CM-SP-PHYv3.0

CM-SP-PHYv3.0 This specification deals with physical layer aspects of the technology. 5 42 MHz or 85 MHz frequency band is used. Operating DOCSIS 3.0 frequencies varies between implementations. The physical media portion of the document defines the electrical characteristics and signal processing operations used between cable modems and CMTS(s) installed at the data center. With regard to gaining access to the physical network DOCSIS 3.0 defines two primary methods: time division multiple access (TDMA) and SCDMA (synchronous code divisionmultiple access). The modulation of IPTV data on to these channels are achieved through the use of a wide variety of modulation schemes. The modulation scheme is dependent on the access method and the direction of the network traffic, namely,upstream or downstream. These schemes deal with the hostilities that are often present on a HFC network when used to deliver real-time data traffic such as IPTV.
Examining the DOCSIS 3.0 MAC Layer DOCSIS 3.0 introduces a number of features that build upon what was present in previous versions of DOCSIS. These enhancements are detailed in the CM-SP-MULPIv3.0 document. These features are explored in the following subsections.
DOCSIS 3.0 has defined new ways of utilizing the MAC frame in order to support some of the new features incorporated in to the technology. A frame is the basic unit of transfer used for communication at the MAC layer between the CMTS and cable modems. The length of the frame is variable and consists of two parts a header and the video payload. The structure of a DOCSIS 3.0 packet header. 

Note that a DOCSIS 3.0 packet also includes overhead details that are used by the upper layers of the protocol stack.

DOCSIS 3.0 MAC Layer Field Descriptions Field Name Size (Byte) Description of Functionality FC 1 Short for frame control, this field identifies the header type and identifies whether an extended header (EHDR) is present or not.
MAC_PARM 1 When an EHDR sequence field is present, this provides the EHDR length.
LEN (SID) 2 This defines the length of the frame.
EHDR 0 240 The extended MAC header is used to support a number of functions, including the tagging of Multicast IPTV packets and the sequencing of out-of-order packets. HCS 2 HCS stands for header check sequence and is used toidentify transmission errors that affect the header.
The ability of DOCSIS 3.0 devices to use a technique calledChannel Bonding.
Channel Bonding to increase the data throughput over a HFC network allows cable operators implement IPTV services. Under this mechanism, multiple smaller channels are bonded together to create a larger logical channel with high bandwidth capabilities. In addition to providing greater throughput when compared to a single channel, this mechanism also reduces the congestion delays associated with sending packets over a single channel. DOCSIS cable modems include multiple tuners,which are used to access the various channels available as part of the group ofbonded channels.
The mechanism operates by concurrently distributing IPTV packets across anumber of channels, which gets delivered to a cable modem. The distribution packets across multiple channels can cause difficulties for the cable modem as some of the channels suffer from jitter and latency. If this is the case the packets will arrive out of order. To mitigate against such problems, DOCSIS 3.0 introduces amethod of tagging or marking each packet with a particular sequence number,which is used by the cable modem to reassemble the original IPTV stream before forwarding onwards to an IP set-top box. The packet sequence number is 2 bytes in length and in the case of downstream traffic is included in the DS-EHDR fieldof the MAC header.
Overall management and allocation of bonded channels is the responsibility of the CMTS. The channel bonding mechanism can be used for transmitting data both upstream and downstream across a cable TV network.

DOCSIS 3.0 builds on the load balancing mechanisms used by LOAD BALANCING previous versions of the technology. Load balancing is a critical feature of large cable TV networks that support hundreds and in some cases thousands of cable modems. The load balancing section of the DOCSIS 3.0 specification makes provisions to cope with the large volumes of traffic that are generated by multimedia applications such as IPTV.
DOCSIS 3.0 allows cable operators to u se IP multicasting as a method of delivering IPTV sessions and services to their subscribers. The use of multicasting is particularly attractive to cable TV providers because it reduces the amount of network ban dwidth required to deliver a multichannel pay TV service.
To facilitate the use of multicasting DOCSIS 3.0 features an identifier called a multicast downstream service ID (MDSID). The MDSID allows cable modems that have been authorized to view a particular IPTV multicast stream to efficiently identify packets associated with the stream. It is 20 bits in length and is added by the CMTS to the packet header. Applying a MDSID to various packets allows cable modems to process only multicast and IP-VoD streams that are intended for transmission onwards to an IPTVCD connected to its home networking port. In addition to tagging network traffic and bonding multiple channels together DOCSIS 3.0 also accommodates support for multicast specific communications models and protocols such as:
. Source Specific Multicast (SSM)
. Version 3 of the Internet Group Membership Protocol (IGMP)
. Multicast Listener Discovery (MLD) versions 1 and 2 Chapter 4 presents further details on these technologies.