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The cable data system comprises many
different technologies and standards. For cable modems to be
mainstreamed, modems from different vendors must be interoperable.
Physical Layer
Downstream Data Channel
At the physical layer, downstream data channel is based on North
American digital video specifications (specifically, International
Telecommunications Union [ITU]-T Recommendation J.83 Annex B) and
includes the following features:
- 64 and 256 QAM
- 6 MHz-occupied spectrum that
coexists with other signals in the cable plant
- Variable-length interleaving
support, both latency-sensitive and latency-insensitive data
services
- Contiguous serial bit stream with
no implied framing, providing complete physical (PHY) and
MAC-layer decoupling
Upstream Data Channel
The upstream data channel is a shared channel featuring the
following:
- QPSK and 16 QAM formats
- Data rates from 320 Kbps to 10
Mbps
- Flexible and programmable cable
modem under control of CMTS
- Time-division multiple access
- Support of both fixed-frame and
variable-length protocol data units (PDUs)
Data Link Layer
The data link layer provides the general requirements for many
cable-modem subscribers to share a single upstream data channel for
transmission to the network. Among these requirements are collision
detection and retransmission capability. The large geographic reach
of a cable data network poses special problems as a result of the
transmission delay between users close to headend versus users at a
distance from cable headend. To compensate for cable losses and
delay as a result of distance, the MAC layer performs ranging, by
which each cable modem can assess time delay in transmitting to the
headend. The MAC layer supports timing and synchronization,
bandwidth allocation to cable modems at the control of CMTS, error
detection, handling and error recovery, and procedures for
registering new cable modems.
Network Layer
Cable data networks use IP for communication from the cable modem to
the network. The Internet Engineering Task Force (IETF) DHCP forms
the basis for all IP address assignment and administration in the
cable network.
Transport Layer
Cable data networks support both the Transmission Control Protocol
(TCP) and the User Datagram Protocol (UDP) at the transport layer.
Application Layer
All of the Internet-related applications are supported here. These
applications include HTTP, FTP, e-mail, Trivial File Transfer
Protocol (TFTP), news, chat, and Simple Network Management Protocol
(SNMP). The use of SNMP provides for management of the CMTS and
cable data networks.
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