Tuesday, August 31, 2010

HandOver (HO)

IEEE 802.16e-2005 standard defines a framework for tracking subscriber stations as they move from the coverage range of one base station to another when active or as they move from one paging group to another when idle. The architecture also supports IP-layer mobility using mobile IP.

Three handoff methods are supported in IEEE 802.16e-2005:
1. hard handover (HHO) - Mandatory
2. fast base station switching (FBSS) - Optional
3. macro diversity handover (MDHO) - Optional

Mobility

Scanning: This is a process in which, BS allocates time for each MS to monitor and measure the radio condition of the neighboring BSs. and the time allocated to each MS is called the scanning interval. Each scanning intervalis followed by an interval of normal operation, referred to as the interleaving interval

In order to start the scanning process, the BS issues a MOB_SCN-REQ message that specifies to the MS the length of each scanning interval, the length the of interleaving interval, and the number of scanning events the MS is required to execute

The identity of neighboring BSs and the frequencies that a MS is required to scan are provided in the MOB_NBR-ADV message sent over the broadcast channel

During a scanning interval, the MS measures the received signal strength indicator (RSSI) and the signal-to-noise-plus noise ratio (SINR) of the neighboring BS and it sends MOB_SCN_REP message to serving BS

Power Saving Feature

1. Sleep Mode: Sleep mode is a state in which the MS effectively turns itself off and becomes unavailable for predetermined periods(sleep window-listen window). The periods of absence are negotiated with the serving BS. WiMAX defines three power-saving classes,

Power saving class-I: in Class-I the sleep window is exponentially increased from a minimum value to a maximum value. This is typically done when the MS is doing best-effort(web browsing HTTP) and non-real-time traffic(FTP)
Power saving class-II: Power Save Class 2 has a fixed-length sleep window and is used for UGS service (VOIP)
Power saving class-III: Power Save Class 3 allows for a one-time sleep window and is typically used for multicast traffic or management traffic when the MS knows when the next traffic is expected

To facilitate handoff while in sleep mode, the MS is allowed to scan other base stations to collect handoff-related information.

2. Idle Mode: Idle mode allows even greater power savings, and support for it is optional in WiMAX. Idle mode allows the MS to completely turn off and to not be registered with any BS and yet receive downlink broadcast traffic.

The MS is assigned to a paging group by the BS before going into idle mode, and the MS periodically wakes up to update its paging group. Idle mode saves more power than sleep mode, since the MS does not even have to register or do handoffs

Note: Active connections will be there during Sleep mode, where as there wont be any active connections during Idle mode

CID

Initial Ranging: 0x0000

Basic CID: 1 - no of m (m is no of MS)
for short and very urgnet msgs like HO, RNG_REQ/RSP, SBC_REQ/RSP ARQ_RESET/.. etc.. BCID messages should not be fragmented or packed

Primary CID: m+1 - 2m (for longer and less urgent (delay tolerent) msgs)
REG_REQ/RSP, DSA_xxx etc...

Secondary CID: The secondary management connection is used by the BS and SS to transfer delay tolerant, standards-based messages. These standards are the Dynamic Host Configuraation Protocol (DHCP), Trivial File Transfer Protocol (TFTP), Simple Network Manageement Protocol (SNMP), etc. The secondary management messages are carried in IP datagrams

Transport CID (TCID): 2m+1 - 0xFEFE
for each direction UL/DL per service flow, 1 SFID will be associated for specific QOS requirement. so if there are active connections per MS, then there will be 8 SFIDs and CIDs in UL and 8 SFIDs and CIDs in DL will be allocated

Broadcast CID: 0xFFFF
DLMAP, ULMAP, DCD, UCD etc... MAC msg should not be fragmented on this CID