What is a CMTS?
A CMTS, or connection management switch, is a network device that handles a wide variety of Internet traffic. This device is responsible for controlling bandwidth by providing basic filtering, which is a necessary step in order to protect your network from various attacks. It can also perform traffic shaping to prioritize application traffic. For example, traffic shaping may be based on a subscribed plan, the size of a download, or the quality of a PacketCable-based VOIP service. The CMTS can also function as a bridge, if necessary.
A DOCSIS-compliant CMTS uses a scheduler to allocate transmission time to pending grants. Using a DOCSIS-compliant CMTS, a provider can avoid accidental oversubscription. In addition, the scheduler will ensure that the total upstream utilization is not more than 75% during peak hours.
A DOCSIS-compliant CMTS performs a variety of tasks to optimize the service. For example, a DOCSIS-compliant CMTS will enable a customer’s router to dynamically obtain an IP address from the cable company. It will also act as a proxy that forwards requests to DHCP servers. Additionally, a DOCSIS-compliant CMTS may perform filtering functions to prevent service theft. It may also provide traffic shaping, quality of service, and bridging capabilities.
A DOCSIS-compliant CMTS can also provide different upstream scheduling modes to meet the needs of different applications and packet streams. Each service flow has its own service flow ID and can have its own set of quality of service parameters, including maximum throughput, minimum guaranteed throughput, priority, and so on. A DOCSIS-compliant CMTP server can also define a default scheduling mode for each service flow.
A DOCSIS-compliant CMTS can also enforce fragmentation. A DOCSIS compliant CMTS will place grants into a sequence based on their priority, which can range from zero to seven. It will also force cable modems to fragment large frames.
A DOCSIS-compliant CMTS can also reduce jitter, which is a signaling phenomenon caused by a high number of simultaneous calls. Using a DOCSIS-compliant CMTS allows service providers to ensure that VoIP quality is not affected by oversubscription by preventing the creation of new UGS service flows.
A DOCSIS-compliant CMTS supports multiple protocols for billing and metering purposes. The interfaces used for billing will have a ccmtr-collected-type, which specifies whether the CMTS collects data locally or externally.
The CMTS will then broadcast an MAP message containing the grant for modem B and no instructions to the other modems. This enables the modems to realize that multiple bandwidth requests require more upstream time than they were granted. The CMTS will then grant the additional transmission time, based on the availability of bandwidth.
A DOCSIS-compliant CMTS will also support a CMTS centric subscriber management MIB. The CMTS centric subscriber management MIB will provide a framework for the Cable Modem Termination operator to control IP addresses and protocols. The OID repository is updated regularly, so it is a good idea to check for updated descriptions before implementing a CMTS.
DOCSIS-compliant CMTS software can also be virtualised and deployed on COTS servers. This virtualised CMTS software is cloud-native and supports multiple deployment models, including hybrid deployments. Furthermore, the vCMTS software supports 10G PON and features to improve network efficiency.
DOCSIS-compliant CMTSs support advanced time division multiple access. This advanced technique separates upstream and downstream traffic. This technique allows cable modems to transmit small bursts of data during bandwidth requests. But cable modems must maintain a low initial ranging opportunity.
A DOCSIS-compliant CMTS can also support UGS service flow. During a VoIP call, cable modems that support integrated VoIP ports can ask their CMTS to create an appropriate UGS service flow. The CMTS will then provide the required upstream bandwidth to complete the call.
DOCSIS-compliant HFC CMTS
The DOCSIS standard has helped cable operators develop HFC technology. This type of cable modem network consists of an infrastructure of coaxial cables, optical nodes, and an amplifier. These are connected to a central location called the CMTS, which is the hub of an HFC network. DOCSIS 3.1 improves the spectral efficiency of HFC networks and allows operators to utilize new, higher-range spectrum.
A DOCSIS-compliant HFC CPMTS can identify a DED in a subscriber account by the DED’s MAC address and ranging offset value. Once this information is obtained, a subscriber account alert is issued if more than one DED is associated with different MAC domains, indicating that they are not co-located.
The DOCSIS-compliant HFC CPMTs can manage a wide range of different protocols, including IPv6. They are capable of delivering a variety of different protocols and are also capable of handling multiple MAC layers. This enables them to provide a range of network services to their subscribers.
DOCSIS-compliant HFC CPMTs are the key to delivering IP-based services over a cable network. They must be DOCSIS-certified by the cable operators to ensure that they meet the standards required by cable operators. They are also required to be compatible with a variety of cable technologies.
DOCSIS is a global standard that has been ratified by the International Telecommunication Union (ITU) Telecommunication Standardization Sector. It has been approved for various versions, including 1.0, 2.0, and 3.0. The latter two versions are compatible with existing CMTSs and are also backward-compatible.
DOCSIS 3.1 introduces new modulation and techniques that increase the bandwidth and capacity of an HFC network. DOCSIS 3.1 is the latest specification for HFC networks. Most headends are not capable of supporting the higher DOCSIS 3.1 frequencies. In this situation, DEV Systemtechnik has designed a unique solution that allows operators to leverage existing HFC infrastructure and upgrade it for high-speed services. DEV Systemtechnik’s MODULO HFC handles high-frequency signals and sends them to a fiber node. Additionally, the MODULO HFC also supports an upstream channel towards the CMTS.
The frequency band used by DOCSIS-compliant HFC TMTS is primarily defined by ITU-T J.83 specifications. In North America, DOCSIS services use the ITU-T J.83 standard. This specification uses the Annex B specification, which uses an extra layer of Trellis Coded Modulation channel coding to provide 2 dB of gain. It also uses an upstream and downstream modulation technique that uses a wider bandwidth.
A CMTS can serve different numbers of cable modems and nodes. Some cable operators may need more than one CMTS to meet their needs. Most CMTSs have an Ethernet interface. Downstream traffic from cable modems passes through the Ethernet interface and is then exited via the RF interface connected to the HFC network. Upstream traffic from cable modems to the internet is passed the other way.
Cable modem SNR typically follows a Gaussian distribution, with a higher mean and lower deviation. A typical HFC plant has downstream SNR of 36 dB. DOCSIS 3.0 supports 256QAM modulation. The highest capacity is 8 bits per second per hertz. Using higher order modulation will improve SNR.
The FDX and ESD protocols are not widely deployed yet, but a recent trial of the node+0 architecture showed that they can be used to build full end-to-end systems. Both FDX and ESD have significant challenges to overcome, and it is expected that it will take until 2025 to reach commercialization.