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message is sent by BS when if it determines that SS is authorized, this message initiates a SA among BS and SS. An authorization key (AK) is sent to SS that is encrypted with the public key of the SS. SS then gains the authorization to access the WMAN channel if AK is used properly. 802.16 design supposes that only BS and SS share the AK-AK must never be disclosed to another entity.
Privacy and Key Management Protocol
This protocol establishes a data SA among BS and SS. This is accomplished, via two or three messages sent between BS and SS.

Step 1: This step is non-compulsory and this message is sent only if BS wishes to force rekeying of a data SA or to make a new SA. BS after computing HMAC (1) (hash function-based message authentication code) permits SS to recognize falsification.
Step 2: SS requests SA parameters by sending a message to BS. SS must make use of the SAID from the authorization protocol SAID list. SS sends a separate second message for each data SA. SS computes HMAC (2) too to permit BS to recognize falsification. BS can strongly authenticate SS by HMAC (2).
Step 3: BS sends message 3 when if it finds that HMAC (2) is valid and SAID is certainly one of the SS's SA. Message 3 includes the old TEK (temporal encryption key) that is used to reiterate the active SA, the new TEK is used after the expiration of the existing TEK.

2.9.3 Physical layer
Initially IEEE 802.16 standard supported lots of physical medium interfaces. In 2001 only single carrier modulation was supported by the standard 802.16 but now Orthogonal Frequency Division Multiplexing (OFDM) is also supported, which is a multi-carrier modulation scheme. There are two OFDM-based modes in 802.16 standard: OFDM and OFDMA. Depending on distance and noise, these technologies permit sub-carriers to be modulated (QPSK, 16-QAM, 64-QAM) adaptively. The scalability options in OFDMA offer higher efficiency in bandwidth utilization. Also in the begging standard only used 10-66 GHz bands, but now 2 to11 Gega Hertz and 10 to 66 Gega Hertz bands are used also. This has to do with the modularity of the standard, that permits modifications to happen in the Physical layer with very few variations to the rest of the protocol [24].

In 10 to 66 Gega Hertz 802.16 PHY layer specification, line-of-sight transmission was an essential. Single-carrier modulation was chosen for that reason. The air interface in that case, is known as “Wireless MAN-SC”. Base station sends out TDM signal and SSs assigned with a time slot serially. Uplink although, is carried by TDMA [24]. Afterward, another profile was adopted, based on the demand for burst design. This one permits both TDD, in which channel is shared by uplink and downlink but transmission is not simultaneous, also FDD, where different channels are used for uplink and downlink, sometimes simultaneously.

The 2 to11 Gega Hertz 802.16 PHY layer specification addresses both the licensed and un-licensed bands. The requirement of None Line of Sight operation motivated this design. The two air interface specifications up to now are [10] Wireless MAN-OFDM which make use of OFDM with 256-point transform. It is accessed through TDMA. This is compulsory interface for un-licensed bands [24]. Wireless MAN-OFDMA which make use of OFDMA with a 2048-point transform. In this system, to offer multiple access, a subset of the various carriers to individual r本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。