276 ACCESS NETWORKS
◦ FC PARM: A 5-bit field that contains parameters dependent on the value of the FC TYPE.
◦ EHDR ON: A 1-bit field indicating whether or not an extended header is present. •
MAC-PARM : This 1-byte field contains parameters whose use depend on the value of
the FC field. •
LEN SID : A 2-byte field that gives the length of the extended header if present, plus
the number of bytes that follow after the HCS field. In certain cases, this field is used to carry the value of a SID see Section 11.2.4.
• Extended header EHDR
: This is an optional variable-size extended header. •
Header check sequence HCS : The integrity of the MAC header is ensured by a CRC.
The HCS is a 2-byte field that contains the FCS, which is obtained using the pattern x
16
+ x
12
+ x
5
+ 1. The HCS covers the entire header i.e. it starts from the FC field
and includes the extended header. As stated above, the following MAC frame formats are possible: MAC header with
packet PDU, MAC header with ATM cells, reserved MAC header for future PDU types, and MAC header used for specific control functions.
The MAC header with packet PDU is used to transport Ethernet packets. In this case, the MAC header is followed by a data PDU with the following fields:
• Destination address DA
: A 48-bit field populated with the destination address. •
Source address SA : A 48-bit field populated with the source address.
• Typelen
: A 16-bit Ethernet type length field. •
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 modem’s 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 277
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 mini-
slots are to be used. As can be seen in Figure 11.16, the CMTS issues the first MAP message at time t
1
. The MAP message propagates through the cable network and it arrives at the CMs at time t
2
. For simplicity, we assumed that all of the CMs are at the same distance from the CMTS. The MAP describes how a group of n
1
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 t
3
. 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 t
4
and it arrives to the CMs at time t
5
. This MAP message describes how the second batch of n
2
contiguous mini-slots that the CMTS expects to start receiving at time t
6
should be used. The third MAP message is issued at time t
7
and it describes how the third group of n
3
mini-slots should be used. The first time slot of this group is expected to arrive at the CMTS at time t
9
. The group of mini-slots mapped in a MAP message is divided into intervals of con-
secutive 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:
CMTS
CMs MAP 1
t
1
t
3
t
4
t
6
t
7
t
9
t
8
t
5
t
2
Mini-slots mapped by MAP 1 MAP 2
MAP 2 MAP 3
MAP 3
Figure 11.16 An example of MAP messages.
278 ACCESS NETWORKS
• 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 requestdata interval and the short or long data interval, described below.
• The requestdata IE
: Provides an interval of mini-slots, in which either requests for bandwidth or short data PDUs can be transmitted on a contention basis. Since trans-
missions might collide, the CMTS must acknowledge any data PDUs that it received correctly. Note that this IE is different from the request IE.
• The initial maintenance IE
: The IE provides an interval of mini-slots, during which new CMs can join the network. The transmission during these mini-slots is contention-based.
• The station maintenance IE
: Defines an interval of mini-slots, during which CMs are expected to perform some aspect of routine maintenance, such as ranging or power
adjustment. The transmission during these mini-slots is contention-based. Alternatively, the CMTS can specify a particular CM to perform a network maintenance task.
• The short and long data grants IEs
: Define an interval of mini-slots, during which CMs can transmit short data PDUs and long data PDUs, respectively. These IEs are issued
either in response to bandwidth requests, or to an administrative policy to provide bandwidth to some CMs. A data grant cannot be larger than 255 mini-slots.
• Data acknowledge IE
: Used by the CMTS to acknowledge that a data PDU has been received. This IE is used only in conjunction with contention transmission. It was not
designed to provide a reliable transport between CMs and CMTS. A CM must request a number of mini-slots in order to transmit an entire frame. This
frame might be: a single MAC frame; a MAC frame that has been formed by concatenating multiple MAC frames; or a MAC frame containing a fragment of a long data PDU. The
request must be large enough in order to accommodate the MAC frame and the necessary physical layer overheads. A CM cannot request more than the required number of mini-
slots necessary to transmit one MAC frame, and it can only have one request outstanding at a time.
In Figure 11.17, we give an example of the upstream transmission scheme between the CMTS and a CM. The following events take place:
CMTS
CM MAP
t
1
t
2
t
4
t
8
t
10
t
3
t
5
t
6
Slots mapped by first MAP Second MAP
Request t
7
t
9
t
11
MAP Data PDU
Figure 11.17 An example of the upstream transmission scheme.
THE CABLE-BASED ACCESS NETWORK 279
