Friday, December 11, 2009

The MAC and WiMAX Architecture

The WiMAX DL from the BS to the user operates on a point-tomultipoint basis as illustrated in Figure 1. The WiMAX wireless link operates with a central BS with a sectorized antenna that is capable of handling multiple independent sectors simultaneously. Within a given frequency channel and antenna sector, all stations receive the same transmission. The BS is the only transmitter operating in this direction, so it transmits without having to coordinate with other stations except the overall TDD that may divide time into UL and DL transmission periods. The DL is generally broadcast. In cases where the DL-MAP does not explicitly indicate that a portion of the DL subframe is not a specific SS, all SSs capable of listening to that portion of the DL subframe will listen.

Figure 1: Typical WiMAX architecture for point-to-multipoint distribution

The MAC is connection-oriented. Connections are referenced with 16-bit connection identifiers (CIDs) and may require continuously granted bandwidth or bandwidth on demand. As described previously, both bandwidths are accommodated. A CID is used to distinguish between multiple UL channels that are associated with the same DL channel. The SSs check the CIDs in the received PDUs and retain only those PDUs addressed to them.

The MAC PDU is the data unit exchanged between the MAC layers of the BS and its SSs. It is the data unit generated on the downward direction for the next lower layer and the data unit received on the upward direction from the previous lower layer.

Each SS has a standard 48-bit MAC address, which serves as an equipment identifier because the primary addresses used during operation are the CIDs. Upon entering the network, the SS is assigned three management connections in each direction. These three connections reflect the three different QoS requirements used by different management levels:

  • Basic connection—transfers short, time-critical MAC and radio link control (RLC) messages.

  • Primary management connection—transfers longer, more delay-tolerant messages, such as those used for authentication and connection setup. The secondary management connection transfers standards-based management messages such as Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP), and Simple Network Management Protocol (SNMP). In addition to these management connections, SSs are allocated transport connections for the contracted services.

  • Transport connections—are unidirectional to facilitate different UL and DL QoS and traffic parameters; they are typically assigned to services in pairs.

SSs share the UL to the BS on a demand basis. Depending on the class of service utilized, the SS may be issued continuing rights to transmit, or the BS may grant the right to transmit after receiving a request from the user.

Service Classes and QoS

Within each sector, users adhere to a transmission protocol that controls contention between users and enables the service to be tailored to the delay and bandwidth requirements of each user application. This is accomplished through four different types of UL scheduling mechanisms. These mechanisms are implemented using unsolicited bandwidth grants, polling, and contention procedures. The WiMAX MAC provides QoS differentiation for different types of applications that might operate over WiMAX networks:

  • Unsolicited Grant Services (UGS)—UGS is designed to support constant bit rate (CBR) services, such as T1/E1 emulation and VoIP without silence suppression.

  • Real-Time Polling Services (rtPS)—rtPS is designed to support real-time services that generate variable size data packets, such as MPEG video or VoIP with silence suppression, on a periodic basis.

  • Non-Real-Time Polling Services (nrtPS)—nrtPS is designed to support non-real-time services that require variable size data grant burst types on a regular basis.

  • Best Effort (BE) Services—BE services are typically provided by the Internet today for web surfing.

The use of polling simplifies the access operation and guarantees that applications receive service on a deterministic basis if required. In general, data applications are delay tolerant, but real-time applications, like voice and video, require service on a more uniform basis and sometimes on a very tightly controlled schedule.

For the purposes of mapping to services on SSs and associating varying levels of QoS, all data communications are in the context of a connection. Service flows may be provisioned when an SS is installed in the system. Shortly after SS registration, connections are associated with these service flows (one connection per service flow) to provide a reference against which to request bandwidth. Additionally, new connections may be established when a customer's service needs change. A connection defines both a service flow and the mapping between peer convergence processes that utilize the MAC. The service flow defines the QoS parameters for the PDUs that are exchanged once the connection has been established.

Service flows are the mechanism for UL and DL for QoS management. In particular, they facilitate the bandwidth allocation process. An SS requests UL bandwidth on a per connection basis (implicitly identifying the service flow). The BS grants the bandwidth to an SS as an aggregate of grants in response to per connection requests from the SS.[1]

The modulation and coding schemes are specified in a burst profile that may be adjusted adaptively for each burst to each SS. The MAC can make use of bandwidth-efficient burst profiles under favorable link conditions then shift to more reliable, although less efficient alternatives, as required to support the planned 99.999 percent link availability (QPSK to 16-QAM to 64-QAM).

The request-grant mechanism is designed to be scalable, efficient, and self-correcting. The WiMAX access system does not lose efficiency when presented with multiple connections per terminal, multiple QoS levels per terminal, and a large number of statistically multiplexed users.

Along with the fundamental task of allocating bandwidth and transporting data, the MAC includes a privacy sublayer that provides authentication of network access and connection establishment to avoid theft of service, and it provides key exchange and encryption for data privacy.

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