Wimax standards and Security The Wimax

MAC Functions for the PMP Topology

This section will introduce the major parts of the MAC protocol specified in the IEEE802.16d standard, especially, for the functions and features of the MAC protocol to support PMP topology.
Inside a sector of the WiMAX systems each SS has a 48-bit universal MAC address, which uniquely defines the SS from within the set of all possible equipment types. It is used during the initial ranging process to establish the

appropriate connections for an SS. It is also used as part of the authentication process for the BS and SS to verify each other. Connections are identified by a 16-bit CID. The CID is a connection identifier of the traffic at SSs, including connectionless traffic like IP, because it serves as a pointer to the destination and context information. Requests for transmission are based on these CIDs because the granted bandwidth may differ for different connections.

1. 
   MAC PDU Composition and Transmission
   
   1. 
      MAC PDU Composition
      

Each MAC PDU is the basic unit of information prepared at the MAC layer and delivered to the physical layer. The PDU begins with a fixed-length generic MAC header. The header may be followed by the payload, which consists of zero or more subheaders and zero or more MAC service data units (SDUs) or fragments. The payload may vary in length so that a MAC PDU may represent a variable number of bytes. This allows the MAC to tunnel various higher- layer traffic types without any knowledge of the formats or bit patterns of those messages.
There are two types of MAC headers. The first type is the generic MAC header in each MAC PDU containing either MAC management messages or data made at the convergence layer. The second type is the bandwidth request header for requesting additional bandwidth. Five types of subheaders may be inserted in MAC PDUs immediately following the Generic MAC header. The mesh subheader could exist before all the other subheaders. After this, the Grant Management subheader will come next. And the FAST FEEDBACK Allocation subheader always appears as the last per-PDU subheader. The Packing and Fragmentation subheaders are mutually exclusive and both will not be present in the same MAC PDU. A set of MAC management messages are defined. These messages are carried in the Payload of the MAC PDU. All MAC Management messages begin with a Management Message Type field and may contain additional fields.
Multiple MAC PDUs could be concatenated into a single transmission
unit in either the uplink or downlink. Since each MAC PDU is identified by a unique CID, the receiving MAC entity is able to present the MAC SDU (after reassembling the MAC SDU from one or more received MAC PDUs) to the correct instance of the MAC service access point (SAP). MAC Man- agement messages, user data, and bandwidth request MAC PDUs may be concatenated into the same transmission.
Fragmentation is the process by which a MAC SDU is divided into one or more MAC PDUs. This process is undertaken to allow efficient use of available bandwidth relative to the QoS requirements of a connection’s ser- vice flow. Capabilities of fragmentation and reassembly are mandatory. The authority to fragment traffic on a connection is defined when the connection is created by the MAC SAP. Fragmentation may be initiated by a BS for downlink connections and by an SS for uplink connections.

The MAC protocol can pack multiple MAC SDUs into a single MAC PDU. Packing makes use of the connection attribute indicating whether the con- nection carries fixed-length or variable-length packets. For packing with fixed-length blocks, the request/transmission policy shall be set to allow packing and prohibit fragmentation, and the SDU size shall be included in dynamic service activate request (DSA-REQ) message when establish- ing the connection. The length field of the MAC header implicitly indicates the number of MAC SDUs packed into a single MAC PDU. When packing variable-length SDU connections, the indication of where one MAC SDU ends and another begins is necessary. In the variable-length MAC SDU case, the MAC attaches a Packing subheader to each MAC SDU.

2.