Editing Cable modem termination system (section)

==Connections==
In order to provide high speed data services, a cable company will connect its headend to the Internet via very high capacity data links to a [[network service provider]]. On the subscriber side of the headend, the CMTS enables communication with subscribers' [[cable modem]]s. Different CMTSs are capable of serving different cable modem population sizes—ranging from 4,000 cable modems to 150,000 or more, depending in part on traffic and thus number of channels allocated to each service group and the size of the service groups, although it is recommended for an I-CMTS to service, for example, 30,000 subscribers (cable modems).<ref name="Arris_E6000_main"/> A given headend may have between 1–12 CMTSs to service the cable modem population served by that headend or [[Hybrid fibre-coaxial|HFC hub]].

One way to think of a CMTS is to imagine a [[Router (computing)|router]] with [[Ethernet]] [[Network interface controller|interfaces]] (connections) on one side and [[coaxial cable]] [[Radio frequency|RF]] interfaces on the other side. The Ethernet side is known as the Network Side Interface or NSI.<ref name="DOCSIS_3.0_MAC">Data-Over-Cable Service Interface Specifications DOCSIS® 3.0 MAC and Upper Layer Protocols Interface Specification CM-SP-MULPIv3.0-C01-171207</ref><ref>{{cite web | url=https://www.cablelabs.com/specifications/CM-SP-CMTS-NSI | title=Specifications Search }}</ref>

A service group is a group of customers that share communication RF channels and thus bandwidth. A CMTS has separate RF interfaces and connectors for downlink and uplink signals. The RF/coax interfaces carry RF signals to and from coaxial "trunks" connected to subscribers' cable modems, using one pair of connectors per trunk, one for downlink and the other for uplink. In other words, there can be a pair of RF connectors for every service group, although it is possible to configure a network with different numbers of connectors that service a set of service groups, based on the number of downstream and upstream channels the cable modems in every service group use. Every connector has a finite number of channels it can carry, such as 16 channels per downstream connector, and 4 channels per upstream connector, depending on the CMTS.<ref name="Arris_E6000_main">Arris E6000 manual https://fccid.io/ANATEL/01759-14-07236/Manual-E6000/50DAF2B5-F106-42DF-A563-6008357AC079/PDF</ref> For example, if the cable modems on every service group use 24 channels for downstream, and 2 channels for upstream, then 3 downstream connectors can service the cable modems on two service groups, and be serviced by 1 upstream connector.<ref>The Arris E6000 manual https://fccid.io/ANATEL/01759-14-07236/Manual-E6000/50DAF2B5-F106-42DF-A563-6008357AC079/PDF mentions upstream modules with 96 channels divided by 24 ports, and downstream modules with 128 channels divided by 8 ports</ref> A service group may serve up to 500 households. A service group has channels, whose bandwidth is shared among all members of the service group.<ref>{{cite web | url=https://www.sec.gov/Archives/edgar/data/1141107/000095012311058190/g27481exv99w1.htm | title=Exv99w1 }}</ref><ref>A Side-By Side Comparison of Centralized vs. Distributed Access Architectures. Commscope https://www.commscope.com/globalassets/digizuite/1618-arris-centralized-vs-distributed-access-networks-wp.pdf CMTS/CCAP handles service groups directly </ref><ref>HFC Cable Architecture Wade Holmes one optical node per service group https://courses.cs.duke.edu/spring18/compsci356/slides/cable-hfc-intro.pdf</ref><ref>E6000® Converged Edge RouterRelease 6.0 https://www.commscope.com/globalassets/digizuite/61837-e6000-cer-rel-6-0-data-sheet.pdf is a CCAP (CMTS) that can handle 96 upstream and 96 downstream service groups for a total of 192 streams, divided by 12 available slots in the router's image and in https://fccid.io/ANATEL/02605-15-07236/Manual-E6000/166C8E9C-8C13-4F42-B929-31E3DDBB82CA/PDF is 16 streams per slot, every slot has 16 connectors. The router has 14 slots but 2 are reserved, a service group has separate downlink and uplink cables at the CMTS according to diagram in Data-Over-Cable Service Interface Specifications DOCSIS® 3.1 and the Arris E6000 manual where upstream and downstream ports use different connectors Physical Layer Specification and also to make use of 192 connectors in 12 slots with 16 connectors per slot but only 96 downlink and 96 uplink service groups </ref><ref>E6000® Converged Edge Router
Downstream Cable Access Module 2 (DCAM-2) https://www.normann-engineering.com/products/product_pdf/ccap_cmts/arris/e6000-cer-dcam2.pdf has 16 ports for downstream signals, occupies 1 slot in a 14-slot e6000 chassis where 2 slots are reserved for router system modules</ref><ref>https://archive.nanog.org/sites/default/files/08-Noll.pdf mentions "coax feeder cable" instead of trunk but mentions trunk or distribution amplifiers on the feeder which originates from an optical node or just node according to Optical Node Series (NC) NC4000H4 1.2 GHz Fiber Deep Node https://www.commscope.com/globalassets/digizuite/61985-nc4000h4.pdf</ref> The channels are later regrouped at the cable headend or distribution hub and serviced by CMTSs and other equipment such as Edge QAMs.

The RF signals from a CMTS are connected via coaxial cable to headend RF management modules for RF splitting and combining, with other equipment such as other CMTSs so that several CMTS can service one service group,<ref name="Arris_E6000_main"/><ref name="DOCSIS_3.0_MAC"/> and then to an "optics platform" or headend platform, which has transmitter and  receiver modules that turn the RF signals into light pulses for delivery over fiber optics through an HFC network.<ref>Data-Over-Cable Service Interface Specifications DOCSIS® 4.0 Physical Layer Specification CM-SP-PHYv4.0-I06-221019. Cablelabs.</ref><ref>Data-Over-Cable Service Interface Specifications DOCSIS® 3.1 Physical Layer Specification CM-SP-PHYv3.1-I19-211110</ref> Examples of optics platforms are the Arris CH3000 and Cisco Prisma II. At the other end of the network, an optical node converts the light pulses into RF signals again and sends them through a coaxial cable "trunk". The trunk has one or more amplifiers along its length, and on the trunk there are distribution "taps" to which customers' modems are connected via coaxial cable.

In fact, most CMTSs have both Ethernet interfaces (or other more traditional high-speed data interfaces like [[Synchronous optical networking|SONET]]) as well as RF interfaces. In this way, traffic that is coming from the Internet can be routed (or bridged) through the Ethernet interface, through the CMTS and then onto the RF interfaces that are connected to the cable company's '''hybrid fiber coax''' ([[Hybrid fibre-coaxial|HFC]]). The traffic winds its way through the HFC to end up at the cable modem in the subscriber's home. Traffic from a subscriber's home system goes through the cable modem and out to the Internet in the opposite direction.

CMTSs typically carry only [[Internet Protocol|IP]] traffic. Traffic destined for the cable modem from the Internet, known as downstream traffic, is carried in IP packets encapsulated according to [[DOCSIS]] standard. These packets are carried on data streams that are typically modulated onto a TV channel using either 64-QAM or 256-QAM versions of [[quadrature amplitude modulation]].

Upstream data (data from cable modems to the headend or Internet) is carried in Ethernet frames encapsulated inside DOCSIS frames modulated with [[QPSK]], 16-QAM, 32-QAM, 64-QAM or 128-QAM using [[Time-division multiple access|TDMA]], [[ATDMA]] or S-[[CDMA]] frequency sharing mechanisms. This is usually done at the "subband" or "return" portion of the [[North American cable television frequencies|cable TV spectrum]] (also known as the "T" channels), a much lower part of the frequency spectrum than the downstream signal, usually 5–42&nbsp;MHz in [[DOCSIS|DOCSIS 2.0]] or 5–65&nbsp;MHz in EuroDOCSIS.

A typical CMTS allows a subscriber's computer to obtain an [[IP address]] by forwarding [[Dynamic Host Configuration Protocol|DHCP]] requests to the relevant servers. This DHCP server returns, for the most part, what looks like a typical response including an assigned IP address for the computer, gateway/router addresses to use, DNS servers, etc.

The CMTS may also implement some basic filtering to protect against unauthorized users and various attacks. [[Traffic shaping]] is sometimes performed to prioritize application traffic, perhaps based upon subscribed plan or download usage and also to provide guaranteed [[Quality of service]] (QoS) for the cable operator's own [[PacketCable]]-based [[VOIP]] service. However, the function of traffic shaping is more likely done by a Cable Modem or policy traffic switch. A CMTS may also act as a [[Network bridge|bridge]] or [[Router (computing)|router]].

A customer's cable modem cannot communicate directly with other modems on the line. In general, cable modem traffic is routed to other cable modems or to the Internet through a series of CMTSs and traditional routers. However, a route could conceivably pass through a single CMTS.

A '''CCAP''' (Converged Cable Access Platform) combines CMTS and Edge QAM functionality in a single device so that it can provide both data (internet) with CMTS functionality, and video (TV channels) with Edge QAM functionality.<ref>{{Cite web|url=https://www.lightwaveonline.com/network-design/article/16658207/the-evolution-of-ccap|title=StackPath|website=www.lightwaveonline.com|date=13 September 2013 }}</ref><ref>{{Cite web|url=https://www.nctatechnicalpapers.com/Paper/2015/2015-evolution-of-cmts-ccap-architectures/download|title=Wayback Machine|website=www.nctatechnicalpapers.com}}</ref> Edge QAM (Quadrature Amplitude Modulator/Modulation) converts video sent via IP (internet protocol) or otherwise, into a QAM signal for delivery over a cable network. Edge QAMs are normally standalone devices placed at the "edge" of a network. They can also be connected to a CMTS core, to make up an M-CMTS system which is more scalable. A CMTS core is normally a conventional or I-CMTS that supports operation as a CMTS core in an M-CMTS system.<ref>Cisco DOCSIS 3.0 Downstream Solution Design and Implementation Guide Chapter 3: Solution Overview for I-CMTS www.cisco.com/c/en/us/td/docs/cable/cmts/wideband/solution/guide/release_2-0/ds_solu/overvw_icmts.html</ref><ref>Cisco DOCSIS 3.0 Downstream Solution Design and Implementation Guide Chapter 2: Solution Overview for M-CMTS www.cisco.com/c/en/us/td/docs/cable/cmts/wideband/solution/guide/release_2-0/ds_solu/1overvw.html</ref><ref>Data-Over-Cable Service Interface Specifications DOCSIS® 3.0 Physical Layer Specification CM-SP-PHYv3.0-C01-171207</ref>