Elements of 802.16 - 2 [Under WiMAX]
» MAC connections
MAC is supposed to provide transport for IP/Ethernet packets, ATM cells etc. This transport is managed in terms of "connections" having identifier called (16-bit) CID. Though called connections, it is more of "context" that is mapped to CID. SSs are identified by 32-bit universal MAC address. 802.16 specify two types of connections: Management and Transport as shown below:
Management connections carry Control or Management messages. Transport connections is to carry Data (coming from MAC user). Note that MAC PDU has common format. It can contain management messages or data or both. MAC header(s) contain the information about "payload" that is being carried. MAC is capable of data fragmentation (data fragmented and sent in separate PDUs) and packing (different data being small packed together and sent in one PDU). Another important information that MAC header contain is CID.
Management connections are created for MS during network entry process. MAC address is used for initial identification and during further network entry process, management CIDs are given.
There are three types of management connections:
Basic - Created when MS joins the network and used for short & urgent MAC messages.
Primary - Though created during network entry time, it is used for long and delay tolerant messages.
Secondary - This connection "may be" created once MS joins the network. It is used for delay tolerant messages like DHCP, SNMP etc.
MS network entry process consists of adjustment of timing, power and other parameters [Ranging], negotiation of basic capabilities, authentication [Key exchange], registration, and then optional (IP/ATM) connectivity.
Ranging is pivotal process for 802.16 access. As the name suggests, ranging process is about estimating distance between MS and BS antenna. Ranging process help MS to adjust power/timing parameters to maintain quality of the radio connection. UL subframe has ranging regions (bursts) which are supposed to be used by MSs for ranging purposes. Ranging messages are the first messages used by MS to talk to BS.
Transport connections can be requested by MS or created by BS. Typically, based on subscription, BS will create these connections/service flows after network entry process.
To cater to various types of data traffic characteristics (QoS), 802.16 has a concept of "Service Flow".
» Service Flow
Service flow concept is similar to PDP Context in GPRS. Service flow has associated QoS parameter set and can be activated for a connection. It is identified by (32-bit) SFID. Once activated, it will have corresponding CID association. One CID can have only one service flow and vice versa. Though with the certain combinations of QoS parameters required service can be realised, 802.16 specify five types of scheduling mechanisms or services.
| UGS |
Unsolicited Grant Service In UGS, BS does fixed periodic allocation to MS. UGS is meant for constant bit rate, real time applications like Voice call or VoIP without silent suppression. |
| rtPS |
Real Time Polling Service "Polling" in the name suggest that MS is to be polled. In rtPS, BS provide "periodic" (unicast) opportunities for (individual) MS to request required allocation. rtPS is meant for real time and variable packet length traffic like streaming audio/video (MPEG coded). |
| nrtPS |
Non Real Time Polling Service In nrtPS, BS provide "regular" (unicast) opportunities for (individual) MS to request required allocation. nrtPS is meant for delay tolerant and variable packet length traffic like file transfer (FTP). |
| BE |
Best Effort service This is possibly default service with no specific constraints and so called Best Effort. MS need to go for contention request opportunities to receive an allocation. It is meant for bursty, non real time traffic like web browsing. |
| ErtPS |
Extended Real Time Polling Service This was introduced in 802.16e-2005. This is similar to UGS except that allocations are dynamic like rtPS; BS does variable allocations periodically and so MS need not request allocations. It is meant for real time traffic like VoIP with silence suppression. Possibly similar to AMR in UMTS. |
BS has to employ appropriate scheduling algorithms to take care of QoSs of active service flows.
With this as background, in next article, we will sum up our understanding of how overall 802.16 work.
References: WiMAX Handbook (edited) by Ahson and Ilyas, Fundamentals of WiMAX by Andrews, Ghosh, and Muhamed, WiMAX forum web site, Latest 802.16-2009 (covering amendments 802.16e-2005, 802.16-2004/Cor1-2005, 802.16f-2005 & Std 802.16g-2007 made on 802.16-2004).
| Copyright © Samir Amberkar 2010 | § § |
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