Monday, December 21, 2009

Legacy QoS Mechanisms

The following paragraphs describe legacy mechanisms.


WiMAX incorporates a number of time-proven mechanisms to ensure good QoS. Most notable are TDD, FDD, FEC, FFT, and OFDM. The WiMAX standard provides flexibility in spectrum usage by supporting both FDD and TDD. Thus, it can operate in both FDD/OFDM and TDD/OFDM modes. It supports two types of FDD: continuous FDD and burst FDD.

In continuous FDD, the upstream and downstream channels are located on separate frequencies, and all CPE stations can transmit and receive simultaneously. The downstream channel is always on, and all stations are always listening to it. Traffic is sent on this channel in a broadcast manner using TDM. The upstream channel is shared using TDMA, and the BS is responsible for allocating bandwidth to the stations.

In burst FDD, the upstream and downstream channels are located on separate frequencies. In contrast to continuous FDD, not all stations can transmit and receive simultaneously. Those that can transmit and receive simultaneously are referred to as full-duplex capable stations while those that cannot are referred to as half-duplex capable stations.

A TDD frame has a fixed duration and contains one downstream subframe and one upstream subframe. The two subframes are separated by a guard time called transition gap (TG), and the bandwidth that is allocated to each subframe is adaptive. The TDD subframe is illustrated in Figure 1.

(Source: IEEE)

Figure 1: TDD subframe

Within a TDD downlink subframe, transmissions coming from the BS are organized into different modulation and FEC groups. The subframe header, called the FCH, consists of a preamble field, a PHY control field, and a MAC control field. The PHY control field is used for physical information, such as the slot boundaries, destined for all stations. It contains a map that defines where the physical slots for the different modulation/FEC groups begin.

The groups are listed in ascending modulation order, with QPSK first, followed by 16-QAM and then 64-QAM. Each CPE station receives the entire DL frame, decodes the subframe, and looks for MAC headers indicating data for the station. The DL data is always FEC coded. Payload data is encrypted, but message headers are unencrypted. The MAC control is used for MAC messages destined for multiple stations.

This variation uses burst single-carrier modulation with adaptive burst profiling in which transmission parameters, including the modulation and coding schemes, may be adjusted individually to each SS on a frame-by-frame basis. Channel bandwidths of 20 or 25 MHz (typical United States allocation) or 28 MHz (typical European allocation) are specified. Randomization is performed for spectral shaping and to ensure bit transitions for clock recovery.

Forward Error Correction (FEC)

WiMAX utilizes FEC, a technique that doesn't require the transmitter to retransmit any information that a receiver uses for correcting errors incurred in transmission over a communication channel. The transmitter usually uses a common algorithm and embeds sufficient redundant information in the data block to allow the receiver to correct. Without FEC, error correction would require the retransmission of whole blocks or frames of data, resulting in added latency and a subsequent decline in QoS.

Need QoS? Throw more bandwidth at it! Throughput and latency are two essentials for network performance. Taken together, these elements define the "speed" of a network. Whereas throughput is the quantity of data that can pass from source to destination in a specific time, round-trip latency is the time it takes for a single data transaction to occur (the time between requesting data and receiving it). Latency can also be thought of as the time it takes from data send-off on one end to data retrieval on the other (from one user to the other). Therefore, the better throughput (bandwidth) management, the better the QoS

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