WiMAX MAC Protocol Inside


This page of WiMAX tutorial covers MAC protocol used between base station and subscriber station compliant to fixed wimax IEEE 802.16-2004 PMP profile.

This article describes wimax MAC layer protocols as per IEEE 802.16-2004 or 16d OFDM standard, its functions mainly for schedule management and resource allocation. This article also helps in the design of wimax physical layer as wimax MAC layer plays very important role to determine modulation-code rate and other useful physical layer parameters. It covers wimax MAC layers viz. convergence sub layer, MAC common part sublayer and security sublayer.


Before we start analyzing wimax MAC layer let us summarize protocol layers at L1 and L2 of fixed WiMAX. As mentioned below MAC layer comprises of three sub layers namely convergence sub layer, MAC common part sub layer and security sub layer.

WiMAX protocol stack

The wimax MAC layer comprises three sub layers:
1. Convergence sub layer: Interfaces layer above i.e. network layer and layer below i.e. MAC CPS. This includes classifying external network service data units (SDUs) and associating them to the proper MAC service flow identifier (SFID) and connection identifier (CID). Refer CID and SFID for detailed information. Hence it takes care of delivery of CS PDU to the MAC SAP associated with the service flow for transport to the peer MAC SAP and receipt of the CS PDU from the peer MAC SAP. The optional feature supported by this layer is Payload header compression.
2. MAC common part sub layer: The core MAC functionality is performed by this sub layer. It takes care of access mechanism of SS with BS, Bandwidth allocation, connection establishment and connection maintenance. This is explained in detail in this article.
3. Security sub layer: It takes care of security aspects viz. authentication, secure key exchange and encryption.

WiMAX frame structure

To understand MAC CPS we need to understand WiMAX frame structure which is mentioned below. For the discussion point of view let us assume following system parameters and we will assume that number of subscriber Station (SS) is one. Let us assume that WiMAX system BW is 3.5MHz hence we will have sampling frequency of 4MHz. Let us assume System Frame Duration of 10ms and DL/UL Ratio of 56%, hence we will have total symbols in a WiMAX Frame equal to 125, No. of symbols in DL sub frame 70 and No. of Symbols in UL sub frame 55.

As mentioned in the frame, downlink sub frame is transmitted by Base station (BS) to Subscriber stations (SSs) and Uplink sub frame is transmitted by SSs to BS. TTG and RTG are the turnaround times to allow electronic circuitry in Base station and Subscriber stations to settle down in switching from Tx to Rx and vice versa to avoid loss of the data. Let us understand how the scheduling concept is mentioned in the standard. The standard defines DLFP which is 88 bits long and is always BPSK which is transmitted after L.P. (Long preamble) of 2 symbols. It carries Length of the Downlink Bursts and it is in unit of OFDM symbols.
DLFP=(..., Rate_ID, Length_B1, DIUC, Length_B2, DIUC, Length_B3, DIUC, Length_B4....)
DLFP is decoded by SS based on known modulation-code rate, which is always BPSK1/2. DLFP's extracted parameters are required as scrambler seed. Once SS decodes this it will come to know about Downlink B1 which carries system critical messages namely DLMAP, ULMAP, UCD and DCD as described in the 16d standard. Refer DLMAP vs ULMAP and UCD,UIUC,DCD,DIUC for more.

ULMAP structure with important fields mentioned below.
ULMAP= (..., Allocation start time,...start_time_B1,...duration_B1,...Start_time_B2,...duration_B2,...)
Allocation start time field, which defines starting point of uplink sub frame which is in units of Physical slots (PS), and mentions after how many PSs starting from DL Sub frame, uplink allocation is going to start. Physical slot is the basic unit of resource allocation is fixed WiMAX and it is 4/Fs. For ranging standard defines fixed location after TTG but Bandwidth request can be sent anywhere in the uplink sub frame except region dedicated for ranging request. ULMAP which is transmitted by BS contains start_time_field by which SS will come to know when in the frame they have to initiate the transmission. SS will transmit the burst appending Short preamble (S.P.) as defined in the standard.

As modulation-code rate for SS can vary in the system, request for bandwidth is designed in units of bytes. The Bandwidth Request PDU consists of header alone, which consist of field BR, which defines number of bytes requested by SS. Each MAC PDU starts with Generic MAC Header (GMH) which is 6 bytes long and consists of mainly following fields. As mentioned each MAC messages will have type field by which MAC layer will decide what action need to be taken with the received MAC PDU on both the sides(BS and SS)
GMH = (Header type,...., MAC msg type,..., CID, HCS)

There are two major fields UCD and DCD which is called uplink channel descriptor and downlink channel descriptor respectively. These fields in conjunction with UIUC and DIUC will decide modulation-code rates of the downlink and uplink bursts. As in all the standards and wireless systems, before actual conversation (voice) or data transfer starts there will be exchange of messages between requesting system (Here SS) and serving system (Here BS). Similarly in WiMAX following messages are exchanged between BS and SS before internet connection is established.

WiMAX MAC Layer Messages

Following figure covers various MAC layer messages exchanged between BS and SS which include Broadcast message(DLMAP,ULMAP,UCD and DCD),Ranging request(RNG REQ),Bandwidth request(BW REQ),DSA REQ and DSA ACK. The MAC layer is implementation specific, here MAC layer is implementated into two parts lower MAC layer and upper MAC layer. Lower MAC layer does CRC calculation and HCS calculation for GMH(Generic MAC Header). Upper MAC layer does scheculing and takes care of MAC layer messages.

Following table describes different WiMAX MAC management messages and their type number, message description and connection type.

MAC Management Message Type Message Name MAC Message Description Connection
0 UCD Uplink Channel Descriptor Broadcast
1 DCD Downlink Channel Descriptor Broadcast
2 DLMAP Dwonlink Access Definition Broadcast
3 ULMAP Uplink Access Definition Broadcast
4 RNG REQ Ranging Request Initial Ranging
5 RNG RSP Ranging Response Initial ranging
6 REG REQ Registration request Primary management
7 REG RSP Registration response Primary management
8 - Reserved -
9 PKM REQ Privacy Key Management Request Primary Management
10 PKM RSP Privacy Key Management Response Primary management
11 DSA REQ Dynamic Service Addition Request Primary management
12 DSA RSP Dynamic Service Addition Response Primary management
13 DSA ACK Dynamic Service Addition Acknowledge Primary management
14 DSC REQ Dynamic Service Change Request Primary management
15 DSC RSP Dynamic Service Change Response Primary management
16 DSC ACK Dynamic Service Change Acknowledge Primary management
17 DSD REQ Dynamic Service Deletion Request Primary management
18 DSD RSP Dynamic Service Deletion Response Primary management
19 - reserved -
20 - reserved -
21 MCA REQ Multicast Assignment Request Primary management
22 MCA RSP Multicast Assignment Response Primary management
23 DBPC REQ Downlink Burst Profile Change REquest Basic
24 DBPC RSP Downlink Burst Profile Change REsponse Basic
25 RES CMD Reset Command Basic
26 SBC REQ SS Basic Capability request Basic
27 SBC RSP SS Basic Capability response Basic
28 CLK CMP SS network Clock Comparison Broadcast
29 DREG CMD De/Re-register command Basic
30 DSX-RVD DSx received message Primary management
31 TFTP CPLT Config File TFTP complete message Primary management
32 TFTP RSP Config File TFTP complete rseponse Primary management
33 ARQ Feedback Standalone ARQ feedback Basic
34 ARQ Discard ARQ Discard Message Basic
35 ARQ Reset ARQ Reset Message Basic
36 REP REQ Channel Measurement Report Request Basic
37 REP RSP Channel Measurement Report Response Basic
38 FPC Fast Power Control Broadcast
39 MSH NCFG Mesh Network Configuration Broadcast
40 MSH NENT Mesh Network Entry Basic
41 MSH DSCH Mesh Distributed Schedule Broadcast
42 MSH CSCH Mesh Centralized Schedule Broadcast
43 MSH CSCF Mesh Centralized Schedule Configuration Broadcast
44 AAS FBCK REQ AAS Feedback Request Basic
45 AAS FBCK RSP AAS Feedback Response Basic
46 AAS Beam Select AAS Beam Select Message Basic
47 AAS_Beam_REQ AAS Beam Request message Basic
48 AAS_Beam_RSP AAS Beam Response message Basic
49 DREG_REQ SS De-registration message Basic
50-255 - Reserved -

MAC message flow between BS and SS before data transfer takes place:

1. Broadcast message-DLMAP,ULMAP,UCD and DCD (BS to SS)

2. RNG REQ (SS to BS),RNG RSP (BS to SS), Refer REG REQ and REG RSP page and WiMAX Ranging Procedure for more.

3. BW REQ (SS to BS),SBC REQ (SS to BS),SBC RSP(BS to SS), Refer WiMAX Bandwidth Request procedure for more.

4. BW REQ (SS to BS),REG REQ (SS to BS),REG RSP (BS to SS)

One can refer REG REQ and REG RSP and SBC REQ and SBC RSP MAC management messages for details on fields carried by them.

WiMAX MAC layer messages

At this stage network entry for SS is completed and now it will establish service flow. For Service flow there are two methods, BS initiated and SS initiated. We will talk about BS initiated Service flow for both downlink and uplink connection.
5. DSA REQ for downlink (BS to SS),DSA RSP for downlink (SS to BS)
6. DSA ACK for downlink (BS to SS),DSA REQ for uplink (BS to SS)
7. DSA RSP for uplink (SS to BS),DSA ACK for uplink (BS to SS)
At this stage connection is established and both the sides can transfer the data to each other i.e. Network layer can start communicating thorough MAC-PHY.
Also refer WiMAX Network Entry Procedure for some more insight into wimax subscriber station's camping on the wimax compliant BS.

Links to WiMAX MAC Layer resources

WiMAX Ranging Procedure
WiMAX Bandwidth Request procedure
WiMAX Network Entry Procedure
WiMAX FCH header format
WiMAX TLV Format basics
WiMAX Timers
WiMAX Network Interface types
WiMAX QoS Classes

Frame structure of wireless standards, technologies

Frame structure of various wireless standards/technologies are mentioned below. It include WiMAX, WLAN, Zigbee, GSM, GPRS, UMTS, LTE, TD-SCDMA, GPS, SDH, 11ac WLAN, AMPS, Ethernet, VLAN etc.

WiMAX physical layer Frame Structure as per 802.16d and 802.16e standards
WiMAX MAC layer Frame Structure as per OFDM 802.16d standard
Zigbee RF4CE Frame Structure
Zigbee physical layer Frame Structure
Zigbee MAC layer Frame Structure
GPRS Frame Structure
GPS Frame Structure
LTE Frame Structure
TD-SCDMA Frame Structure
UMTS Frame Structure
SONET Frame Structure
SDH Frame Structure
802.11ac PHY Frame Structure
802.11ac MAC layer Frame Structure
WLAN Frame Structure as per 802.11a,11b,11n,11ac standards
AMPS Frame Structure
Ethernet Frame Structure
VLAN Frame Structure
GSM Frame Structure

Other Standard MAC layer protocol


RF and Wireless Terminologies