JOCHEN SCHILLER - AllOnline

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Mobile communications
An optional direct link phase can be inserted between the downlink and
the uplink phase. The access to the common physical medium is always controlled
by the CC (typically in an AP).
HiperLAN2 defines six different so-called transport channels for data transfer
in the above listed phases. These transport channels describe the basic
message format within a MAC frame.
● Broadcast channel (BCH): This channel conveys basic information for the
radio cell to all MTs. This comprises the identification and current transmission
power of the AP. Furthermore, the channel contains pointers to the
FCH and RCH which allows for a flexible structure of the MAC frame. The
length is 15 bytes.
● Frame channel (FCH): This channel contains a directory of the downlink
and uplink phases (LCHs, SCHs, and empty parts). This also comprises the
PHY mode used. The length is a multiple of 27 bytes.
● Access feedback channel (ACH): This channel gives feedback to MTs
regarding the random access during the RCH of the previous frame. As the
access during the RCHs is based on slotted Aloha, collision at the AP may
occur. The ACH signals back which slot was successfully transmitted. The
length is 9 bytes.
● Long transport channel (LCH): This channel transports user and control
data for downlinks and uplinks. The length is 54 bytes.
● Short transport channel (SCH): This channel transports control data for
downlinks and uplinks. The length is 9 bytes.
● Random channel (RCH): This channel is needed to give an MT the opportunity
to send information to the AP/CC even without a granted SCH. Access is
via slotted Aloha so, collisions may occur. Collision resolution is performed
with the help of an exponential back-off scheme (ETSI, 2001b). The length is
9 bytes. A maximum number of 31 RCHs is currently supported.
BCH, FCH and ACH are used in the broadcast phase only and use BPSK with
code rate 1/2. LCH and SCH can be used in the downlink, uplink or (optional)
direct link phase. RCH is used in the uplink only for random access (BPSK, code
rate 1/2). HiperLAN2 defines further how many of the channels are used within
a MAC frame. This configuration may change from MAC frame to MAC frame
depending on the connection QoS, resource requests, number of MTs etc. Figure
7.37 shows valid combinations of channels/transfer phases within MAC frames.
It is required that the transport channels BCH, FCH and ACH are present plus at
least one RCH. While the duration of the BCH is fixed (15 byte), the duration of
the others may vary (either due to a variable size of the channel or due to the
multiple use of channels). However, the order BCH-FCH-ACH-DL phase-UL
phase-RCH must be kept from an MT’s point of view (centralized mode). For the
direct mode the DiL phase is inserted between the DL and UL phases.
Data between entities of the DLC layer are transferred over so-called logical
channels (just another name for any distinct data path). The type of a logical