METROPOLITAN AREA NETWORKS
BW (bandwidth) request messages. Here the BW special request header is used.
These requests can be up to 32 kb long with a single message. They may be incre-
mental or aggregate, as indicated by the MAC header. For non-UGS services only,
the request may be piggybacked. They are placed in the GM (grant management)
and are always incremental and may be up to 32 kb long per request in the CID
Poll-me bit is for UGS services only and is used by the subscriber station (SS) to
request a bandwidth poll for non-UGS services.
Figure 13.18 is a typical BW (bandwidth) request message.
To maintain QoS in the Grant per Subscriber Station operation, the following measures
Use of a semidistributed approach.
A base station sees the requests for each connection. Based on this, it grants bit
rate capacity to the subscriber station (SS) while maintaining QoS and fairness. The
SS scheduler maintains QoS among its connections and is responsible to share the
bandwidth among the connection, again while maintaining QoS and fairness. The
algorithms in the BS and SS can be very different; while the subscriber station may
use the bandwidth in a way unforeseen by the base station.
During SS initialization, there is a scan for a downlink channel and synchronization is
established with the BS. The SS must then obtain the transmit parameters from the uplink
channel descriptor (UCD) message. Ranging is then performed followed by negotiation
of basic capabilities. The subscriber station (SS) is then authorized and there is a key
exchange carried out. Registrations are then performed and IP connectivity is established;
time of day is exchanged. Operational parameters are then transferred and connections
are set up.
Ranging is an important element in uplink and downlink communication. For uplink
transmissions, times are measured at the base station. At startup, the SS sends a RNG-
REQ (ranging-request) in a ranging window. The BS now measures arrival time and
signal power, then calculates required (time) advance and power adjustment. The BS
sends adjustment in RNG-RSP (range response). The SS adjusts its advance and its power
and then sends a new RNG-REQ message. This loop continues until power and timing
Registration is a form of capability negotiation. The Subscriber Station (SS) sends
a list of capabilities and parts of the configuration file to the Base Station (BS) in the
BR msb (8)
CID msb (8)
BR Isb (8)
CID Isb (8)
Typical bandwidth request message.