5. MAC service definition
5.1 Overview of MAC services5.1.1 Data service
5.1.1.1 General
This service provides peer LLC entities with the ability to exchange MSDUs. To support this service, the
local MAC uses the underlying PHY-level services to transport an MSDU to a peer MAC entity, where it is
delivered to the peer LLC. Such asynchronous MSDU transport is performed on a connectionless basis. By
default, MSDU transport is on a best-effort basis. However, the QoS facility uses a traffic identifier (TID) to
specify differentiated services on a per-MSDU basis. The QoS facility also permits more synchronous
behavior to be supported on a connection-oriented basis using TSPECs. There are no guarantees that the
submitted MSDU will be delivered successfully. Group addressed transport is part of the data service
provided by the MAC. Due to the characteristics of the WM, group addressed MSDUs may experience a
lower QoS, compared to that of individually addressed MSDUs. All STAs support the data service, but only
QoS STAs in a QoS BSS differentiate their MSDU delivery according to the designated traffic category or
traffic stream (TS) of individual MSDUs.
Because operation of certain functions of the MAC may cause reordering of some MSDUs, as discussed in
more detail below, in non-QoS STAs, there are two service classes within the data service. By selecting the
desired service class, each LLC entity initiating the transfer of MSDUs is able to control whether MAC
entities are or are not allowed to reorder those MSDUs.
There are two service classes available in a QoS STA: QoSAck and QoSNoAck. The service classes are
used to signal if the MSDU is to be transmitted with or without using the MAC-level acknowledgment.
In QoS STAs either associated in a BSS or having membership in an IBSS, the MAC uses a set of rules that
tends to cause higher UP MSDUs in a BSS to be sent before lower UP MSDUs in the BSS. The MAC
sublayer entities determine the UPs for MSDUs based on the TID values provided with those MSDUs. If
a TSPEC has been provided for a TS, via the MAC sublayer management entity, the MAC attempts to
deliver MSDUs belonging to that TS in accordance with the QoS parameter values contained in the TSPEC.
In a BSS with some STAs supporting the QoS facility and others not supporting the QoS facility, in
delivering an MSDU to a non-QoS STA, the QoS STA uses the access category (AC) corresponding to the
UP of the MSDU.
