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User data: A variable-length field that contains user data of up to 1500 bytes.
CRC: This 32-bit field contains the FCS obtained using the Ethernet-prescribed CRC used to protect the data PDU.
The MAC header with ATM cells is to be defined in the future. The following MAC headers are used for specific control functions:
MAC header for timing: Used in the downstream direction to transport the global timing reference, to which all of the cable modems synchronize. In the upstream direction, it is used as part of the ranging message needed for the cable modems timing and power adjustment.
MAC header for requesting bandwidth: This header is the basic mechanism that a cable modem uses to request bandwidth. It is only used in the upstream direction. There is no data PDU following this MAC header. Because there is no data PDU, the LEN field is not required and is replaced with a SID. This is the SID allocated to a CM by the CMTS for the upstream service flow.
MAC management header: Used to transport all management messages.
MAC header for fragmentation: In the upstream direction, a large data PDU can be split into a number of smaller pieces, and each piece can be transmitted individually. The pieces are then reassembled into the original data PDU at the CMTS. This MAC header provides the mechanism to do this fragmentation.
MAC header for concatenation: Used to transport several MAC frames (MAC header plus optional data PDU) in a single transmission.
THE CABLE-BASED ACCESS NETWORK
11.2.3 The DOCSIS MAC Protocol Operation
As mentioned above, the upstream channel is divided into mini-slots. The access to the mini-slots by the CMs is controlled by the CMTS. This is done through a MAC management message known as the MAP management message (see Figure 11.16). The CMTS issues continuously MAP messages to describe how groups of contiguous minislots are to be used. As can be seen in Figure 11.16, the CMTS issues the first MAP message at time t1. The MAP message propagates through the cable network and it arrives at the CMs at time t2. (For simplicity, we assumed that all of the CMs are at the same distance from the CMTS). The MAP describes how a group of n1 contiguous mini-slots are to be used by the CMs and how the CMs should access these mini-slots. The first time slot of this group of mini-slots is expected to arrive at the MCTS at time t3. Since it takes some delay before the CMs receive a MAP message and process it, the second MAP message is issued by the CMTS at time t4 and it arrives to the CMs at time t5. This MAP message describes how the second batch of n2 contiguous mini-slots that the CMTS expects to start receiving at time t6 should be used. The third MAP message is issued at time t7 and it describes how the third group of n3 mini-slots should be used. The first time slot of this group is expected to arrive at the CMTS at time t9.
The group of mini-slots mapped in a MAP message is divided into intervals of consecutive mini-slots, referred to as intervals. Each of these intervals is designated by the CMTS for different type of use. For instance, one interval can be used by the CMs to transmit bandwidth requests, another interval can be used by the CMs to transmit data PDUs, and a third interval can be used by new CMs to join the access network.
Within an interval of mini-slots, the CMs can start transmitting at the beginning of any mini-slot, with the possibility that their transmissions might collide. A contention resolution algorithm based on the exponential back-off algorithm is used to decide when the CMs that collided can re-transmit again. Alternatively, within an interval each CM transmits over a number of consecutive mini-slots allocated to it by the CMTS without collision. The allocation of mini-slots to CMs is done based on requests for bandwidth that the CMTS receives from the CMs. The CMs send bandwidth requests to the CMTS using the MAC header for requesting bandwidth described above. The CMTS allocates contiguous time slots to the requesting CMs using a scheduling algorithm, which is outside the scope of DOCSIS.
The different intervals and how they can be accessed by the CMs are described in the MAP management message by different information elements (IE). Each information element describes a particular type of interval. The following are some of the supported information elements:
Mini-slots mapped by MAP 1 MAP 2
Figure 11.16 An example of MAP messages.
The request IE: Specifies an interval of contiguous mini-slots, during which the CMs can send bandwidth requests to the CMTS. The transmission during these mini-slots is contention-based, so that multiple CMs might potentially transmit at the same time. A contention-resolution algorithm is used to recover from collisions. Alternatively, the CMTS can specify a particular CM to transmit its bandwidth request, in which case there will be no collisions. (Requests for bandwidth can also be transmitted during other intervals, such as the request/data interval and the short or long data interval, described below.)