Service Flows and Classes

WiMAX services are the providing of information transfer to or between users. Information transfer services can have a variety of characteristics that can be selected and varied by the operator. The WiMAX system uses services flows to identify specific transmission characteristics for specific user services and a single user may have multiple service flows. Common sets of service characteristics may be defined by service classes.

Service Flows

Service flows are communication channels (e.g. a stream of packets) that have particular service characteristics associated with the transfer of data. For example, a communication link might have several service flows associated with it; a real time service flow for voice communication, a high-integrity service flow (low error rate) for data file transfer and a best effort service flow for Internet web browsing.

Each service provided on a WiMAX system is associated with a service flow. Service flows are requested, established and ended. When subscriber stations request services, the system can evaluate and negotiate the requested characteristics at any time.

A single WiMAX subscriber station may have multiple service flows for each connection (service flows can be different in different directions). Service flows can be dynamically added, changed and ended.

Service flows are uniquely identified by a service flow identifier (SFID). A SFID is associated with a specific connection identifier to determine the service characteristics a specific user will receive on that particular device

Unsolicited Grant Service (UGS)

Unsolicited grant service is a service flow in which the transmission system automatically and periodically provides a defined number of timeslots and fixed packet size that is used by a particular receiver. UGS is commonly used to provide services that require a constant bit rate (CBR) such as audio streaming or leased line (e.g. T1 or E1) circuit emulation.

UGS provides a constant bit rate for a single connection. A subscriber device may need additional bandwidth for an additional service that is added to a connection or to temporarily provide more bandwidth on the UGS connection. To request more bandwidth on a UGS connection, a poll me bit or slip indicator bit may be used.

A poll me bit is a signaling message in a data field within the header of a data packet that indicates that the device would like to be polled. The poll me bit indicates to the base station that the subscriber device needs to be polled for a service other than for the current UGS service.

For transmission to synchronous connections, timing inaccuracies may result in the need to transfer additional bits if the clock of one connection is slightly faster than the other connection. When the buffer of the faster connection indicates the number of bits to be transmitted may soon run out, a slip indicator bit may be used. The slip indicator is a signaling message within the header of a data packet that indicates that the data transmission queue of that device is changing (slipping) and that the device needs more bandwidth to keep up with the transmission queue. This allows the base station to temporarily assign additional bandwidth until the transmission buffer has caught up.

Figure 1 shows how WiMAX unsolicited grant service (UGS) operates. Subscriber stations are assigned to receive and transmit during assigned time intervals. The subscriber station may use the poll me bit in the header to indicate it wants to be polled so it can send data for another service. When the base station receives the poll me bit, it sends a polling message which allows the subscriber station to send a packet of data that is independent of the UGS packets.

Figure 1: Wireless Unsolicited Grant Service (UGS)

Scheduling Services

Overview

Scheduling services are the medium access control functions (data flow control) that define how and when devices will receive and transmit on a communication system. The types of services that WiMAX can provide range from guaranteed bandwidth with low delay unsolicited grant service (UGS) to random access best effort (BE) service. WiMAX systems use a grant management system to coordinate the request for new services and changes to existing services (such as requesting more bandwidth). The WiMAX system uses a combination of time division multiple access, polling and contention based flow control to provide specific types of services to users.

Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a subscriber communicates on a WiMAX system using TDMA, he/she is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems can guarantee a constant data rate with a minimal amount of flow control overhead.

Polling is the process of sending a request message (usually periodically) for the purpose of collecting events or information from a network device. The receipt of a polling message by a device starts an information transfer operation for a specific time period. Polling may be performed with specific units (unicast), to groups of units (multicast) or to all units (broadcast).

Unicast polls are requests for data transmission or responses to commands that are only sent between a sender (polling device) and receiver (polled device). When a subscriber station is responding to a unicast polling message, no other devices are allowed to transmit.

Multicast polls are requests for data transmission or responses to commands that are sent from a polling device to several receiving devices which are part of a multicast group. When a device receives a multicast polling message for its group, it will respond if it has data to send. When a subscriber station is responding to a multicast polling message, others may also have information to transmit. For multicast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

Broadcast polls are requests for data transmission or responses to commands that are sent from a polling device to all devices that are able to receive its broadcasted polling message. When a device receives a broadcast polling message, it will respond if it has data to send. For broadcast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

The amount of time between polling messages is called the polling cycle. The time between polling cycles is a balance between delay (more polling messages is less delay) and overhead (more polling messages increases the percentage of data that is used for control messages).

Figure 1 illustrates the different types of polling that are used in the WiMAX system. A device that is part of a multicast group, has received a multicast polling message, must compete for access to send its data. Finally, for a broadcast polling message, any device that has data will compete for access to send its data.

Figure 1: WiMax Polling Types

Contention based access control is the independent operation (distributed access control) of communication devices (stations). In a contention-based system, communication devices randomly request service from channels within a communication system. Because communication requests occur randomly, two or more communication devices may request service simultaneously. The access control portion of a contention based session usually involves requiring the communication device to sense for activity before transmitting and listening for message collisions after sending its service request. If the requesting device does not hear a response to its request, it will wait a random amount of time before repeating the access attempt. The amount of time waited between retransmission requests increases each time a collision occurs.

The WiMAX system defines time periods that subscriber stations can use for contention based access. When subscriber units desire to initiate requests to the system that are not scheduled from a polling message, they must access the process during the contention time slots period. The contention time slot period is periodically broadcast on the downlink channel along with other channel access control information.

Figure 2 shows how contention based access control can be performed on a WiMAX system. Channel descriptors are periodically broadcasted on the downlink radio channel that provides the time intervals for the contention slots. Subscriber devices that use contention based access must compete during these time periods. The WiMAX subscriber station will initially attempt to access the system at a relatively low power level. If the subscriber station does not hear a response to its request, it will wait a random amount of time, increase its transmitted power level and attempt access again. The subscriber station will continue to wait increasing amounts of time each time and increases its transmitted power level each time an access attempt fails until it receives a response from the system.

Figure 2: WiMax Contention Based Access Control

The WiMAX system uses a grant management process for the requesting and allocation (granting) of resources (such as transmission time or bandwidth). Subscriber stations can request changes to the type of services they require (e.g. increases or decreases in bandwidth) by transmitting a bandwidth request header and the system can decide to grant, adjust or not authorize the grant request.

The WiMAX system can grant resources based on a connection or based on a specific subscriber station. A grant per subscriber station is the allocation of transmission bandwidth that affects the transmission for all the connections associated with a subscriber station. A grant per connection is the assignment of bandwidth which only affects the transmission for a specific connection on a subscriber device.

Bandwidth requests can be in aggregate or incremental form. An aggregate request is a message that defines the amount of a resource (such as transmission bandwidth) that is requested to provide for a combined group of applications or services. An incremental request is a message that defines the additional amount of a resource (such as transmission bandwidth) that is requested to provide for an application or service. Bandwidth request messages may be sent as stand alone messages or they may be piggybacked in the payload of another packet of data.

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.

Medium Access Control | WiMAX Operation

Medium access control is the process used by communication devices to gain access to a shared communications medium or channel. The methods for controlling access to WiMAX systems may be assigned (“non-contention based”) or random (“contention based”).

When the WiMAX system uses contention free access control the subscriber station must wait for polling messages before responding. If contention based access control is used (e.g. best effort service), the subscriber device must compete for access to send its packets. The WiMAX system can mix contention free and contention based access on the same radio channel.

Contention free access is provided by defining time periods that specific devices will use when communicating with the system. Because all the devices listening to the WiMAX radio channel can hear these messages, devices will not transmit during the assigned time periods.

Contention based access is provided through the use of contention slots and the collision sense multiple access (CSMA) process. The WiMAX channel descriptors define specific time periods (“contention slots”) that contention based WiMAX devices must use when accessing the WiMAX system. Contention slots are dedicated time intervals (time slots) on a communication channel that can be used to allow devices to randomly request service from a system.

When contention based WiMAX subscriber stations access the system, they first obtain the contention time slot interval and the system access parameters (e.g. initial access transmit power level). After the contention slot time period has started, the subscriber station begins to transmit an access message at a low RF power level. If the subscriber station hears a positive response to its access request message, it can transmit its package. If the subscriber device does not hear a response (e.g. another device has transmitted at the same time), it must stop transmitting and wait a random amount of time before attempting to access the system again. Each time the device attempts to access the system and fails, it must wait a longer amount of time before attempting to access the system again. This prevents the possibility of many collisions between devices that are attempting to access the system at approximately the same time.

Figure 1 illustrates how the WiMAX system can mix contention free and contention based access control on a WiMAX radio channel. This diagram shows that the downlink channel contains downlink and uplink descriptor messages that define when subscriber stations are allowed to transmit. For unicast polled devices (contention free), they are assigned specific time periods to transmit from a polling message. For multicast polled, broadcast polled or best effort devices (contention based), they compete during the contention time slot periods.

Figure 1: WiMax Access Control

Initial Ranging | WiMAX Operation

Initial ranging is the process of estimating the distance or propagation time between a transmitter and receiver. Ranging information may be used to assist in the establishment of operating parameters for the transmitter and receiver. The transmitter power level and packet transmission delay time ensure packets do not overlap with transmission from other devices.

During the initial ranging process, the base station is assigned the basic CID that will be used to control the radio operations of the subscriber device. After the basic CID is assigned, a primary management CID may be assigned to allow for authentication and the establishment of other CID channels. A secondary CID may be assigned to allow the downloading of configuration files and the assignment of an IP address using dynamic host configuration protocol (DHCP).

Figure 1 depicts the basic channel acquisition processes that may be used in the WiMAX system. The subscriber station begins by scanning a set of potential WiMAX frequencies. If it finds a WiMAX radio channel, it synchronizes with the RF channel and acquires the downlink channel descriptor (DCD) and uplink channel descriptor (UCD) messages to determine how to access the system. The subscriber station then sends initial ranging request messages to get the attention of the system and to receive timing adjustment information. This process starts by transmitting at a lower RF power level and gradually increasing until the system responds with an assignment of basic and primary control identifiers (CID). The subscriber station then sends its transmission capabilities to the base station and the WiMAX system responds with an authorization or denial of service for these transmission capabilities.

Figure 1: 802.16 Channel Acquisition and Initial Ranging

Channel Acquisition | WiMAX Operation

Channel acquisition is the process of finding and acquiring access to a communication channel. When WiMAX devices initialize (e.g. when they are turned on), they begin a channel scanning process. Channel scanning is the process of searching through multiple radio channels to find signals that indicate a channel is available on which to communicate. The WiMAX device will typically have a stored list of frequency channels for it to scan in order to reduce the amount of scanning time. These frequency channels may be preprogrammed by or for a WiMAX system operator so the WiMAX device will initially try to connect to a specific WiMAX system.

When the WiMAX device has found one or more WiMAX radio channels, the device will decode the channel and look for packets of data that have a frame control header that contains a downlink channel description (DCD) message and an uplink channel description (UCD) message. The DCD message contains parameters that are necessary or that will assist it to access the device in receiving information from the downlink channel on the communication system. The UCD message provides the device with the parameters that are necessary to access the communication system.

Logical Channels | WiMAX Radio

Logical channels are a portion of a physical communications channel that is used for a particular (logical) communications purpose. The WiMAX physical channel can have up to 65,535 logical channel connections and each connection can multiple service flows associated with it.

Connection ID (CID)

WiMAX logical channels are identified by a connection identifier (CID). A CID is a unique name or number that is used to identify a specific logical connection path in a communication system. Some connection channel IDs are reserved for control (management connection) and other connections are used for transporting user data.

Each type of connection has its own CID. A two-way connection requires two CIDs. For basic, primary and secondary connections, CID codes are assigned in pairs and they are the same for the downlink and uplink connections.

An initial ranging connection identifier is a code that is used during the initial connection to a wireless system to determine how much transmission timing adjustment is required. For WiMAX systems, the initial ranging CID is 0000 for standard transmission systems and FEFF for adaptive antenna systems.

A basic CID is a logical channel that is assigned during the initial ranging process. Basic CID connections are used for time sensitive MAC control messages such as RF power control and time alignment. The range of CIDs that can be assigned for basic CIDs is from 0001 to some number (m) selected by the operator.

A primary management CID is a logical channel that is used to transfer link control messages. The range of CIDs that are assigned as primary CIDs ranges from the address above the highest basic CID (m+1) to double the number of basic CIDs (2m).

A secondary management CID is a logical channel that is used for upper layer control messages such as DHCP and TFTP messages. The range of CIDs that is assigned as secondary management CIDs ranges from the address above the highest primary management CID (2m+1) up to connection ID FEFE.

A transport CID is a logical channel that is used to transfer user data. The range of CIDs that are assigned as transport CIDs ranges from the address above the highest primary management CID (2m+1) up to connection ID FEFE. Transport connections can use different CIDs in the uplink and downlink directions.

Multicast polling connection identifiers are used to prompt subscriber stations which are part of a multicast group that have data to transmit to attempt to transmit their data using a contention control process. The multicast polling CIDs range from FF00 to FFFC.

A broadcast connection identifier is used to transfer broadcast messages to all devices that are listening to the radio channel. The broadcast CID is FFFF.

Figre 1 outlines some of the CID codes that are used in the WiMAX system. The table shows that CID 0000 is reserved for initial ranging and the basic, primary, secondary and transport CIDs are dynamically assigned as needed. Other reserved CIDs include FEFF for adaptive antenna initial ranging, FF00 through FFFC for multicast polling, FFFD for fragmental broadcast messages, FFFE for padding messages and FFFF for broadcast messages.

Figure 1: WiMax CID Codes

Service Flow ID (SFID)

A service flow identifier is a unique number that is assigned by a system that is used to identify the flow of a communication channel that is used for a specific service type. A WiMAX device may have multiple SFIDs per connection (per CID).

Figure 2 illustrates that the WiMAX system has logical connection and service flow channels. Each subscriber station has at least one connection channel and service flows may be assigned to the connections. In this diagram, a WiMAX base station has setup connections with 3 WiMAX subscriber stations. For home #1, the WiMAX transceiver connection is providing one type of service flow for an Internet web browser over a single connection. For home #2, the WiMAX base station has setup a single connection with two services flows; one for a web browsing computer and the other for an IP telephone. For the office user, the WiMAX base station has setup 3 connections on a single subscriber device (3 CIDs). Of these, 2 connections have 2 service flows providing web browsing and IP telephone service and the 3rd connection has a single service flow for web browsing service.

Figure 2: WiMax Logical Channels

Retransmission Policy | Wimax Radio

Retransmission policy (automatic repeat request – ARQ) is the set of rules or processes used by networks to define if, when, and how retransmissions of data or information will occur. Some types of services (such as real time digital audio) do not use retransmissions because the delay for retransmission would take too long to offer any benefit.

Retransmission uses error detection, feedback, retransmission processes and the retransmission of blocks of data in packets. The WiMAX system uses two forms of ARQ; hybrid automatic repeat request (HARQ) and selective repeat (SR).

Selective Repeat (SR)

Selective repeat automatic repeat request is a data transmission control process that allows the receiver to request the retransmission of selective blocks of data.

Data to be transmitted is grouped into blocks and given a block sequence number (BSN). The maximum size of a data block in the WiMAX system is 2040 bytes. When blocks are transmitted using ARQ, each block is given a sub-packet identifier (SPID). An SPID is an index value that can be used to identify specific packets that are awaiting conformation in an automatic repeat request (ARQ) process.

As blocks are transferred between the sending device and the receiving device, acknowledgement messages are sent. WiMAX acknowledgement message types include selective, cumulative and cumulative with selective. ARQ messages can be sent as separate messages or the may be combined with other messages.

Hybrid Automatic Repeat Request (HARQ)

Hybrid automatic repeat request is a data transmission flow control process that uses a combination of the physical layer (PHY) and medium access control (MAC) layer to allow the receiver to stop and restart the retransmission of data over a transmission channel.

HARQ is a variation of a stop and wait ARQ. Stop and wait ARQ is a flow control process that allows the data flow to stop when packets are not received and wait until a successful retransmission is received before the data flow is restarted.

Sub Channelization (Sub-carriers)

Sub channelization is the dividing of communication channels into smaller sub-parts. The air interface portions of the WiMAX system divide a wide radio channel into several sub-carriers. A sub-carrier is a modulation signal that is imposed on another carrier that can be used to independently transfer information from other sub-carriers located on the radio channel. The WiMAX system sub-carrier signal types include a pilot sub-carrier (reference signal), guard sub-carrier (interference protection) and data sub-carrier (user information).

A pilot sub-carrier is a reference signal that serves as a control signal for use in the reception of other sub-carrier signals. A guard sub-carrier is one or more sub-carriers on a communication channel that is not used (null channel). The guard sub-carrier is dedicated to the protection of a communication channel from interference due to radio signal energy or time overlap of signals. Data sub-carriers are transmission channels that carry user information or data. Sub-carrier signals are referenced from the center of the radio channel. The sub-carrier that is located at the center of the radio channel is called the DC sub-carrier.

The number of sub-carriers in the WirelessMAN-OFDM system is 256. Of these, 55 are reserved as guard bands and 8 sub-carriers are used to transfer reference pilot signals. This allows up to 192 sub-carriers to be used for data transfer.

For a 20 MHz WiMAX OFDMA radio channel, there may be 2048 subcarriers. Of these, approximately 70% can be used as data carriers, 25% are reserved to protect from interference (guard bands) and approximately 15% are used as reference signals (pilot channels).

Figure 1 depicts how the WirelessMAN-OFDM system divides a wide radio channel into several independent orthogonal channels with smaller bandwidth. The sub-channel in the center of the RF channel is called the DC subcarrier. Some of the sub-carriers are used as reference pilot channels and some are reserved for guard bands.

Figure 1: WiMax OFDMA Sub Channelization