Network Topology Acquisition

1 Network Topology Advertisement

A BS broadcasts information about the network topology using the MOB_NBR-ADV (Neighbour ADVertisement) MAC management message. This message provides channel information about neighbouring BSs normally provided by each BS's own DCD/UCD message transmissions. The MOB_NBR-ADV does not contain all the information of neighbouring BSs, UCD and DCD. The standard indicates that a BS may obtain that information over the backbone and that availability of this information facilitates MS synchronisation with neighbouring BS by removing the need to monitor transmission from the neighbouring (handover target) BS for DCD/UCD broadcasts. The BSs will keep mapping tables of neighbour BS MAC addresses and neighbour BS indexes transmitted through the MOB_NBR-ADV message, for each configuration change count, which has the same function as for the DCD message.

BSs supporting mobile functionality must be capable of transmitting a MOB_NBR-ADV MAC management message at a periodic interval to identify the network and define the characteristics of the neighbour BS to a potential MS seeking initial network entry or handover. The standard indicates that the maximum value of this period is 30 seconds.

2 MS Scanning of Neighbour BSs

A scanning interval is defined as the time during which the MS scans for an available BS. A BS may allocate time intervals to the MS for the purpose of MS seeking and monitoring suitability of neighbour BSs as targets for a handover. MS scanning of neighbour BSs is based on the following MAC Management messages: MOB_SCN-REQ, SCaNning interval allocation REQuest, MOB_SCN-RSP, SCaNning interval allocation Response. MOB_SCN-REP and SCaNning result REPort.

The MOB_SCN-REQ message is sent by the MS to request a scanning interval for the purpose of seeking available BSs and determining their suitability as targets for HO. In the MOB_SCN-REQ message the MS indicates a group of neighbour BSs for which only Scanning or Scanning with Association are requested by the MS. The Neighbour_BS_Index of the MOB_SCN-REQ message corresponds to the position of BSs in the MOB_NBR-ADV message. In this message, the MS may also request the scanning allocation to perform scanning or noncontention Association ranging. Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbour BSs (see the following section).

Upon reception of the MOB_SCN-REQ message, the BS responds with a MOB_SCN-RSP message. The MOB_SCN-RSP message can also be unsolicited. The MOB_SCN-RSP message either grants the requesting MS a scanning interval that is at least as long as that requested by the MS or denies the request. In the MOB_SCN-RSP message the BS indicates a group of neighbour BSs for which only Scanning or Scanning with Association are recommended by the BS.

Following reception of a MOB_SCN-RSP message granting the request, an MS may scan for one or more BSs during the time interval allocated in the message. When a BS is identified through scanning, the MS may attempt to synchronise with its downlink transmissions and estimate the quality of the PHY channel.

The BS may negotiate over the backbone with a BS Recommended for Association (in the MOB_SCN-REQ message) the allocation of unicast ranging opportunities. Then the MS will be informed on Rendez vous time to conduct Association ranging with the Recommended BS. When conducting initial ranging to a BS Recommended for Association, the MS uses an allocated unicast ranging opportunity, if available.

The serving BS may buffer incoming data addressed to the MS during the scanning interval and transmit that data after the scanning interval during any interleaving interval or after exit of the Scanning mode. When the Report mode is 0b10 (i.e. event-triggered) in the most recently received MOB_SCN-RSP, the MS scans all the BSs within the Recommended BS list of this message and then transmits a MOB_SCN-REP message to report the scanning results to its serving BS after each scanning period at the time indicated in the MOB_SCN-RSP message. The MS may transmit a MOB_SCN-REP message to report the scanning results to its serving BS at any time. The message will be transmitted on the Primary Management CID.

3 Association Procedure

Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbour BSs. The function of Association is to enable the MS to acquire and record ranging parameters and service availability information for the purpose of proper selection of a handover target BS and/or expediting a potential future handover to a target BS. Recorded ranging parameters of an Associated BS may be further used for setting initial ranging values in future ranging events during a handover.

Upon completion of a successful MS initial ranging of a BS, if the RNG-RSP message (sent by the BS) contains a service level prediction parameter set to 2, the MS may mark the BS as Associated in its MS local Association table of identities, recording elements of the RNG-RSP to the MS local Association table and setting an appropriate ageing timer.

There are three levels of Association as follows:

§  Association Level 0: Scan/Association without coordination. The serving BS and the MS negotiate the Association duration and intervals (via MOB_SCN-REQ). The serving BS allocates periodic intervals where the MS may range neighbouring BSs. The target BS has no knowledge of the MS. The MS uses the target BS contention-based ranging allocations.

§  Association Level 1: Association with coordination. Unilaterally or upon request of the MS (through the MOB_SCN-REQ message), the serving BS provides Association parameters to the MS and coordinates Association between the MS and neighbouring BSs. The target BS reserves a CDMA initial ranging code and an initial ranging slot (transmission opportunity) in a specified dedicated ranging region (rendezvous time). The neighbouring BS may assign the same code or transmission opportunity to more than one MS, but not both. There is no potential for collision of transmissions from different MSs.

§  Association Level 2: network assisted association reporting. The MS may request to perform Association with network assisted Association reporting by sending the MOB_SCN-REQ message, including a list of neighbouring BSs, to the serving BS with scanning type = 0b011. The serving BS may also request this type of Association unilaterally by sending the MOB_SCN-RSP message with the proper indication. The serving BS will then coordinate the Association procedure with the requested neighbouring BSs in a fashion similar to Association Level 1. With Level 2, the MS is only required to transmit the CDMA ranging code to the neighbour BSs. The MS does not wait for RNG-RSP from the neighbour BSs. Instead, the RNG-RSP information on PHY offsets is sent by each neighbour BS to the serving BS over the backbone. The serving BS may aggregate all ranging information into a single MOB_ASC_REPORT, MOB_ASC-REP, Association result REPort, message.

4 CDMA Handover Ranging and Automatic Adjustment

For OFDMA PHY, 802.16e defines the handover ranging^]. An MS that wishes to perform handover ranging must take a process similar to that defined in the initial ranging section with the following modifications. In the CDMA handover ranging process, the CDMA handover ranging code is used instead of the initial ranging code. The code is selected from the handover ranging domain. The handover ranging codes are used for ranging with a target BS during the handover. Alternatively, if the BS is pre-notified for the upcoming handover MS, it may provide bandwidth allocation information to the MS using Fast_Ranging_IE to send an RNG-REQ message.

Mobility, Handover and Power-Save Modes

Handover Considerations

One of the major goals of the 802.16e amendment is to introduce mobility in WiMAX. Consequently, mobile WiMAX profiles are based on 802.16e. Mobility is based on handover. Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call. data session or other). The handover can be due to mobile subscriber moves, to radio channel condition changes or to cell capacity considerations. Handover is a mandatory feature of a cellular network. The handover (HO) is described as defined in 802.16e.

In 802.16e, the two known generic types of handover are defined:

§  Hard handover, also known as break-before-make. The subscriber mobile station (MS) stops its radio link with the first BS before establishing its radio link with the new BS. This is a rather simple handover.

§  Soft handover, also known as make-before-break. The MS establishes its radio link with a new BS before stopping its radio link with the first BS. The MS may have two or more links with two or more BSs, which gives the soft handover state. The soft handover is evidently faster than the hard handover.

Two types of soft handover are then defined in 802.16e:

§  Fast BS Switching (FBSS). This is a state where the MS may rapidly switch from one BS to another. The switch is fast because the MS makes it without realising the complete network entry procedure with regard to the new BS.

§  Macro Diversity HandOver (MDHO). Transmissions are between the MS and more than one BS.

In the mobile WiMAX profiles, only the hard handover is mandatory. The FBSS and MDHO are optional. The 802.16 standard also indicates that the support of the MDHO or FBSS is optional for both the MS and the BS.

Handover has challenging objectives. First, it has to be fast enough, of the order of 50 ms or 150ms. There are also security requirements, as some attacks are possible at the occasion of the handover procedure. Finally, the handover does not have only Layer 2 considerations. Layer 3 considerations are also needed. Hence, the handover is not independent of the architecture.

Network Functionalities | WiMAX Architechture

This section describes the main functions achieved by the WiMAX end-to-end system.

1 Network Discovery and Selection

This function is required for nomadic, portable and mobile WiMAX services where in the same geographical area the MS may have radio coverage access to an ASN managed by a single NAP and shared by several NSPs or coverage access to several ASNs managed by several NAP/NSPs. To perform network discovery and selection, the MS (which has been pre-provisioned with configuration information at subscription) performs a four-step process:

§  NAP discovery. During the scanning of the DCD of the different BSs in the coverage reach from the MS, the MS detects the ‘operator ID’ in the BSID field.

§  NSP discovery. The MS discovers the available NSP through the list NSP ID which is broadcast by the ASN as part of the system information identity message. NSP discovery is also possible via solicited request/response messages.

§  NSP enumeration and selection. Based on the dynamic information obtained in the coverage area and the configuration information from the subscription, the MS selects the appropriate NSP. Manual configuration of the NSP may also be available in the case of a visited NSP.

§  ASN attachment. After selection of the NSP and associated ASN, the MS indicates its NSP selection by sending an NAI (Network Access Identifier) message used by the ASN to determine the next hop AAA where an MS AAA packet should be routed to.

2 IP Addressing

WiMAX networks support IPv4 and IPv6 addressing mechanisms. At the end of the procedure, a PoA (Point of Attachment) IP address is delivered to the MS. The IP allocation address modes depend on the WiMAX access service types.

In the case of IPv4, the dynamic PoA configuration is based on DHCP. The DHCP proxy may reside in the ASN and the DHCP server in the CSN. In the case of IPv6, stateful IP address allocation is based on DHCPv6. The DHCP server resides in the CSN and the DHCP proxy may reside in the ASN. For the stateless CoA (Care of Address), IP address allocations RFC 2462 and RFC 3041  are used.

3 AAA Framework

The AAA framework follows the IETF specifications and includes the following services:

§  authentication: device, user or combined user/device authentication;

§  authorisation: user profile information delivery for sessions, mobility and QoS;

§  accounting: delivery of information for pre-paid/post-paid services.

Authentication and authorisation procedures are based on the EAP (Extensible Authentication Protocol). Between the MS and the ASN (the authenticator function), EAP runs over PKMv2, which enables both user and device authentication. Between the AAA server and the ASN, the EAP runs over RADIUS.

Accounting is also based on RADIUS messages. WiMAX Release 1 offers both offline (post-paid) and online (pre-paid) accounting. In the case of offline accounting, the ASN generates UDRs (Usage Data Records), which are a combination of radio-specific parameters and IP-specific parameters. The serving ASN then sends the UDR to the RADIUS server.

4 Mobility

The mobility procedures are divided into two mobility levels:

§  ASN anchored mobility procedures. This refers to MS mobility where no CoA address update is needed, also known as micromobility.

§  CSN anchored mobility procedures. The macromobility between the ASN and CSN is based on mobile IP protocols running across the R3 interface.

CSN anchored mobility implies that, in the case of IPv4, the MS changes to a new anchor FA (Foreign Agent). WiMAX systems must support at least one of the following mobile IP schemes:

§  Proxy-MIP. In this case, the MS is unaware of CSN mobility management activities and there is no additional signalling/overhead over the air to complete the CSN mobility.

§  Client MIP (CMIPv4). In this case, the MIP client in the MS participates in inter-ASN mobility.

5 End-to-End Quality of Service

The IEEE standard defines the QoS framework for the air interface. The WiMAX architecture specifications extend the QoS framework to the complete network where many alternatives for enforcing the QoS on Layer 2 or Layer 3 may exist.

The end-to-end QoS framework relies on functions implemented in the CSN (PF (Policy Function) and AF (Application Function)) and in the ASN (SFM (Service Flow Management) and SFA (Service Flow Authorisation)). In the CSN, the AF triggers a service flow trigger to the PF based on the information sent by the MS with whom it communicates. The PF then evaluates service requests against a policy database in the NSP. In the ASN, the SFA communicates with the PF and is responsible for evaluating the service request against user QoS profiles. The SFM (located in the BS and responsible for creation, admission, modification and release of 802.16 service flows) mainly consists of an admission control function, which decides, based on available radio resource and other local information, whether a radio link can be created.