Quality of Service (QOS)

QOS refers to meeting certain requirement - e.g. throughput, packet error rate, delay, and jitter - associated with given application. Broadband wireless networks must support a variety of applications, such as voice, data, video, and multimedia, and each of these has different traffic patterns and QoS requirements. The variability in the QoS requirements across applications, services, makes it a challenge to accommodate all these on a single-access network, particularly wireless networks, where bandwidth is at a premium.

The problem of providing QoS in broadband wireless systems is one of managing radio resources effectively. Effective scheduling algorithms that balance the QoS requirements of each application and user with the available radio resources need to be developed. In other words, capacity needs to be allocated in the right proportions among users and applications at the right time. This is the challenge that the MAC-layer protocol must meet: simultaneously handling multiple types of traffic flows - bursty and continuous, varying throughputs and latency requirements. Also needed are an effective signaling mechanism for users and applications to indicate their QoS requirements and for the network to differentiate among various flows.

Before any data transmission happens, the BS and the MS establish a unidirectional logical link, called a connection, between the two MAC-layer peers. Each connection is identified by a connection identifier (CID), which serves as a temporary address for data transmissions over the particular link.

WiMAX also defines a concept of a service flow. A service flow is a unidirectional flow of packets with a particular set of QoS parameters and is identified by a service flow identifier (SFID). The QoS parameters could include traffic priority, maximum sustained traffic rate, maximum burst rate, minimum tolerable rate, scheduling type, ARQ type, maximum delay, tolerated jitter, service data unit type and size, bandwidth request mechanism to be used, transmission PDU formation rules, and so on. These parameters are managed using the DSA and DSC messages. The base station is responsible for issuing the SFID and mapping it to unique CIDs.

To support a wide variety of applications, WiMAX defines five scheduling services:

1. Unsolicited grant services (UGS): This is designed to support fixed-size data packets at a constant bit rate (CBR). Examples of applications that may use this service are T1/E1 and VoIP.

2. Real-time polling services (rtPS): This service is designed to support real-time service flows, such as MPEG video, that generate variable-size data packets on a periodic basis.

3. Non-real-time polling service (nrtPS): This service is designed to support delay-tolerant data streams, such as an FTP, that require variable-size data grants at a minimum guaranteed rate.

4. Best-effort (BE) service: This service is designed to support data streams, such as Web browsing, that do not require a minimum service-level guarantee.

5. Extended real-time polling service (ErtPS): This service is designed to support real-time applications, such as VoIP with silence suppression, that have variable data rates, but require guaranteed data rate and delay. This service is defined only in IEEE 802.16e-2005, not in IEEE 802.16-2004.

Network Entry and Initialization

When an MS acquires the network after being powered up a WiMAX network undergoes various steps. An overview of this process, also referred to as network entry, is shown in Figure

 

- Scan and Synchronize Downlink Channel:

When an MS is powered up, it first scans the allowed DL frequencies to determine whether it is presently within the coverage of a suitable WiMAX base station. Each MS stores a nonvolatile list of all operational parameters, such as the DL frequency used during the previous operational instance. The MS first attempts to synchronize with the stored DL frequency. If this fails, the MS it scans other frequencies in an attempt to synchronize with the DL of the most suitable BS.

During the DL synchronization, the MS listens for the DL frame preambles. When one is detected, the MS can synchronize itself with respect to the DL transmission of the BS. Once it obtains DL synchronization, the MS listens to the various control messages, such as FCH, DCD, UCD, DL-MAP, and UL-MAP, that follow the preamble to obtain the various PHY and MAC related parameters corresponding to the DL and UL transmissions.

- Obtain Uplink Parameters:

Based on the UL parameters decoded from the control messages, the MS decides whether the channel is suitable for its purpose. If the channel is not suitable, the MS goes back to scanning new channels until it finds one that is. If the channel is deemed usable, the MS listens to the UL MAP message to collect information about the ranging opportunities.

- Ranging:

At this stage, the MS performs initial ranging with the BS to obtain the relative timing and power-level adjustment required to maintain the UL connection with the BS. Once the a UL connection has been established, the MS should do periodic ranging to track timing and power-level fluctuations. These fluctuations can arise because of mobility, fast fading, shadow fading, or any combinations thereof. Since the MS does not have a connection established at this point, the initial ranging opportunity is contention based.

MS sends a RNG-REQ message with the CID set to initial ranging CID. If it does not receive any response from the BS within a certain time window,
then the MS considers the previous ranging attempt to be unsuccessful and enters the contention-resolution stage. Therein, the MS sends a new CDMA ranging code at the next ranging opportunity, after an appropriate back-off delay. If Ranging process is successful then, BS sends RNG-RSP message with basic CID(BCID) and Primary CID(PCID) allocated to perticular MS. From here on, the basic and primary management CID is used by the MS and the BS to send most of the MAC management messages

- Negotiate Basic Capabilities:

After initial ranging, the MS sends an SBC-REQ message informing the BS about its basic capability set, which includes various PHY and bandwidth-allocation-related parameters. On the reciept of this message, the BS responds with an SBC-RSP, providing the PHY and bandwidth-allocation parameters to be used for UL and DL transmissions.

- Register and Establish IP Connectivity:

After negotiating the basic capabilities and exchanging the encryption key, the MS registers itself with the network. In WiMAX, registration is the process by which the MS is allowed to enter the network and can receive secondary CIDs. The registration process starts when the MS sends a REG-REQ message to the BS. The message contains a hashed message uthentication code (HMAC), which the BS uses to validate the authenticity of this message. Once it determines that the request for registration is valid, the BS sends to the MS a REG-RSP message in which it provides the secondary management CID. In the REG-REQ message, the MS also indicates to the BS its secondary capabilities not covered under the basic capabilities, such as IP version supported, convergence sublayer supported, and ARQ support.

After receiving the REG-RSP message from the BS, the SS can use DHCP to obtain an IP address.

- Establish Service Flow:

The creation of service flows can be initiated by either the MS or the BS, based on whether initial traffic arrives in the uplink or the downlink.

When it an MS chooses to initiate the creation of a service flow, an MS sends a DSA-REQ message containing the required QoS set of the service flow. On receipt of the DSA-REQ message, the BS first checks the integrity of the message and sends a DSX-RVD message indicating whether the request for a new service flow was received with its integrity preserved. Then the BS checks whether the requested QoS set can be supported, creates a new SFID and sends an appropriate DSA-RSP indicating the admitted QoS set. MS completes the process by sending a DSA-ACK message.