IEEE 802.11 and HIPERLANIEEE 802.11 and HIPERLAN · The communication range cannot be determined precisely in wireless ... BER in wireless network is about 10E 4 compared to ... IEEE

IEEE 802.11 WLAN and HIPERLAN

Dmitri A. Moltchanov

E-mail: [email protected]

http://www.cs.tut.fi/kurssit/ELT-53306/

ELT-53306 D.Moltchanov, TUT

NGN BACKBONE

3G MOBILE SYSTEMS

AD HOC NETWORKS

WLAN

BAN/PAN

WMAN

Lecture: IEEE 802.11 WLAN and HIPERLAN 2


• WLAN technical challenges and design issues:

• Overview of IEEE 802.11;

– IEEE 802.11 task groups;

– Development and layered structure of IEEE 802.11.

• Physical layer;

• MAC layer mechanism;

• Comparison of IEEE 802.11a and 802.11b;

• Comparison of IEEE 802.11g and 802.11b;

• System design for networking in IEEE 802.11;

• The HIPERLAN set of standards:

– HIPERLAN/1;

– HIPERLAN/2.



1. WLAN technical challenges and design issuesProblems making WLAN design a complicated task:

• Address is not a physical location:

The station is not always stationary. The address does not give any information about location.

• Dynamically changed topology:

The network connectivity is partial at times.

• Medium boundaries are soft:

The communication range cannot be determined precisely in wireless networks.

• Erroneous medium:

BER in wireless network is about 10E − 4 compared to 10E − 9 in fixed networks.

• Hidden and exposed terminal problems:

Some nodes should (not) be allowed to communicate at a certain time.

TASK: build a reliable network using unreliable channels.



What criteria need to be met?

• Operational simplicity:

Mobile use MUST be able to quickly set up and access network services in a SIMPLE manner.

• Power efficient operation:

The main resource of MT is the power. Design of WLAN must use power saving features.

• Licence-free operation:

Lost cost installation is required for widespread usage of WLAN, e.g., ISM band.

• Tolerance to interference:

There are a lot of technologies operating in ISM band causing interference between them.

• Security:

The inherent broadcast nature make the WLAN vulnerable to different attacks.

• Compatibility:

Compatibility with other technologies and applications is required for a commercial success.

ON TOP OF THIS: global usability, safety, quality of service.



2. Overview of IEEE 802.11What is important about IEEE WLAN standards:

• the IEEE 802.11 standards are de-factor standards for WLANs;

• set 802.11x specifies the physical and the medium access control (MAC) layers only!

• interfaces to higher layer is the same as those in IEEE 802.x standards;

• MAC layer should be able to work with multiple physical layers.

2.1. IEEE 802.11 task groups

A number of task groups have been defined to work on different networking aspects of WLANs:

• 802.11 WG:

– first WG in 802.11 set;

– aims: develop MAC layer and physical layer specifications;

– released in 1997.



• 802.11a WG:

– aims: develop a standard for WLAN operations in the 5GHz frequency band;

– released in 1999 (rates up to 54Mbps).

• 802.11b WG:

– aims: develop a standard for operations in 2.4GHz (ISM) frequency band;

– released in 1999;

– rates up to 11Mbps;

– referred to as Wireless Fidelity (Wi-Fi).

• 802.11c WG:

– aims: develop a standard for bridging and access points operations;


• 802.11d WG:

– aims: definition and requirements for 802.11 operation in different countries;




• 801.11e WG:

– aims: extend the 802.11 to QoS provision;

– work is in progress.

• 802.11f WG:

– aims: inter access point protocols for operation in ESS;


• 802.11g WG:

– aims: extensions to support up to 54Mbps, compatible with 802.11b;


• 802.11h WG:

– aims: MAC layer to be in compliance with European standards;


• 802.11i WG:

– aims: security extensions for 802.11.



• 802.11j WG:

– aims: extensions for operation in 4.9GHz band in Japan.

• 802.11n WG:

– aims: extensions for MAC layer to achieve very high data rates (up to 600Mbps);

– work is in progress.

Additional notes about 802.11 WGs:

• Initially, IEEE 802.11 was released, IEEE 802.11b/a/g/n appeared later;

• IEEE 802.11b was the most successful among family (early entrance to the market);

• IEEE 802.11a first appeared on the marked (not compatible with IEEE 802.11b);

• IEEE 802.11g appeared on the marked (compatible with IEEE 802.11b);

• IEEE 802.11n is backward compatible with 802.11b/g

Why is 802.11 WLANs are so successful:

• simplicity of the basic access protocol;

• good start back in 90s.



2.2. Development and layered structure of IEEE 802.11

Development of IEEE was as follows:

• IEEE 802.11 WG examined new challenges (∼10 years):

– wireless connection management;

– link reliability management;

– power and security management.

• IEEE 802.11b and IEEE 802.11a reasonably fast released their standards.

FHSS PHY DSSS PHY Infrared PHY

IEEE 802.11 MAC

IEEE 802.3 LLC

Figure 1: Layered architecture of IEEE 802.11.



802.11

FHSS

IEEE 802.11 MAC

IEEE 802.3 LLC

802.11

DSSS

802.11b

DSSS

802.11a

OFDM

802.5

PHY

802.3

PHY

802.5

MAC

802.3

MAC

802.1

Managem

ent

TCP/IP

Applications

802.11g

OFDM

Figure 2: Whole stack of 802 standardization effort for LAN.



3. Physical layerIEEE standard supports three options for medium to be used for communication:

• one is based on infrared;

• two others are based on radio.

The physical layer is logically divided into two layers:

• physical medium-dependent sublayer (PMD);

• physical layer convergence protocol (PLCP).

PMD: FHSS PMD: DSSS PMD: Infrared

IEEE 802.11 MAC

IEEE 802.3 LLC

PLCP

Figure 3: Physical layer of IEEE 802.11.



PMD HANDLES FUNCTIONS RELATED TO MEDIUM ADAPTATION:

• encoding;

• decoding;

• modulation.

Three choices for PMD in IEEE 802.11 standard are:

• FHSS PMD:

– operates in 2.4GHz ISM band;

– uses 2-level GFSK for 1Mbps and 4-level GFSK for 2Mbps.

• DSSS PMD:

– operates in 2.4GHz ISM band;

– uses DBPSK for 1Mbps and DQPSK for 2Mbps.

• INFRARED PMD:

– operates in 850-950nm range;

– provides data rates of 1Mbps and 2Mbps using pulse position modulation (PPM).



PLCP ABSTRACTS THE FUNCTIONALITY OF PMD PROVIDING:

• service access point (SAP):

– This SAP is independent of the used transmission technology;

– The SAP abstracts the channel.

• clear channel assessment (CCA) carrier sense signal:

– CCA is used by the MAC layer to implement CSMA/CA medium access scheme.

EXTENSIONS FOR IEEE 802.11 DEFINE THE FOLLOWING PMDS:

• IEEE 802.11b:

– operates in 2.4GHz ISM band, use DSSS with CCK to provide up to 11Mbps.

• IEEE 802.11a

– operates in 5GHz, use OFDM to provide up to 54Mbps.

• IEEE 802.11g

– operates in 2.4GHz, use OFDM to provide 20∼ 54Mbps (DSSS with CCK if <20Mbps).

• IEEE 802.11n: 2.4 or 5GHz, OFDM + MIMO spatial streams, wider 40MHz channel.



4. MAC layer mechanismsMAIN FUNCTIONS OF THE MAC LAYER:

• to arbitrate transmission requests of wireless stations operating in the area;

• to multiplex transmission requests of wireless stations operating in the area;

• to provide roaming support;

• to authentication wireless stations;

• to conserve power consumption.

THE FOLLOWING SERVICES ARE SUPPORTED:

• asynchronous data service is mandatory:

– unicast and multicast packets in infrastructure-based and ad-hoc modes.

• real-time service service is optional:

– infrastructure-based mode where AP controls access to the shared medium.



TWO MEDIUM ACCESS METHODS ARE DEFINED:

• The Distributed Coordination Function (DCF):

– primary access method defined in IEEE 802.11;

– based on CSMA/CA that use RTS-CTS mechanism.

• Point Coordination Function (PCF):

– is implemented on top of DCF to provide real-time service;

– AP controls medium access avoiding simultaneous transmissions.

Controlled delivery Contention delivery

PCF

DCF

Figure 4: Correspondence of DCF and PCF to delivery services.



4.1. Interframe spacing: priorities in frame transmission

THERE ARE FOLLOWING IFSs DEFINED IN IEEE 802.11:

• Short inter-frame spacing (SIFS), shortest:

– the shortest ISF, highest priority;

– used for RTS/CTS frames and ACKs;

– these frames are allowed to transmit just after SIFS.

• PCF inter-frame spacing (PIFS): is the waiting time between SIFS and DIFS (real-time);

– used by PCF in contention-free operation;

– transmission of contention -based stations is just preempted.

• DCF inter-frame spacing (DIFS): used by stations in DSF mode (asynchronous data);

– minimum idle time for contention-based transmissions;

– station is allowed to transmit after DIFS if it has been idle more than this DIFS.

• Extended inter-frame spacing (EIFS): longest (least priority access).

– used when there is an error in frame transmission.



4.2. Medium access mechanism for contention-based access

THERE WERE TWO CHOICES FOR SHARED MEDIUM ACCESS:

• CSMA/CD:

– +: successfully used in wired IEEE 802.3 networks;

– −: collisions in wireless channels are harder to detect;

– −: collisions leads to usage of bandwidth (this is a scarce resource).

• CSMA/CA was adopted.

HOW TO PROVIDE CARRIER SENSING:

• physical carrier sensing:

– direct sensing of the PHY;

– expensive, provided by the physical layer, complexity depends on the PHY.

• virtual carrier sensing:

– provided by the network allocation vector (NAV);

– NAV indicates how long the medium is reserved;

– NAV is set according to fields (durations) indicated in most frames.



RTS

CTS

FRAME

ACK

NAV (RTS)

SIFS

SIFS

SIFS

DIFS

Figure 5: Example of NAV in one-hop neighborhood of the sender.

HOW CSMA/CA PERFORMS:

• if the medium is sensed to be free for DIFS the node access medium for transmission;

• if the medium is busy, the node backs off (defers access) for a contention time;

• CW ∈ {CWmin, CWmax} where CW is the integer multiply of slot times;

• when back off time expires the station can access the medium;

– during back off if the node detect a channel busy, it freezes the CW;

– CW is resumed when the channel is sensed to be free for a DIFS.



DIFS DIFS

BACK-OFF

BUSY, BACK-OFF

FROZEN

DIFS

BACK-OFF

RESUMED

DATA

ACK

SIFS

BUSY

SOURCE:

DESTINATION:

Figure 6: Illustration of the frozen back off timer.

HOW TO SET CONTENTION WINDOW SIZE:

• if CW is small in size:

– values are close to each other at different MTs;

– increase in the number of collisions on the shared medium.

• if CW is very large:

– an unnecessary delay is introduced.

• initially: contention window in set to a random value between (0,CWmin);

• collision occurs: CW doubles up to CWmax.



31DIFS

63DIFS

127DIFS

255DIFS

511DIFS

1023 slotsDIFS

1023 slotsDIFS

initial transmission

1st retransmission

2nd retransmission

5th

6th

3rd

4th

Figure 7: Evolution of the contention window with increasing of transmission attempts.

CWmax: 2 in a certain power −1 slots (slot is medium dependent).



4.3. Acknowledgements

WHY WE HAVE TO USE ACKs:

• you may expect that frequently frames are incorrectly received.

ACKs ARE POSITIVE! HOW IT IS DONE:

• if a packet is correctly received then priority transmission is organized for ACK (SIFS);

• the receiver accesses the medium after waiting for a SIFS and sends ACK.

4.4. Error detection

WHAT IS IMPLEMENTED TO CONCEAL ERRORS:

• Error detection:

– to detect errors CRC code is used.

• Error correction:

– if no ACK is received by the sender, frame is retransmitted;

– the number of retransmissions is limited;

– if the limit is exceeded the error to higher layer is reported.



4.5. RTS-CTS mechanism

receiver senderhidden terminal

collisionpackets packets

Figure 8: Illustration of the hidden terminal problem.

THE RTS-CTS MECHANISM WORKS AS FOLLOWS:

• the sender sends an RTS packet to the receiver including:

– the intended receiver of the data packet;

– the whole expected duration of transmission.



• RTS packet is received by all MTs in one-hop neighborhood of the sender:

– they set their network allocation vector (NAV);

– NAV specifies the earliest time when the station is permitted to attempt transmission.

• the intended receiver of a packet does the following:

– waits for SIFS (high priority!);

– response with clear-to-send (CTS) packet;

– CTS contains the duration field.

• CTS packet is received by all MTs in one-hop neighborhood of the receiver:

– they set their network allocation vector (NAV);

– if the set of stations receiving RTS and CTS are different, hidden terminals exist.

• All stations are informed and the medium is reserved for one sender exclusively;

• The sender starts its transmission after waiting for SIFS;

• The receiver receives packets, waits for SIFS and responds with ACK;

• The NAV in each node marks the medium as free.



BACK-OFF RTS

SIFS CTS

SIFS DATA

SIFS ACK

DIFS BACK-OFF

NAV FROM RTS

NAV FROM CTS

NAV FROM DATAAccess to medium is differed:

SOURCE:

DESTINATION:

OTHER NODES:

Figure 9: Illustration of the RTS-CTS algorithm.

WHAT ARE SHORTCOMINGS AND ADVANTAGES:

• +: completely and reliably removes the hidden terminal problem;

• −: introduces significant overhead and sometimes is not performed;

• +: performs well in overloaded networks.



SOMETIMES RTS-CTS IS NOT PERFORMED:

• execution of RTS-CTS depends on the packet size and RTS threshold:

– packet size is greater than RTS threshold: a four-way RTS-CTS-DATA-ACK is performed;

– packet size is less than RTS threshold: a two-way DATA-ACK is performed.

• −: performs bad in overloaded networks.

• +: performs well in moderately loaded networks.

BACK-OFF DATA

SIFS ACK

DIFS BACK-OFF

SOURCE:

DESTINATION:

OTHER NODES:

Figure 10: When RTS-CTS is not used.



4.6. Fragmentation and reassembly

WHAT IS THE PURPOSE:

• a way to decrease the number of incorrectly received frame due to bit errors;

• the length of the fragments are equal to each other within a single packet;

• the length of final fragment can be less;

• fragments contain information needed to resemble the initial packet.

RTS

CTS

FR 0

NAV (RTS)

SIFS

SIFS

ACKSIFS

FRAG 1SIFS

ACKSIFS

NAV (FRAGMENT 0)

NAV (CTS)

NAV (ACK)

Figure 11: Fragmentation into a number of fragments.



4.7. Frame format

Frame control (2 bytes):

type;

retry bit;

more fragments bit;Address 1,2,3,4 (6 bytes):

48 bits MAC address;

Most common is to use 3 addresses:

source;

destination;

BSSID.

Duration/ID (2 bytes):

3 formats available;

when bit 15 is 0: NAV.

Sequence control (2 bytes):

fragment number;

sequence number.

Body (up to 2304 bytes)

FCS (4 bytes):

CRC



4.8. Other MAC functions

THERE ARE FOLLOWING ADDITIONAL FUNCTIONS OF THE MAC:

• Point Coordination Function (PCF):

– aim: provide QoS parameters: max access delay, minimum transmission bandwidth, etc.

– idea: AP acts as a centralized coordinator;

– how: AP determines and informs a node that has a right to transmit next.

• Synchronization:

– requirement: each station has a clock, all clocks have to be synchronized;

– aim: power management, PCF coordination, hopping synchronization when using FHSS;

– how: timing coordination function.

• Power management:

– usage of batteries requires power conservation/managment functions.

To switch off the transceiver when carrier sensing is not needed! Two state are defined:

– sleep: in this state MT cannot transmit and receive packets (invoked periodically);

– awake: in this state MT may perform all operations.



• QoS in IEEE 802.11 environment:

– Hybrid coordination function (HCF):

∗ AP polls stations in a weighted manner.

– Extended DCF (EDCF):

∗ higher priority MTs are allowed to choose the back off from a smaller CW.

• Support for roaming:

– AP has a range of up to several hundreds meters;

– The roaming between AP is done using the following:

∗ when the stations begins to experience a poor single quality it scans for a new AP.

THERE ARE TWO SCANNING METHODS:

• Active scanning: sends a probe on each channel and waits for a response;

• Passive scanning: listening to the medium to find other networks.

THE INFORMATION ABOUT JOINING THE NETWORK IS OBTAINED:

• from beacon frames used for synchronization;

• probe frames used when PCF is employed.



5. IEEE 802.11a and IEEE 802.11bWhat are differences:

• Power efficiency:

– 802.11b: DSSS;

– 802.11a: OFDM;

– Result: 802.11b is more power efficient than 802.11a.

• Frequency:

– 802.11b: 2.4GHz ISM bandwidth, highly overloaded;

– 802.11a: 5GHz, less overloaded but higher absorbtion rate;

– Result: each has its own advantages and drawbacks.

• Communication range:

– 802.11b: communication range is around ≈ 150m;

– 802.11a: shorter communication range compared to 802.11b (≈ 50m);

– Result: more transceivers are required for 802.11a.



• Data-rate:

– 802.11b: up to 11Mbps;

– 802.11a: up to 54Mbps, (up to 72Mbps) (realistically, less, fastly decreases with distance);

– Result: 54Mbps looks promising if one can get it working.

• Cost efficiency:

– 802.11b: well-established manufacturing;

– 802.11a: components are more expensive, more transmitters are required;

– Result: 802.11b is cheaper.

• Compatibility:

– 802.11b: this was the first available WLAN;

– 802.11a: is not compatible with 802.11b;

– Result: not so good for 802.11a.

• Number of users:

– 802.11a: can accommodate more users due to increase in channels and bandwidth;

– Result: capacity of 802.11a is higher.



6. IEEE 802.11g and IEEE 802.11bWhat are differences:

• Power efficiency:

– 802.11b: DSSS;

– 802.11g: OFDM;

– Result: 802.11b is more power efficient than 802.11a and IEEE 802.11g.

• Frequency:

– 802.11b: 2.4GHz ISM bandwidth, highly overloaded;

– 802.11g: 2.4GHz ISM bandwidth, highly overloaded;

– Result: poor performance in overloaded environment (802.11a is better: 5GHz).

• Communication range:

– 802.11b: communication range is around ≈ 150m;

– 802.11g: shorter communication range compared to 802.11b, higher than 802.11a;

– Result: only slightly more transceivers are required for 802.11g than for 802.11b.



• Data-rate:

– 802.11b: up to 11Mbps;

– 802.11g: up to 54Mbps (realistically ≈ 10− 20);

– Result: less than 802.11a, more than 802.11b.

• Cost efficiency:

– 802.11b: well-established manufacturing;

– 802.11g: components are more expensive than 802.11b, less expensive than (802.11a);

– Result: 802.11b is cheaper.

• Compatibility:

– 802.11b: this was the first available WLAN;

– 802.11g: compatible with 802.11b;

– Result: from this point of view 802.11g is a nice choice.

• Number of users:

– 802.11g: the same number of channel as in 802.11b;

– Result: capacity of 802.11a is higher.



7. System design for networking in IEEE 802.117.1. Types of networks based on IEEE 802.11

THERE ARE TWO BASIC CONCEPTS IN 802.11 SYSTEM:

• Basic Service Set (BSS): a set of stations communicating with each other;

• Basic Service Area (BSA): area in which stations communicate.

IEEE 802.11 NETWORK MAY OPERATE IN TWO MODES:

• independent BSS: IBSS (ad-hoc mode):

In this mode MT communicates directly with other MTs without APs.

Figure 12: Illustration of IBSS.



• infrastructure BSS:

In this mode an MT communicates via AP.

– +: less complex configuration;

– +: AP assists station in power savings.

Figure 13: Illustration of infrastructure BSS.

Note: to obtain network services stations must be associated with AP.

Note: station can be associated with only one AP.



• extended service set (ESS):

Is created by linking BSSs using backbone network.

– +: provide service in larger areas (via layer 2, AP acts as a bridge);

– +: does not specify a particular technology;

– +: just requires backbone to provide a specific set of services.

Backbone

BSS1BSS2

BSS3

Figure 14: Illustration of ESS.



7.2. Components of infrastructure BSS/ESS

IEEE 802.11 NETWORK CONSISTS OF THE FOLLOWING COMPONENTS:

• Distribution system: backbone network (e.g. IEEE 802.3);

• Access point: device performing wireless-to-wired mapping functions;

• Wireless medium: air interface;

• Station: end-user equipment using IEEE 802.11.

Distribution system

Access point

StationWireless

medium

Figure 15: Components of IEEE 802.11 WLAN.



7.3. Distribution system

Include not only backbone network but bridging capabilities of APs.

IT PROVIDES:

• relay of frames within ESS;

• inter AP protocol (IAPP):

provides a way for AP to know where (to which AP) to forward a frame.

Bringing capabilities

Distribution system

Wireless medium

Figure 16: Illustration of distribution system.



7.4. Network services

SERVICES OFFERED BY 802.11 ARE DIVIDED INTO:

• services offered by AP;

• services offered by MT.

THE FOLLOWING SERVICES ARE OFFERED BY AP:

• Association:

The address of MT must be known by AP before communication. This is done via association.

• Reassociation:

The established association is transferred from one MT to another using reassociation.

• Disassociation:

When node leaves AP or shuts down it enforces disassociation.

• Distribution:

This refers to the distribution of traffic within, in and out the network.

• Integration:

This service is evoked when transmission via non-IEEE 802.11 network is required.



THE FOLLOWING SERVICES ARE OFFERED BY MT:

• Authentication:

This is used in order to establish the identity of stations to each other. Authentication imple-

mentation may range from insecure handshake procedures, to public key encryption schemes.

• Deauthentication:

This procedure is evoked to terminate existing authentication.

• Privacy:

The contents of messages os sometimes encrypted using a certain protocol to prevent unau-

thorized reading.

• Data delivery:

IEEE 802.11 networks provide a way to transmit and receive data. The transmission is not

guaranteed to be completely reliable.



7.5. Mobility support

THREE TYPES OF TRANSITIONS BETWEEN APs ARE SUPPORTED:

• No transition:

– station is not moving or moving within a BSS.

• BSS transition:

– movement between BSS within ESS;

– station continuously monitors the signal level;

– use reassociation to be associated with another AP;

– requires cooperation between APs, should be standardized in IAPP.

• ESS transition:

– movement between two ESSs;

– does not support this type of transition (except for allowing to associate again);

– higher-layer connection will be interrupted;

– result: seamless transition is not supported.



8. The HIPERLAN set of standardsThe HIgh PERformance Radio LAN (HIPERLAN)

BASIC INFORMATION:

• European counterpart to the IEEE 802.11;

• defined by European Telecommunications Standards Institute (ETSI).

FOUR SET OF STANDARDS HAVE BEEN DEFINED

• HIPERLAN/1:

– radio LAN, 5.15GHz and the 17.1GHz, max rate: 23.5Mbps.

• HIPERLAN/2:

– short-range wireless access to IP, 5GHz, rates: 6Mbps to 54Mbps.

• HIPERACCESS:

– to cover the so-called ’last mile’, rate: 25Mbps.

• HIPERLINK:

– point-to-point static interconnections, 17GHz, range: up to 150m.



9. HIPERLAN/1WHAT HIPERLAN/1 PROVIDES:

• allows nodes to be deployed in a pre-arranged and ad-hoc manners;

• provides forwarding mechanism e.g., multi-hop routing;

• provides data rate of around 25Mbps;

• has the capability to support both multimedia and asynchronous data;

COMPARING WITH IEEE 802.11:

• IEEE 802.11 is seen as dumb but simple;

• HIPERLAN is clever but more sophisticated.

PROTOCOL STACK: TWO LAYERS OF OSI:

• physical layer;

• data-link layer:

– medium access control (MAC) sublayer;

– channel access control (CAC) sublayer.



9.1. Physical layer

WHAT ARE FUNCTIONALITIES:

• modulation and demodulation;

• forward error corrections;

• signal strength measurement;

• synchronization between the sender and a receiver;

• channel sensing (idle/busy) using CCA scheme this is similar to IEEE 802.11.

9.2. MAC sublayer


• processing packets from the higher layer;

• scheduling packets according to the QoS requests;

• forwarding packets and power conservation features;

• communication confidentiality using encryption-decryption schemes.



TWO SPECIFIC FUNCTIONS ARE PROVIDED:

• multi-hop forwarding:

– in ad hoc mode;

– topology-related information is exchanged between nodes.

• QoS priorities assignment (!!!):

– computes access priority for each PDU received from the higher layer;

– maps this priority to the channel access mechanism (CAM) priority.

9.3. The CAC sublayer


• CAC sublayer offers a connectionless data service to MAC sublayer;

• The MAC layer uses the service to specify the CAM priority;

• CAM priority is a single QoS parameter for the CAC layer;

• the packet selected for transmission is transmitted.



9.4. Channel access scheme

ELIMINATION YIELD NON-PREEMPTIVE MULTIPLE ACCESS

• dynamic listen-then-talk channel access protocol;

• similar to CSMA/CA used in IEEE 802.11;

• exception: provides service differentiation!

CHANNEL ACCESS CONSISTS OF THE FOLLOWING CYCLES:

• . . .

• synchronization;

• prioritization;

• contention;

• transmission;

• synchronization;

• . . .



• PRIORITIZATION:

Aim: to detect nodes having packets with the highest CAM priority. Two stages:

– priority detection:

A node listens channel for a number of slots proportional to the CAM priority of its packet.

– priority assertion:

A node asserts its priority sending a signal in the slot corresponding to the packet priority.

Nodes having packets with low CAM priority detects nodes with the higher priority packets.

• CONTENTION:

Aim: eliminate as many nodes as possible to minimize the collision:

– Elimination phase:

∗ a node transmits signal for geometrically distributed number of slots (0.5k,k is the CAM);

∗ the it senses the media for one slot;

∗ if transmissions in this slot are detected, a node stops contention process;

∗ if no, it goes to yield phase.

– Yield phase:

A node listens channel for a number of slots. If it is idle, the node is chosen for transmission.



• TRANSMISSION:

The successful delivery is acknowledged using ACK packets.

1 2 3 4 5AP

1

2

3

4

Prioritization

Data

Data ACK

ACK

Contention Transmission

Cycle

Elimination phase Yield phase

Elimination survival identification interval

Figure 17: Illustration of the EY-NPMA algorithm.



9.5. Power conservation

HIPERLAN/1 ALLOWS POWER TO BE CONSERVED AT:

• MAC layer:

At the MAC layer two modes are defined namely:

– sleep;

– awake.

To implement this function nodes are divided into:

– p-savers: those nodes that want to implement sleep function;

– p-supporters: neighbors to p-savers chosen to sleep later.

Note: p-savers must be active only on pre-determined slots.

• Physical layer:

A physical signal consists of:

– low bit rate (LBR) burst: uses FSK modulation (lower rate/less power);

– high bit rate (HBR) burst: uses GMSK modulation (higher rate/more power).

The LBR contains the dest. address and precedes HBR, node decides whether to receive HBR.



10. HIPERAN/2HIPERLAN/2 WAS DRIVEN BY NEEDS TO:

• support higher data rates than IEEE 802.11;

• support QoS guarantees;

• support handover procedures;

• provide integration with cellular networks;

• seamlessly support of IP and ATM networks.

CORE NETWORK: IP, ATM, UMTS

APAP

AP

MT MT

MT

MTMT

Figure 18: Illustration of the typical HIPERLAN/2 configurations.



THE AP IN HIPERLAN/2 CONSISTS OF:

• one of many transceivers called access point transceivers (APT);

• access point controlled (APC) controlling APTs.

TWO MODES OF OPERATION ARE DEFINED FOR HIPERLAN/2:

• Business environment:

– the ad-hoc architecture of HIPERLAN/1 extended to support centralized communication.

• Home environment:

– ad-hoc mode of communications controlled by a central entity elected from nodes.

THE PROTOCOL STACK OF HIPERLAN/2 CONSISTS OF:

• physical layer;

• data-link control layer (DLC);

• convergence layer (CL).



10.1. Physical layer

CLASSIC FUNCTIONALITIES AND(!!!):

• choice of suitable modulation using link adaptation scheme (6-54Mbps).

10.2. Data-link control layer

PROVIDES: the logical connection-oriented link between AP and MTs.

THE DLC IS DIVIDED TO:

• Radio link control (RLC) sublayer on the control plane;

• Error control sublayer on the user plane;

• MAC sublayer.

1. ERROR CONTROL SUBLAYER:

• error detection: CRC;

• error correction: Selective-Repeat ARQ (SR-ARQ).

Note: packet discard mechanism can be provided specifying the maximum delay.



2. THE RLC SUBLAYER:

• Association control function (ACF):

– registration and authentication function.

• DLC user connection control (DCC):

– setup, modify, and terminate connections.

• Radio resource control:

The RRC is responsible for efficient utilization of available frequency resources:

– Dynamic frequency selection:

∗ allows to choose the best suitable frequency (channel) for communications;

∗ unique to HIPERLAN/2.

– Handover:

∗ sector handover (moving to another sector of the same antenna of an APT);

∗ radio handover (handover between two APTs under the same APC);

∗ network handover (handover between two APs in the same network).

– Power saving: these features are similar to those used in HIPERLAN/1.



3. THE MAC SUBLAYER

Based on: TDMA/TDD with centralized AP scheduling providing:

• collision free transmission;

• QoS support.

This protocol simultaneously supports connection-oriented:

• AP-MT multicast and unicast transmission;

• MT-MT peer communication.

10.3. Convergence layer


• to adapt requirements of higher layer protocols to services provided by the lower layers;

• to convert the higher layer packets into fixed size ones used by HIPERLAN/2.

CL IS UNIQUE FOR EVERY SUPPORTED CORE NETWORK:

• packet-based CL: for variable length packets such as IP;

• cell-based CL: for fixed size ATM cells.


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