, there are different packet formats

need to create in order to allow a base station communicates with a subscriber station. I have

set up some packet format in OPNET for the future use; please see Appendix A for the some

packet formats that have been developed in OPNET. Up until now, I have created six different

types of packet formats; generic MAC header, bandwidth (BW) request header, downlink MAP



(DL-MAP) message, uplink MAP (UL-MAP) message, downlink channel descriptor (DCD) massage

and uplink channel descriptor (UCD) message. In addition, I have provides some description for

those packet formats that I have created.

Figure 14 – WiMax conceptual scenario utilising the MAC layer (network model)

7. Current progress and results



At the time being, I am doing some more researches on how to implement the WiMax model,

and continue learning the OPNET programming language and other functionalities form the

software. As mentioned in the first part of this document, we started with a simple model

before actually implementing the full WiMax standard. Therefore, a simple source-sink model

was built. The following sections cover the information of the model that has currently been

completed. It describes in details of the implementation of the network model, node model

and process model of my design.

7.1 Source-sink model with two nodes

7.1.1 Network model

Figure 15 shows the network model for the source-sink scenario. This model basically

generates packets from one node (every second) and sends it across to the other node. There is

a sink module at each node as well, which accepts and discards the incoming packets. Notice

that this model is implemented in both directions. In other words, both nodes (i.e. node 1 and

node 2) are able to send and discard packets from its opposite node.

Figure 15 – Network model for source-sink scenario with two nodes

7.1.2 Node models

Figure 16 shows the node model of this scenario. Each node from Figure 15 has four

components, a source, a sink, a transmitter and a receiver. A source uses to generate packets,

while a sink uses to discard packets.

Figure 16 – Node model for source-sink

7.1.3 Process models



Figure 17 and Figure 18 shows the source and sink process model respectively. The source

process model was built inside the source node model, and the sink process model was built

inside the sink node model. The OPENT codes as well as all the necessary settings (include

state variables, temporary variables, header block, link model and model attributes) can be

found in Appendix B. For the source model case, at the “start” stage it allows the user to enter

the packet format, start and stop time before the simulation can run. Then it generates an

interrupt and it goes to either “Pkt_Gen” (packet generation) state and “End” (end of process)

state depends on its interrupt code, which is received in the “idle” state.

Figure 17 – Process model for source module

Figure 18 – Process model for sink module

7.2 Results



Based on the model that was built in the previous section, we obtained some statistics on the

end-to-end delay, traffic received in packet/sec, packets, bits/sec and bits, point-to-point

utilization, throughput and delay. All the result figures can be found in Appendix C. Notice that

in this case, all the statistics are stable, since we kept generating and discarding the packets

continuously. However, this will not be the case for the WiMax model, as all the sources

generate different types of traffic with different data rates.

8. Future directions and developments There are still lots of work to be done until the completion stage of the project, and they are

described in the following. For more information, please see the previous section on “Project

plan” (section 3.4).

o Keep learning OPNET programming and its functionalities within the software

o Develop model with a BS and firstly a single SS and then extend to multiple SSs.

Allows sending packets to either directions, then obtain statistics.

o Develop MAC layer protocol and functionalities, and then obtain appropriate

statistics.

o Produce a web site, presentation and final report.

Since lots of progresses are going in parallel in the project, so more tasks will be added in the

near future.

9. Conclusion The IEEE802.16 is a very complicated standard, featuring high adaptiveness to maximize

air-link usage. Therefore, it requires sophisticated algorithms. However, we can see that

WiMax can us bring lots of advantages that make us more convenient in terms of using

broadband Internet access. This report summaries several aspects of the IEEE802.16 (WiMax)

standard. This report stated the concept and understanding of this standard, and especially

went into deep details in the MAC layer as well as the method of implementation in OPNET.

WiMax is expected to have a signification growth in the telecommunications industry. This year

(2005) is actually called “WiMax” year, meaning that there is a milestone to the next stage

regarding this aspect. However, lots of other technologies still have to be invented and fully

tested before coming out to the global market. Since it’s is the early stage of this standard,

fully testing is essential. The best way before inventing the actual commercial product is to

simulate this standard in software. Doing so we can see how this standard works, what

outcome to expect etc.

10. References



[1] Ahmad, A., Xin, C., He, F. & McKormic, M. (2004) “Multimedia performance of IEEE802.16

MAC”, Computer Science Department, Norfolk State University

[2] Antila, J. (2004) “CURRENT TOPICS IN IP NETWORKS”, Helsinki University of Technology,

P.14 - 21

[3] Ganz, A. & Wongthavarawat K. (2004) “Multimedia wireless Networks Technologies,

Standard, and QoS” Prentice Hall, America

[4] IEEE802.16-01/58r1, “The 802.16 Wireless/manTM MAC: It’s Done, but what is it?”, 2001

[5] IEEE802.16-2001, “IEEE Standard for Local and Metropolitan Area Networks – Part 16: Air

Interface for Fixed Broadband Wireless Access System”, April 2002

[6] IEEE802.16.3, “Initial PHY proposal for the IEEE802.16.2 Air Interface Standard”, October

2000

[7] Ghosh, A & Wolter D. R. (2005) “Broadband Wireless Access with WiMax/802.16: Current

Performance Benchmarks and Future Potential”, SBC Laboratories Inc., America

[8] Ramachandran, S., Bostian C. W. & Midkiff, S. F. (2004) “Performance Evaluation of

IEEE802.16 for Broadband Wireless Access”, Centre for Wireless Telecommunications, Virginia

Tech

[9] Wolnicki, J. (2005) “IEEE 802.16: WiMax Broadband Wireless Access: Physical Layer, MAC,

and RRM ”, Institute for Communication Networks (LKN)

[10] Wolnicki, J. (2005) “The IEEE802.16 WuiMax Broadband Wireless Access: Physical Layer

(PHY), Medium Access Control Layer (MAC), Radio Resource Management (RRM)”, Institute for

Communication Engineering (LNT)

[11] http://comet.columbia .edu/~campbell/e6951

[12] http://developer.intel.com/technology/itj/index.htm

[13] http://grouper.ieee.org/groups/802/16/index.html

[14] http://www.ieee.org

[15] http://wimaxforum.org

[16] http://opnet.org

[17] http://WirelessMAN.org

[18]http://www.intel.com/technology/itj/2004/volume08issue03/art04_ieee80216mac/p01_

abstract.htm

Appendix A. Packet/frame formats & descriptions in OPNET


http://developer.intel.com/technology/itj/index.htm

http://grouper.ieee.org/groups/802/16/index.html

http://www.ieee.org/

http://opnet.org/

http://wirelessman.org/

http://www.intel.com/technology/itj/2004/volume08issue03/art04_ieee80216mac/p01_ abstract.htm

http://www.intel.com/technology/itj/2004/volume08issue03/art04_ieee80216mac/p01_ abstract.htm


A.1 MAC Generic Header

Figure A.1 – Generic MAC header format

Name Type Length (original) Descriptions

Type int 8 (6) bits Indicates subheaders & special payload types

Length int 16 (11) bits Length in bytes of MAC PDU

CID Int 16 (16) bits Connection identifier field

HCS int 8 (8) bits Header check sequence field

Header Type int 8 (1) bits Shall be set to zero

Table A.1 – Generic MAC header field’s descriptions

A.2 BW Request Header

Figure A.2 – BW request header format




Type Int 8 (3) bits Indicates type of BW request header

BR Int 16 (16) bits Bandwidth request field

CID Int 16 (16) bits Connection identifier field

HCS Int 8 (8) bits Header check sequence field

Header Type Int 8 (1) bits Equal to 1

Table A.2 – BW request header field’s descriptions

A.3 DL-MAP Message

Figure A.3 – DL-MAP Message format


Type Int 8 (8) Indicates the MAC management message type

PHY synchronisation Int 32 (32)

DCD count Int 8 (8)

BS ID (low) Int 32 (32)

BS ID (high) Int 16 (16)

Element Count Int 16 (16)

Table A.3 – DL_MAP Message field’s descriptions

A.4 UP-MAP Message



Figure A.4 – UP-MAP Message format

Name Type Length (Original) Descriptions

Type int 8 (8) bits Indicates the MAC management message type

Uplink channel ID int 8 (8) bits Indicates uplink channel

UCD count int 8 (8) bits Describes uplink burst profiles

Allocation start time double 32 (32) bits Start time of uplink allocation

Element count int 16 (16) bits A pointer to OPNET list

Table A.4 – UP-MAP field’s descriptions

A.5 Downlink Channel Descriptor (DCD) Message

Figure A.5 – Downlink Channel Descriptor (DCD) Message format


Type int 8 (8) bits Indicates the MAC management

message type

Downlink channel ID int 8 (8) bits Indicates downlink channel

CCC int 8 (8) bits Configuration change count

Table A.5 – Downlink Channel Descriptor (DCD) field’s descriptions

A.6 Uplink Channel Descriptor (UCD) Message



Figure A.6 – Uplink Channel Descriptor (UCD) Message format


Type int 8 (8) bits Indicates the MAC management message type

Uplink channel ID int 8 (8) bits Indicates uplink channel

CCC int 8 (8) bits Configuration change count

RngBS Int 8 (8) bits Ranging backoff start

RngBE int 8 (8) bits Ranging backoff end

ReqBS int 8 (8) bits Request backoff start

ReqBE int 8 (8) bits Request backoff start

Table A.6 – Uplink Channel Descriptor (UCD) field’s descriptions

Appendix B. OPNET codes and setting for source-sink model University of Newcastle, Australia 26


B.1 “Start” state – enter executive

B.2 “Idle” state – enter and exit executive



B.3 “Pkt_Gen” state – enter executive

B.4 “End state” – enter executive

B.5 Link model

B.6 State variable (SV)



B.7 Temporary variable (TV)

B.8 Header block (HB)

B.9 Model attributes

Appendix C. Result from source-sink model



Figure C.1 – End-to-End delay

Figure C.2 – Traffic received at sink (packet/sec)



Figure C.3 – Traffic received at sink (packets)

Figure C.4 – Traffic received at sink (bits/sec)



Figure C.5 – Traffic received at sink (bits)

Figure C.6 – Point-to-point utilization (central link)



Figure C.7 – Point-to-point throughput (central link)

Figure C.8 – Point-to-point queuing delay (central link)


Documents

Abstract 1 (i-Baron Technology Limited, Hong Kong )