Download - 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
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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
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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.
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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
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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.
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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
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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
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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
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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).
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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).
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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
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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.
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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.
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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
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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
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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:
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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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
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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
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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.
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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)
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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
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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
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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
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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
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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
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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.
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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.
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