Error Rate | QoS

Error rate is a ratio between an amount of information that is received in error as compared to the total amount of information that is received over a period of time. Error rate may be expressed in the number of bits that are received in error on the number of blocks of data (packets) that are lost over a period of time. WiMAX error rates can be affected by a number of factors including signal quality and system configuration. Some of the common error rate measures for WiMAX include bit error rate (BER) and packet loss rate (PLR).

Bit Error Rate (BER)

BER is calculated by dividing the number of bits received in error by the total number of bits transmitted. It is generally used to denote the quality of a digital transmission channel. Bit errors can occur randomly over time (random errors) or in group (burst errors).

Random errors are bits in a received digital signal that are received in error that occur in such a way that each error can be considered statistically independent from any other error. Burst errors are the distortion or failure of a digital receiver to correctly decode groups of digital bits. Burst errors typically have a high bit error ratio (BER) compared to the overall BER of a communication link or channel.

Packet Loss Rate (PLR)

Packet loss rate is a ratio of the number of data packets that have been lost in transmission compared to the total number of packets that have been transmitted. Some applications (such as digital television) are more sensitive to packet loss rate than bit error rates.

Quality of Service (QoS)

QoS is one or more measurements of desired performance and priorities of a communications system. The WiMAX system is designed with the ability to apply different QoS levels to downlink and uplink connections as well as provide multiple service types on a single connection to each user. QoS measures for WiMAX systems may include service availability, data throughput, delay, jitter, and error rate.

Service Availability

Service availability is the ratio of the amount of time an authorized user is able to access the services compared to the total time service is supposed to be available. Service availability can be affected by a variety of factors including admission control and oversubscription.

Admission control is the process of reviewing the service authorization level associated with users and determining the extent to which network resources will be allocated if they are available. Admission control is used to adjust, limit or assign the use of limited network resources to specific types or individual users. Admission control may allow for the assignment of higher access level priority for specific types of users such as public safety users.

Oversubscription is a situation that occurs when a service provider sells more capacity to end customers than a communications network can provide at a specific time period. This provides a benefit of reduced network equipment and operational cost.

Oversubscription is a common practice in communications networks as customers do not continuously use the maximum capacity assigned to them and they access the networks at different time periods. Unfortunately, over-subscription in telecommunications can cause problems when customers do attempt to access the network at the same time. For example, when customers open their presents at a holiday event (e.g. Christmas) and attempt to access the Internet at the same time.

Radio Link Control (RLC) | WiMAX Operation

Radio link control protocol is a layer 2 (link layer) that is used to coordinate the overall flow of data packets across the radio link. RLC uses error detection and data retransmission to increase the reliability of the radio link while reducing the error rate. WiMAX radio link control functions include power level control, periodic ranging, burst profile changes and bandwidth requests.

Power control is the process of adjusting the power level in a wireless system where the base station receiver monitors the received signal strength of mobile radios. Control messages are transmitted from the base station to the mobile telephone commanding it to raise and lower its transmitter power level as necessary to maintain a good radio communications link.

Ranging may need to be performed after the subscriber station has been inactive for a while. A timer (the T4 timer) that is continuously reset as the subscriber station communicates with the system helps determine this. If the subscriber station (SS) has not communicated with the system in awhile, the timer will not be reset and it will expire. If the timer expires, the SS must again perform ranging with the system.

The base station is responsible for assigning burst profiles. However, the subscriber station may request changes to the burst profile. This may occur as a result of an increase in the bit error rate of the received signal due to fading or interference. The subscriber station may request a change in burst profile that is more robust or offers a higher data transmission rate. The base station may grant the request, negotiate parameters or reject the request.

During a WiMAX communication session, changes in bandwidth may be requested. The subscriber station may send bandwidth request messages to the base station to increase or decrease its allocated bandwidth. Bandwidth request messages may be sent as independent messages or they may be piggybacked with other messages.