part is to segment packets received from the SSCS, and reassemble segmented

756 
21 Wireless Local Area Networks
for one or two hops of an end-to-end connection. The cellular radio access in the 
BRAIN uses HIPERLAN/2 and supports QoS on a per connection basis.
An IP convergence layer is used to provide the functions required for map-
ping the QoS requirements of the individual connection to the QoS parameters 
available in DLC connections. The convergence layer offers a QoS interface to 
support different IP QoS schemes. By using IP QoS parameters, the convergence 
layer establishes DLC connections in which IP QoS parameters are mapped into 
DLC connections for priority, radio bandwidth reservation, appropriate ARQ 
scheme, and handover strategy. This procedure is realized by mapping IP packets 
into different DLC connection queues based on respective code point and desti-
nation address fi elds. The convergence layer associates a specifi c link scheduling 
priority, discarding time and/or bandwidth reservation to each DLC queue. The 
convergence layer segments IP traffi c to fi xed length packets. The segmentation 
and reassembly causes extra complexity in the convergence layer but enables a 
better bandwidth reservation policy.
MAC enables full rescheduling in every 2 ms and dynamically adjusts uplink 
and downlink capacity. Radio link control (RLC) provides connection-oriented 
secured link service to the convergence layer. There are up to 63 unicast data 
connections per terminal that can be supported with various QoS parameters. 
Error correction (EC) provides a selective repeat ARQ mode for each connection. 
Alternatively, for delay intolerant and multicast services a repetition mode can be 
used. Thus, DLC provides means for executing several IP QoS techniques such as 
prioritizing, on-demand-based bandwidth reservation, and delay guarantee. DLC 
also provides dynamic frequency selection (DFS), link adaptation, power control, 
and power saving.
The physical layer uses orthogonal frequency division multiplexing (OFDM) 
to combat frequency selective fading and randomize the burst errors caused by a 
wide band fading channel. There are seven modes with different coding and mod-
ulation schemes available in physical layer; all of them can be adapted dynami-
cally by a link adaptation scheme. There is a strong interaction between the PHY 
modes, retransmission load and utilization of the radio link, delay, and overall 
throughput. 
The OFDM transmits broadband, high data rate information by dividing the 
data into several interleaved, parallel bit streams, and lets each one of them modu-
late on a separate subcarrier. Various coding and modulation schemes are used 
by a link adaptation mechanism. This is to adapt to current radio link quality 
and meet the requirements for different physical layer properties as defi ned by 
the transport channels within DLC. Table 21.15 provides the different PHY 
modes and their transmission rates. The seven PHY modes use BPSK, QPSK, 
and 16-QAM as mandatory subcarrier modulation schemes whereas 64-QAM 
is optional. Forward error correction is performed by a convolutional code of a 
rate of 1/2 and a constraint length of 7. Other code rates of 3/4 and 9/16 can be 
obtained by puncturing.
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The DLC scheduling algorithm deals with the properties of the HIPERLAN/2 
radio access that are dependent on ARQ and link adaptation. ARQ reacts on trans-
mission errors and initiates retransmission. When the error check bit detects error(s) 
in the transmission, ARQ sends a request for retransmission of the error packet 
data unit (PDU). Thus, for a poor radio link, retransmission will cause large trans-
mission delay. Selective repeat ARQ uses a transmission window at the transmitter 
and receiver. The receiver notifi es the transmitter of the sequence number below 
which all PDUs are received correctly and points out which PDU is not correct.
Based on the current radio link conditions, the link adaptation in the DLC 
layer assigns a specifi c PHY mode to the PDU dedicated to a connection. Each con-
nection and its direction are addressed individually and the assignment varies from 
one MAC frame to another. The link adaptation scheme adapts the PHY mode 
based on link quality measurements. ARQ and link adaptation reduce packet loss 
rate but introduce additional overhead and delay to the radio access system.
Total system throughput, transmission delay, and bit error rate are the 
important parameters in determining the performance of the HIPERLAN/2 radio 
access. There is a strong interaction between PHY modes and these parameters. 
The MAC protocol functions are used for organizing the access and 
transmission of data on the radio link. Since HIPERLAN/2 uses a central resource 

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