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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.
Ch21-P373580.indd 756
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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
