Category list

Part Three: WiMAX Multiple Access (MAC Layer) and Qos Management

Chapter List

Chapter 7: Convergence Sublayer (CS)
Chapter 8: MAC Functions and MAC Frames
Chapter 9: Multiple Access and Burst Profile Description
Chapter 10: Uplink Bandwidth Allocation and Request Mechanisms
Chapter 11: Network Entry and Quality of Service (QoS) Management

Chapter 7: Convergence Sublayer (CS)

7.1 CS in 802.16 Protocol Architecture

The service-specific Convergence Sublayer (CS), often simply known as CS, is the top sublayer of the MAC Layer in WiMAX/802.16 (Figure 7.1). The CS accepts higher-layer PDUs from the higher layers and transmits them to the MAC CPS where classical type MAC procedures are applied (see Chapter 8). Classifying and mapping the MSDUs into appropriate CIDs (Connection IDentifier) made by the CS are basic functions of the QoS mechanisms of WiMAX/802.16. Among other functions of the CS is the optional Payload Header Suppression (PHS), the process of suppressing repetitive parts of payload headers at the sender and restoring the headers at the receiver. The classification and mapping made by a QoS management module allow full advantage to be taken of the different PHYsical layer features presented in the two previous chapters of this book.

image from book
Figure 7.1: Protocol layers of the 802.16 BWA standard. (From IEEE Std 802.16-2004 ^[1] . Copyright IEEE 2004, IEEE. All rights reserved.)

In the present version of the 802.16-2004 standard, two CS specifications are provided and described in Section 5 of standards ^[1] and ^[2] . The first CS specification is the ATM CS. The Asynchronous Transfer Mode (ATM) CS is a logical interface that associates different ATM services with the MAC CPS SAP. The ATM CS accepts ATM cells from the ATM layer, performs classification and, if provisioned, PHS. Then the ATM CS delivers CS PDUs to the appropriate MAC SAP.

The other available CS specification is the packet CS. The packet CS is used for the transport of all packet-based protocols such as the Internet Protocol (IP), IPv4, IPv6. Point-to-Point Protocol (PPP) and the IEEE standard 802.3 (Ethernet). Classification and, if provisioned, PHS are also defined for the packet CS.

The standard states that other CSs may be specified in the future. For the moment, no implementation of the ATM CS is planned, although it is detailed in the standard. In the rest of this chapter only the packet CS will be considered .

^[1] IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems , October 2004.

^[2] IEEE 802.16e, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1 , February 2006 (Approved: 7 December 2005).

7.2 Connections and Service Flow

The CS provides any transformation or mapping of external network data received through the CS Service Access Point (SAP) into MAC SDUs received by the MAC Common Part Sublayer (CPS) through the MAC SAP (see Figure 7.1). This includes classifying external network Service Data Units (SDUs) and associating them with the proper MAC Service Flow Identifier (SFID) and Connection Identifier (CID). Classification and mapping are then based on two 802.16 MAC layer fundamental concepts:

Connection. A connection is a MAC Level connection between a BS and an SS (or MS) or inversely. It is a unidirectional mapping between a BS and an SS MAC peers for the purpose of transporting a service flow's traffic. A connection is only for one type of service (e.g. voice and email cannot be on the same MAC connection). A connection is identified by a CID (Connection IDentifier), an information coded on 16 bits.
Service flow. A Service Flow (SF) is a MAC transport service that provides unidirectional transport of packets on the uplink or on the downlink. A service flow is identified by a 32-bit SFID (Service Flow IDentifier). The service flow defines the QoS parameters for the packets (PDUs) that are exchanged on the connection.

Figure 7.2 shows the relation between the SFID and CID. The relation between the two is the following: only admitted and active service flows (see the definitions below) are mapped to a CID, i.e. a 16-bit CID. In other terms:

Figure 7.2: Correspondence between the CID and SFID
A SFID matches to zero (provisioned service flows) or to one CID (admitted or active service flow).
A CID maps to a service flow identifier (SFID), which defines the QoS parameters of the service flow associated with that connection.

The definitions of connection and service flow in the 802.16 standard allow different classes of QoS to be found easily for a given element (SS or BS), with different levels of activation (see Figure 7.3). More details will now be given about connections (and CIDs) and service flows.

image from book
Figure 7.3: Illustration of service flows and connections

7.2.1 Connection Identifiers (CIDs)

A Connection IDentifier (CID) identifies a connection where every MAC SDU of a given communication service is mapped into. The CID is a 16-bit value that identifies a unidirectional connection between equivalent peers in the MAC layers of a BS and an SS.

All 802.16 traffic is carried on a connection. Then, the CID can be considered as a connection identifier even for nominally connectionless traffic like IP, since it serves as a pointer to destinations and context information ^[1] . The use of a 16-bit CID permits a total of 64K connections within each downlink and uplink channel. There are several CIDs defined in the standard (see Table 7.1). Some CIDs have a specific meaning. Some of the procedures introduced in this table, such as ranging , basic, primary and secondary management. AAS and others, will be introduced in different chapters later in this book.

Table 7.1: CID ranges as defined in Reference ^[1] . Values are between 0000 (the 16 bits are equal to zero) and FFFF (the 16 bits are equal to one). It seems probable that the BS decides for a number m of CIDs for each of the basic and primary management connections that may be requested , i.e. a total of 2 m connections. The CID value of basic, primary and secondary management connections for each SS are assigned in a ranging message (see Chapter 11). (From IEEE Std 802.16-2004 ^[1] . Copyright IEEE 2004, IEEE. All rights reserved.)
Open table as spreadsheet
CID	Value	Description
Initial ranging	0 × 0000	Used by SS and BS during the initial ranging process
Basic CID	0 × 0001 – m	Each SS has a basic CID and has a short delay. The same CID value is assigned to both the downlink and uplink connections
Primary managemen	m + 1 − 2 m	The primary management connection is used to exchange longer, more delaytolerant MAC management messages
Transport CIDs and secondary management CIDs	2 m + 1 − 0 × FE9F	Used for data transfer and for secondary management connection
Multicast CIDs	0 × FE9F − 0 × FEFE	For the downlink multicast service, the same value is assigned to all SSs on the same channel that participate in this connection
AAS initial ranging CID	0 × FEFF	A BS supporting AAS (Advanced Antenna System) uses this CID when allocating an AAS ranging period (using AAS_ Ranging_Allocation_IE)
Multicast polling CIDs	0 × FFOO − 0 × FFF9	An SS may be included in one or moremulticast polling groups for the purposes of obtaining bandwidth via polling. These connections have no associated service flow
Normal mode multicast CID	0 × FFFA	Used in DL-MAP to denote bursts for transmission of downlink broadcast information to normal mode SS
Sleep mode multicast CID	0 × FFFB	Used in DL-MAP to denote bursts for transmission of downlink broadcast information to sleep mode SS. May also be used in MOB_TRF-INO messages
Idle mode multicast CID	0 × FFFC	Used in DL-MAP to denote bursts for transmission of downlink broadcast information to idle mode SS. May also be used in MOB_PAG-ADV messages
Fragmentable broadcast CID	0 × FFFD	Used by the BS for transmission of management broadcast information with fragmentation. The fragment subheader should use an II-bit long FSN on this connection
Padding CID	0 × FFFE	Used for transmission of padding information by the SS and BS
Broadcast CID	0 × FFFF	Used for broadcast information that is transmitted on a downlink to all SSs

Security associations (SAs) exist between keying material and CIDs, as described in Chapter 15.

7.2.2 Service Flows

A Service Flow (SF) is a MAC transport service that provides unidirectional transport of packets on the uplink or on the downlink. It is identified by a 32-bit SFID (Service Flow IDentifier).

A service flow is characterised by a set of QoS parameters. The QoS parameters include details of how the SS may request uplink bandwidth allocations and the expected behaviour of the BS uplink scheduler. The main QoS parameters of the 802.16-2004 standard are given in Section 7.4 below. The service flow attributes are now given.

7.2.2.1 Service Flow Attributes

A service flow is partially characterised by the following attributes:

Service Flow ID. An SFID is assigned to each existing service flow. The SFID serves as the identifier for the service flow in the network.
CID. Mapping a CID to an SFID exists only when the connection has an admitted or active service flow (see below).
ProvisionedQoSParamSet. This defines a QoS parameter set that is provisioned via means that the standard assumes to be outside of its scope. The standard states that this could be part of the network management system. For example, the service (or QoS) class name is an attribute of the ProvisionedQoSParamSet. There are five QoS classes, the fifth having been added by the 802.16e amendment.
AdmittedQoSParamSet. This defines a set of QoS parameters for which the BS, and possibly the SS, are reserved resources. The principal resource to be reserved is bandwidth, but this also includes any other memory or time-based resource required to subsequently activate the flow.
ActiveQoSParamSet. This defines a set of QoS parameters defining the service actually being provided to the service flow. Only an active service flow may forward packets. The activation state of the service flow is determined by the ActiveQoSParamSet. If the ActiveQoSParamSet is null, then the service flow is inactive.
Authorisation module. This is a logical function within the BS that approves or denies every change to QoS parameters and classifiers associated with a service flow. As such, it defines an ‘envelope’ that limits the possible values of the AdmittedQoSParamSet and ActiveQoSParamSet (see Section 11.5.4 for more details about the authorisation module).

7.2.2.2 Types of Service Flow

The standard has defined three types of service flow:

Provisioned service flows. This type of service flow is known via provisioning by, for example, the network management system. Its AdmittedQoSParamSet and ActiveQoSParamSet are both null.
Admitted service flow. The standard supports a two-phase activation model that is often used in telephony applications. In the two-phase activation model, the resources for a call are first ‘admitted’ and then, once the end-to-end negotiation is completed, the resources are ‘activated’.
Active service flow. This type of service flow has resources committed by the BS for its ActiveQoSParamSet. Its ActiveQoSParamSet is non-null.

Each service flow class is associated with the corresponding QoSParametersSets. These three types of service flows can be seen as complementary. Figure 7.4 shows the possible transitions between these different service flows. A BS may choose to activate a provisioned service flow directly, or may choose to take the path to active service flows by passing the admitted service flows. The model structure of these three service flow types is shown in Figure 7.5, taken from the standard.

image from book
Figure 7.4: Possible transitions between service flows ^[1]

More details about the activation of a service flow are given in Section 11.5. Having introduced the concepts of CID and SFID and their attributes, it is now possible to describe the process of classification and mapping made in the CS.