DOCSIS CM-SG

Cable Modem Service Group (CM-SG)：

A "Cable Modem Service Group" (CM-SG) is formally defined as the complete set of CMTS channels—both
upstream and downstream—that reach a single cable modem.

In an HFC deployment, all CMs reached by the same
fiber node are reached by the same set of channels.

Furthermore, in most HFC deployments, each fiber node has a
different set of either upstream or downstream channels that reach it. Thus, a CM-SG usually corresponds to the
channels reaching a single fiber node, and the term "CM-SG" can generally be considered to be synonymous with
"fiber node".

 

DS-SG：

A "Downstream Service Group" (DS-SG) is formally defined as the complete set of CMTS downstream channels
that may be received by a single CM.

A CM is reached by a single Downstream Service Group. A DS-SG represents
a unique combination of DOCSIS Downstream RF Channels, each operating at a different center frequency. A DSSG
may be combined in the electrical domain and then be electrically and/or optically split to multiple fiber nodes.
A DS-SG is a set of channels defined by the topology configuration of the CMTS, and is independent of the MAC
Domain configuration.

US-SG：

An "Upstream Service Group" (US-SG) is formally defined as the complete set of upstream channels in a CMTS
that may receive the transmissions of a single CM.

A US-SG is a physical-layer concept; it is defined only by the
physical combining of the upstream RF transmission from CMs. If the upstream fiber signals from different fiber
nodes are not combined, each fiber node usually corresponds to a single US-SG.

 

在C-DOCSIS解决方案中，CMC设备移到传统HFC网络的光节点处，C-DOCSIS网络不经过光纤分配网。

所以，DOCSIS协议中的光节点(Fiber Node)已不在C-DOCSIS的范畴中，从而服务组(Service Group)相关概念可以得到简化。

CMC所有的上行和下行信道构成一个服务组，连接该CMC同轴网络的所有CM可以访问这个服务组的所有上行和下行信道，也就是说这些信道资源对该同轴网络的CM都是“可达”的。

同时，CMC的所有上行和下行信道包含在一个MAC Domain中。
所以在C-DOCSIS中，服务组定义SG 、CM-SG 以及MD-CM-SG都是一样的，

下行信道服务组定义DS-SG与MD-DS-SG是一样的，

上行信道服务组US-SG与MD-US-SG是一样的。