英语论文网

IPv6, and virtual local area network (VLAN).

The MAC Common Part Sub-layer (MAC CPS) is the heart of the standard. This sub-layer defines the rules for connection management, bandwidth allocation and mechanism for system access. Also uplink scheduling, bandwidth request and grant, connection control, and automatic repeat request (ARQ) functions are defined. In the MAC layer the communication among the CS and the MAC CPS is done by MAC service Access Point (MAC SAP). Communication is very easy as only four basic actions can be used: creation of connection, modification of connection, deletion of connection and transport of data over the connection.

The sub-layer among MAC CPS and Physical Layer (PHY layer) is Privacy Sub-layer. It is the sub-layer that is accountable for the encryption and decryption of data that is incoming and leaving the PHY layer and is also used for authentication and secure key exchange.

PHY layer is the layer which is mainly adaptable to the requirements of the protocol. This means that the flexibility of the protocol permits the designers to make changes to it concerning modulation, an example is the addition of the 2-11 GHz band and the OFDM modulation in the 802.16a-2003 standard[5]. The PHY layer also supports different forward error corrections (FEC) like Reed-Solomon GF (256) with changeable block size and error correction capabilities and Block Turbo Codes. Layering of 802.16 protocol is shown in the figure 2.6.

From a security perspective the most significant layers in the MAC sub-layer are the MAC Common Part Sub-layer and Privacy Sub-layer. IEEE when refers to “the MAC” generally means the MAC CPS so from this point when MAC is referred, it implies the MAC CPS [3].

2.9.1 MAC layer
The nucleus of the standard is MAC sub-layer. It functions in the similar way to IEEE 802.11. There are several subscriber stations and one Base Station (BS). The BS is seen as the Access Points (AP's) in 802.11, while the both standards are entirely dissimilar in the way that they use the airwaves. MAC is designed to support point to multipoint technology[24]. On the contrary to 802.11 CSMA/CA scheme, 802.16 make use of UL and DL maps to guarantee crash free access. To share uplink, subscriber stations use Time Division Multiple Access (TDMA), whereas base station make use of TDM. UL and DL schedules are swapped in each frame by means of UL-MAP and DL-MAP messages[3][24].

2.9.1.1 MAC connections
IEEE 802.16 media access control is connection oriented. Each service is mapped to a connection, and each connection is identified through 16-bit connection identifier and might need constantly contracted bandwidth on demand [9][24]. Media access control layer links can be seen in a way similar to TCP links. Similar to TCP links, where a computer might have at the same time lots of separate active links in diverse ports, in MAC links the subscriber station might have several links to a base station for diverse services such as management of network or transport of user data (The management messages are carried by management links and where as other traffic, normally user data is carried by transport links. Though the main distinction is that in MAC links, each link might have dissimilar parameters for bandwidth, security and priority. Each link is recognized via its connection identifier that is allotted via the base station[24]. As