seminar report ‘04

HIPERLAN/2 Seminar report ‘04

1. INTRODUCTION

Recently, demand for high-speed Internet access is rapidly increasing and a lot of

people enjoy broadband wired Internet access services using ADSL ( Asymmetric

Digital Subscriber Line) or cable modems at home. On the other hand , the cellular phone is getting very popular and users enjoy its location-free and wire -free services. The cellular phone also enables people to connect their laptop

computers to the Internet in location-free and wire-free manners. However ,

present cellular systems like GSM (Global System for Mobile communications)

can provide much lower data rates compared with those provided by the wired

access systems, over a few Mbps(Mega bit per second).Even in the next generation

cellular system, UMTS ( Universal Mobile Telecommunications System), the

maximum data rate of its initial service is limited up to 384 kbps; therefore

Department of ECE Govt. Engg. College,Thrissur1


even UMTS cannot satisfy users’ expectation of high-speed wireless Internet

access. Hence, recently, Mobile Broadband System (MBS) is getting popular

and important and wireless LAN (Local Area Network) such as ETSI (European

Telecommunication Standardization Institute) standard HIPERLAN (High

PErformance Radio Local Area Network) type2 (denoted as H/2) is regarded

as a key towards providing high speed wireless access in MBS. H/2 aims

at providing high speed multimedia services, security of services , handover

when roaming between local and wide area as well as between corporate

and public networks. It also aims at providing increased throughput of

datacom as well as video streaming applications. It operates in the 5 GHz

band with a 100 MHz spectrum. WLAN is W-ATM based and is designed

to extend the services of fixed ATM networks to mobile users. H/2 is

connection oriented with a connection duration of 2 ms or multiples of

that. Connections over the air are time-division multiplexed . H/2 allows



interconnection into virtually any type of fixed network technology and

can carry Ethernet frames, ATM cells and IP packets. Follows dynamic

frequency allocation. Offers bit rates of 54 Mbps.

2.HISTORY

Currently WLANs provide wideband wireless connectivity between PCs and

other consumer electronic devices as well as access to the core network and

other equipment in corporate, public , and home environments. The demand

for mobile broad-band communication caused the European Telecommunications

Standards Institute ( ETSI ) to create the Broadband Radio Access Networks

(BRAN) Project. The project makes available various technologies for the

access to wired networks in private as well as in public environments until the

year 2000 and offers bit rates upto 155 Mbps.



The project started with the goal of specifying a W-ATM

( Wireless- Asynchronous Transfer Mode ) based air interface for applications.

The W-ATM idea has been strictly followed when specifying the DLC

(Data Link Control) layer of H/2 where a user data packet handled

in the MAC (Medium Access Control) layer is one ATM cell. It was planned

from the start that an ATM-based WLAN should be able to support any

broad-band network-based service upto a WLAN’s bandwidth limitations

according to the service classes known from ATM networks.

To be able to establish a wireless system to support any type of transport

network , both connection- and packet-oriented data transmission are taken

into account.

An earlier initiative of the ETSI/BRAN project was HiperLAN/1, a connectionless packet-based broad-band WLAN standard at 5 GHz in

1996 that has not found acceptance to date owing to the lack of products.

H/2 is connection-oriented high-performance radio technology, specifically

suited for operating in LAN environments. This system operates in the

unlicensed 5-GHz frequency band that has been specifically allocated to



WLANs . In contrast to the IEEE 802.11 Ethernet technology H/2 is

connection oriented with a connection duration of 2ms or multiples of that.

connections over the air are time-division multiplexed(TDM). H/2 allows

interconnection into virtually any type of fixed network technology and

can carry Ethernet frames, ATM cells and IP packets.

3. H/2 SYSTEM ARCHITECTURE

H/2 system provides wireless access to wired networks for users by a mobile

Terminal(MT) inside buildings , outside in free terrain, or in the proximity of

buildings. The system is cellular.An AP (Access Point) is typically connected

to a core network or a distributed system consists of an APC (Access Point

Controller) and one or more APTs (Access Point Transceiver). An APT

operates one frequency carrier and covers a certain area, called the radio

cell. The APC(AP Controller) is responsible for the management of its APTs.



Two operation modes are defined for the H/2 DLC (Data LinkControl ):

centralised mode and direct mode . In the direct mode , MTs communicate

directly over direct links with each other. In both modes, the AP assigns

the radio resources and controls the communcation in the radio Cell.

In HIPERLAN, each communicating node is given a HIPERLAN ID (HID)

and a Node ID (NID). The combination of these two IDs uniquely identifies

any station, and restricts the way it can connect to other HIPERLAN nodes.

All nodes with the same HID can communicate with each other using a



dynamic routing mechanism denoted Intra-HIPERLAN Forwarding.

The HIPERLAN/2 Access Points (APs) have a built-in support for

automatic transmission frequency allocation within the AP's coverage area.

This is performed by the Dynamic Frequency Selection (DFS) function. An

appropriate radio channel is selected based on both what radio channels are

already in use by other AP's and to minimize interference with the

environment. Thus, there is no need for manual frequency planning as in

cellular networks like GSM. DFS algorithm is described below, where each

AP selects a channel with the least interference level.

Step 1: APs are randomly ordered.

Step 2: According to the order, select an AP.

Step 3: The AP estimates total power of interference signals from other APs

which have already selected a channel and selects a channel with the least

interference signal power. ( Repeat step 2 and 3 until all APs select their

channels.) Interference signals from MHs are not take into account in this

algorithm. And we assume that each AP selects a channel only once and



never changes the channel.

4.H/2 SERVICE MODEL

The H/2 service model comprises the physical and the DLC ( Data Link

Control) layer for both the MT and the AP. Various network types like IP,

Ethernet, IEEE1394 and ATM can be connected to the DLC layer by the

Convergence layer that performs the adaptation of the packet formats to

the requirements of the DLC layer. For higher layers other than ATM, the



convergence layer contains a SAR ( Segmentation And Reassembly) function.

The physical layer provides the basic transport functions for the DLC

(Data Link Control) PDU (Protocol Data Unit).The physical layer includes

the medium or air interface through which the data is transmitted.In the physical

layer the PDUs are Orthogonal Frequency Division Multiplexed(OFDM).



4.a.Data Link Control (DLC) layer

The DLC layer is vertically sub-divided into the control plane and the user

plane. In the user plane , the data transport function is fed with user data

packets from the higher layers via the U-SAP ( User Service Access Point).



This part contains the Error control that applies the ARQ ( Automatic

Repeat reQuest) protocol. The control plane consists of the RLC (Radio Link

Control ) protocol that includes the DCC ( DLC Connection Control), the

RRC (Radio Resource Control), and the ACF (Association Control Function).

Both the user plane and the control plane access the physical medium

via the MAC ( Medium Access Control ) protocol.

4.b.PHYSICAL LAYER

H/2 systems are meant to operate as private or public systems in the license

exempt spectrum in the 5-6 GHz band.The channel grid is 20 MHz.The H/2

sampling frequency is choosen equal to 20 MHz at the output of a typically

used 64-point Inverse Fourier Transformation . 52 subcarriers are used per

channel to facilitate implementation of filters and to achieve sufficient



adjacent channel suppression. 48 subcarriers carry the actual data and 4 are

pilots that facilitate phase tracking for coherent demodulation.A key feature

of the physical layer is to provide several PHYSICAL modes with different

coding and modulation schemes that are selected by a link adaptation

mechanism. The channeling is implemented by Orthogonal Frequency

Division Multiplexing (OFDM) due to its excellent performance on highly

dispersive channels. The basic idea of OFDM is to transmit broadband, high

data rate information by dividing the data into several interleaved, parallel

bit streams, and let each bit stream modulate a separate sub-carrier. The

channel spacing is 20 MHz , which allows high bit rates per channel yet has

reasonable number of channels. The independent frequency sub-channels are

used for one transmission link between the AP(Access Points) and the MTs.



MAC ( Medium Access Control): MAC protocol functions are used for

organising access to and transmission of data on the radio link. The control

is centralised to the AP (Access Point) that informs the MTs (Mobile Terminal)

at what point in time in the MAC Frame they are allowed to transmit their

so- called PDU ( Protocol Data Units ) trains. The length of the PDU

trains vary depending on the Resource Requests (RRs) received at the AP

from the MTs. The air interface is based on time-division duplex (TDD)

and dynamic time-division multiple access ( TDMA ), which allows for

simultaneous communication in both downlink and uplink within the same

time frame, i.e. the MAC frame.



The MAC frame format consists of four elements: Broadcast Channel

(BCH), Down Link (DL), Up Link ( UL ), and Random Access (RA) . Except

for the broadcast control, the duration of the fields is dynamically adapted

to the current traffic situation. The whole DLC is based on scheduling

efficiently MAC frame. The MAC frame and the transport channels form

the interface between the DLC and the physical layer.

The broadcast phase carries the BCCH (Broadcast Control Channel), the

FCCH (Frame Control Channel) and the RFCH (Random Access Feedback

Channel). The BCCH ( downlink only ) transmits control information in each



MAC frame and to all MTs. It provides information about transmission

power levels, starting point and length of the Frame channel & Random

channel. It also transmits the AP identifier and the wakeup indicator.

The FCCH ( downlink only) transmitted in the Frame Channel (FCH) contains

an exact description of how the current MAC Frame resources have been

allocated in the downlink , uplink and the direct link phases.

The downlink phase carries user-specific control information and

the user data,transmitted from an AP to one or more MTs. The uplink phase

carries control and user data from the MTs to the AP. The direct link phase

carries user data from the MTs to the AP.

In the random access phase (RAP) the MTs that do not have capacity



allocated in the current uplink phase may use the Random channel (RCH)

to transmit a resource request.Non-associated MTs first get in contact with a

AP via the Random channel that is also used by MTs during handover to

have access to a new AP.

4.c.CONVERGENCE LAYER:



The Convergence Layer(CL) adapts the core network to the H/2 DLC layer.For

each network supported a specific convergence layer has been defined. The

convergence layer provides all functions needed for connection setup and

mobility support.

There are two types of convergence layers defined: Cell based

and packet based.The packet based convergence layer(CL) is defined to integrate

H/2 into existing packet-based networks and support IP, IEEE 802.3 and

point-to-point protocol. It provides among others a SAR ( Segmentation And

Reassembly) function to fit IP packets into the fixed length payload of the

H/2 Long transport channel PDUs ( Protocol Data Unit ). The cell based



convergence layer(CL) provides the mapping between ATM connection setup

procedures and the corresponding H/2 functions.A SAR is not necessary as

the ATM cell payload and all the necessary fields of ATM cell header fit

into the 54-byte H/2 packet. Nevertheless a compression of the ATM cell

header is necessary.



5. PROTOCOL DATA UNIT

Two kinds of PDU (Protocol Data Units) are defined: Short transport channel

PDU and Long transport channel PDU.A long transport channel PDU is 54

bytes long and contains 48 byte payload, a 24 bit CRC ( Cyclic Redundancy

Check) for error correction, 12 bits for convergence layer(CL) information and a

10 bit sequence number for the ARQ ( Automatic Repeat reQuest ) protocol.

A short channel PDU is 9 bytes long. In order to reduce overhead, all the

long transport channel and short transport channel PDUs in an MAC frame



belonging to connections of the same MT are combined to a PDU train.

In the Ethernet CL , we have to consider three kinds of over-heads. First

overhead is an overhead of Ethernet headers. In the Ethernet CL, IP packet

is handled as a payload of an Ethernet frame. SSCS-PDU (Service Specific

Convergence Sublayer Protocol Data Unit) is constructed by 6 bytes of the

destination address field, 6 bytes of the source address field and 2 bytes of

the type/length field. These fields are added to each payload.

The second overhead is the padding ( PAD ) field and the

trailer field for SAR ( Segmentation And Reassembly ) function. And the

third overhead is the SAR header whose length is 12 bits . In DLC layer,

DLC header and CRC ( Cyclic Redundancy Code ) field are added to each

SAR-PDU. The DLC header is 12 bits length and the CRC field is



24 bits length. Therefore the overhead of DLC layer is 4.5 bytes length and the

total length of DLC-PDU becomes 54 bytes. In the MAC layer, BCH

( Broadcast CHannel), FCH ( Frame Channel ), ACH ( Access feedback

CHannel) and Random channel do not convey any user data therefore

these channels also decrease the system performance.

6. LINKING PROCESS

The Mobile Terminal (MT) has to scan for the Beacon signal sent in the Broadcast

control channel of every MAC Frame containing among others the AP-ID and the

NET-ID of the APT ( Access Point Transceiver ). The MT waits for the



NETWORK-OPERATOR -ID broadcast periodically in theRLC broadcast channel

to check whether access to this particular network is feasible or not and

then continues the association procedure by transmitting a request for a

MAC-ID. A MAC-ID is assigned by the AP used for addressing the MT during

the whole session at this AP and is valid only in the radio cell of one APT.

During the link capability procedure , the MT sends its own

parameters to the AP containing:

a)The Data Link Control version running in the MT.

b)A flag set,if the MT supports the direct mode.

c)The Convergence Layer services supported.

d)Authentication and encryption procedures supported.

The AP will respond with its own set of parameters and select the (CL)Convergence

layer services and Encryption&Authentication procedures for the session.



The disassociation procedure may be initiated by either the AP(Access P) or the

MT.During explicit disassociation the AP and the MT discuss the disassociation

shortly. Implicit disassociation occurs when the MT and AP lose their radio

link completely.

DCC ( Data Link Control Connection Control ) functions are

responsible for setting up , maintaining , renegotiating and closing a DUC

(DLC User Connection) at the DLC (Data Link Control) layer and may be

initiated by either the AP or the MT. An MT requesting the establishment

of a DUC, will propose the connection characteristics but the AP will decide

upon the DUCs characteristics and attribute a unique ID that together with the

MAC ID uniquely identifies a connection in a radio cell.



7. PROPERTIES

1. High speed transmission: H/2 has a transmission rate of 54 Mbps. To achieve

this, H/2 makes use of a modulation method called OFDM(Orthogonal Frequency

Division Multiplexing) for transmission harmonised with IEEE 802.11. OFDM

is particularly efficient in time-dispersive environments, i.e. where the radio

signals are reflected from many points, e.g. in offices. The basic idea of OFDM

is to transmit broadband, high data rate information by dividing the data into

several interleaved, parallel bit streams, and let each bit stream modulate a

separate subcarrier. The channel spacing is 20 MHz, which allows high bit rates

per channel yet has reasonable number of channels: 52 subcarriers are used

per channel (48 subcarriers for data, 4 subcarriers tracking the phase for coherent

demodulation). The independent frequency subchannels are used for one

transmission link between the AP and the MTs.



2. Connection oriented: Data are transmitted on connections between the MT(Mobile

Terminal ) and the AP ( Access Point ) that have been established prior to the

transmission, using signaling functions of the H/2 control plane. Point-to-

point connections are bidirectional , point-to-multipoint and broadcast

connections are unidirectional from the AP toward the MTs in the radiocell.

Connections are realised by means of logical channels.

3. QoS support: The connection orientation of H/2 is a prerequisite for the support of

QoS.An H/2 WLAN is able to support all the QoS classes defined for ATM networks

and thus is ideally suited to also support the QoS requirements of IP networks



that are less stringent than those of ATM networks. The IP convergence layer

provides the functions needed for mapping the IP QoS requirements to the

QoS parameters available from H/2 for its DLC connections.Each connection

may be assigned a specific QoS parameter set,in terms of throughput ,delay,

delay variation , bit error rate etc. In an environment where the connection

characteristics are not available ,QoS is supported by assigning a priority level relative

to other connections.

4. Automatic frequency allocation: H/2 does not need a manual frequency planning

like conventional cellular networks.The APs in H/2 automatically select an appropriate

radio channel for the transmission within each AP’s coverage area by DFS (Dynamic

Frequency Selection). An AP listens to neighbour APs as well as to other radio

sources in the environment and selects a radio channel based on its current

load aiming to minimise interference with other radio cells.

5. Security support: H/2 supports authentication and encryption.The AP and MT may

authenticate each other to ensure autherised access.The user traffic on established

connections may be encrypted to protect against eavesdropping and man-in-middle



attacks.Authentication relies on a supporting function ,such as directory service that is

outside the scope of H/2.

6. Mobility support: The MT uses the AP with the best radio signal performance

as measured by the SNR .Thus, as the MT moves it may detect an alternative AP with

better radio performance than current AP. The MT will then initiate a handover to this

AP and all its connections will be moved to the new AP.

7. To allow MTs to save power, an MT may at any time request the AP to enter a low

power state,and may request a specific sleep period.At the end of the sleep period the MT

searchs for the presence of any wakeup indication from the AP. An AP will delay

any pending data to an MT until the corresponding sleep period has expired.If

no wake-up indication is received, the MT returns to its low power state for the

next sleep period.



8. COEXISTENCE AND RESOURCE SHARING

The guarantee of a certain QoS for wireless multimedia services if WLANs are sharing

the spectrum rather than operating in their own frequency bands.H/2 is likely to

share the spectrum with other system types like IEEE802.11a.H/2 specifies a centrally

controlled air interface with a 2 ms MAC frame, IEEE 802.11a in contrast, applies

CSMA/CA ( Carrier Sense Multiple Access/Collision Avoidance), a network contention

protocol that listens to a network in order to avoid collisions, unlike CSMA/CD that

deals with network transmissions once collisions have been detected. CSMA/CA

contributes to network traffic because, before any real data is transmitted, it has to

broadcast a signal onto the network in order to listen for collision scenarios and to

tell other devices not to broadcast), an LBT (Listen-Before-Talk) scheme with variable

packet lengths.The use of a distributed MAC makes IEEE 802.11a more suitable for

ad hoc networking and non-real-time applications. For the coexistence the following

points have to be noted namely that H/2 applies DFS (Dynamic Frequency Selection)


http://www.webopedia.com/TERM/C/CSMA_CD.html

http://www.webopedia.com/TERM/C/protocol.html

http://www.webopedia.com/TERM/C/contention.html

http://www.webopedia.com/TERM/C/network.html


and supports TPC(Transmitter Power Control).

The IEEE 802.11a system ( Which is commensurate to H/2 system ) keeps

operating the same carrier once it has selected it and does not apply DFS nor

TPC.Both systems use the same Physical layer protocols,carrier bandwidth and apply

LA(Link Adaptation),a flexible interference-dependent selection of a Physical layer

mode. Based on these schemes an FSR(Frequency Sharing Rule) may allow operation

in a common spectrum. An FSR defines techniques for radio channel management for

the systems operating in a common spectrum.HiperLAN/2 achieves higher throughput

as compared to IEEE 802.11a. This is due to the use of centrally controlled



medium access. This MAC protocol is also more suitable for time-bounded

applications.

9. COLLISION RESOLUTION

The number of concurrently receivable signals is restricted by the antenna system and is

interfernce limited. With the number of simultaneously transmitting MTs ( Mobile

Terminals ) increasing , the carrier to interference ratio decreases and a correct

reception of a burst becomes less likely . The present interference situation depends



on the number of simultaneous transmissions taking place,MTs’ positions and the

channel characteristics. Since no further restrictions can be imposed on the initial

access to RCH(Random Channel) that is the interference situation could not be taken into

account,some MTs might not succeed in transmitting via the RCH. To control the

retransmission attempts of these collides MTs, a collision resolution algorithm has

to be applied that can make use of the enhanced reception capabilities.In H/2, the

MTs may use the Random Channel (RCH) to transmit their Resource request to

the Access Point(AP).Especially for delay sensitive devices, this access should be

carried out as fast as possible.

10. CONCLUSION

H/2 system supports IP(Internet Protocol) over wireless ATM(Asynchronous Transfer



Mode) with a guaranteed QoS ( Quality of Service ). H/2 also provides a

convergence layer(CL) to connect directly wireless IP based applications to an IP

network without involving any ATM related signalling(User Network Interfacing)

or ATM fixed infrastructure.H/2 has the ability to support any ATM class of

services with less stringent requirements of QoS (Quality of Service).

To be able to rate the system completely H/2 has to be compared

with IEEE 802.11a system. The physical layer of HiperLAN/2 is very similar to the one

that 802.11a defines. While 802.11a uses Carrier Sense Multiple Access with Collision

Avoidance (CSMA/CA) to transmit packets, HiperLAN/2 uses Time Division

Multiple Access (TDMA). With CSMA/CA, all stations share the same radio channel

and contend for access. For example when an 802.11 station (client) needs to send a

packet, the station first listens for other transmissions and then attempts to send



frames when no other station is transmitting. If another station happens to be transmitting

, all other stations will wait until the channel is free.

The use of TDMA in HiperLAN/2, however , offers a regular time relationship for

network access. TDMA systems dynamically assign each station a time slot based on the

station's need for throughput. The stations then transmit at regular intervals during their

respective time slots, making more efficient use of the medium and improving support of

voice and video applications. The true usable maximum throughput of

HiperLAN/2,however, is 42Mbps, while the maximum usable throughput of 802.11a is

only around 18 Mbps (based on Ethernet packets with an average size of 512 bytes).

This puts HiperLAN/2 well ahead of 802.11a in terms of throughput capacity of each

Access Point(AP). HiperLAN/2 is presumably more cost effective than 802.11a. While



the initial HiperLAN/2 products will probably cost more than 802.11a counterparts,

supporters say that the better throughput will outweigh the slight price difference.

REFERENCES

1.”IP OVER WIRELESS MOBILE ATM-GUARANTEED WIRELESS QoS BY HiperLAN/2”-Published in the 89th volume of Procceedings of the IEEE.Authors:Bernhard .H.Walke,Norbert Esseling,Jorg .H,A.Hettich Stephan Mangold and Ulrich Vornefeld.

2. ETSI Project BRAN-Jamshid Khun Jush (Ericsson) & Gilles Straub(THOMSON Multimedia).

3. B.Walke, D.Petras and D.Plassmann,”wireless ATM:Air interface and network protocols of the mobile broadband system”IEEE Communication.



4. D.Raychaudhari,”Wireless ATM networks:Technology status and future directions”,Procceedings of the IEEE.

5. Evaluation of HiperLAN/2 scalability for mobile broadband systems-ByKen’ichi ishii(Networking Laboratories,NEC,Japan) and A.H. Aghvami(Center for Telecommunications research,King’s College,London).

6. HiperLAN/2-Janne Korhonen,Dept of computer science and engineering,Helsinki University of technology.

7. HiperLAN/2-An efficient high speed WLAN by Jim Geier.


seminar report ‘04

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