Network Access and Initialization in WiMAX

In this section I am going to explain briefly at the activities of a subscriber station in the framework of gaining network access and initialization. The PKM Protocol is used during the network access and initialization phase, in the authentication and authorization steps. A WiMAX Customer Premise Equipment (CPE) must perform a number of tasks before gaining access to a WiMAX network. These tasks are discussed in detail below.

Scanning and Synchronization in WiMAX technology

First the WiMAX Customer Premise Equipment (CPE) searches for a downlink signal from the WiMAX Base Station (BS) and try to coordinate with it. If previous downlink channel existed, the WiMAX Customer Premise Equipment (CPE) will try reusing those functional parameters. If not, the WiMAX Customer Premise Equipment (CPE) must look into all the potential channels in the downlink frequency band. When a channel has been selected, the WiMAX Customer Premise Equipment (CPE) attempts to synchronize with the downlink transmission by detecting the cyclic frame preambles. (David Johnston, Hassan Yaghoobi, 2004)

Uplink & Downlink Parameters Detection

After synchronization has been established at the physical layer, the Subscriber Station (SS) then continues to search for the Downlink Channel Descriptor (DCD) and the Uplink Channel Descriptor (UCD) messages that are regularly broadcasted by the WiMAX Base Station (BS). The Downlink Channel Descriptor (DCD) and Uplink Channel Descriptor (UCD) messages hold information regarding the physical layer features of both the downlink and uplink channels. Among others, these messages then permit the WiMAX Customer Premise Equipment (CPE) to learn about the modulation type and Forward Error Correction (FEC) method of the carrier. Depending on the PHY specification selected for a given settings, the BS also frequently transmits uplink-map (UL-MAP) and downlink-map (DL-MAP) messages that describe their burst start times. It is through the DL-MAP and UL-MAP messages that the WiMAX Base Station (BS) can assign access to the individual channels. (Shyam Parekh, 2006)

Ranging & Subscriber Station Capabilities in WiMAX Technology

In this stage, the WiMAX Customer Premise Equipment (CPE) performs ranging, which is the process of aligning the WiMAX Customer Premise Equipment (CPE) transmission timing-wise to the start of a slot during contention for access. This process is part of framing and media access in 802.16 and consists of initial ranging and periodic ranging.

The early ranging contention slot is used for network entry. Here, the WiMAX Customer Premise Equipment (CPE) sends a ranging request packet (RNG-REQ) in the primary ranging contention slot. If this message is received correctly by the WiMAX Base Station (BS), it then replies to the WiMAX Customer Premise Equipment (CPE) with a ranging response packet (RNG-RSP) unfolding the timing and power correction information to the WiMAX Customer Premise Equipment (CPE). This permits the WiMAX Customer Premise Equipment (CPE) to adjust the timing and power of its signal as received by the WiMAX Base Station (BS). The response will also tell the WiMAX Customer Premise Equipment (CPE) about the connection IDs (CID) chosen by the WiMAX Base Station (BS). The other type of ranging, namely, periodic ranging, provides chances for WiMAX Customer Premise Equipment (CPE) to send ranging-request messages to the WiMAX Base Station (BS) in order to adjust power levels, time, and frequency offsets. (Derrick Boom, 2004)

After ranging is completed, the WiMAX Customer Premise Equipment (CPE) reports its physical layer capabilities to the WiMAX Base Station (BS). This consists of the modulation and coding schemes holds by the WiMAX Customer Premise Equipment (CPE), and whether the WiMAX Customer Premise Equipment (CPE) within the WiMAX Frequency Division Duplexing (FDD) supports half-duplex or full-duplex. The WiMAX Base Station (BS) has the choice of accepting or rejecting these capabilities of the WiMAX Customer Premise Equipment (CPE). (Derrick Boom, 2004)

Subscriber Station Authentication in WiMAX, Authorization & Registration

During subscriber station authentication in WiMAX, the WiMAX Customer Premise Equipment (CPE) must be authenticated by the WiMAX Base Station (BS) and obtain authorization from the WiMAX Base Station (BS) by using the PKM Protocol. Each WiMAX Customer Premise Equipment (CPE) device is assigned to an X.509 digital certificate, which is physically bound to the device hardware during manufacturing. One achievable implementation is to include the devices MAC-address in its certificate. The MAC address in WiMAX is the usual 48bit address used in other IEEE 802 standards such as Ethernet. It is essential to note that just as in Data Over Cable Service Interface Specifications (DOCSIS) compliant cable modem devices, the digital certificate and the private key are allocated during manufacturing of the WiMAX Customer Premise Equipment (CPE) device. The private key must be embedded in the hardware in such a way that it is complicated or infeasible for the user to access or extract. (Michaela Greiler, 2007)

After verification of authentication and authorization, the WiMAX Customer Premise Equipment (CPE) continues with the registration stage. Here, the WiMAX Customer Premise Equipment (CPE) sends a registration request message to the WiMAX Base Station (BS), who answers with a registration response message including among others a secondary management connection ID for the WiMAX Customer Premise Equipment (CPE) and the IP version used for the secondary management connection. The appearance of the registration response message from the WiMAX Base Station (BS) tells to the WiMAX Customer Premise Equipment (CPE) that it has been registered in the network and therefore allowed to enter the network. (Michaela Greiler, 2007)

IP Connectivity in WiMAX Technology

At the completion of registration, the WiMAX Customer Premise Equipment (CPE) can now obtain an IP address through the DHCP protocol, obtain current time information (e. g., through the Internet Time Protocol), and also obtain other parameters from the WiMAX Base Station (BS). (Michaela Greiler, 2007)

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