Data Over Cable Service Interface Specification

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Data Over Cable Service Interface Specification (DOCSIS) is an international standard for the exchange of packets across a community antenna television (CATV) transmission infrastructure. Today, that means the hybrid fiber/coax networks deployed by the cable television companies. Developed by CableLabs (and a variety of contributing companies), DOCSIS defines the protocol relationship between a cable modem and a cable modem termination system (CMTS).

Originally intended to support the addition of broadband Internet services to existing CATV systems, today DOCSIS also supports the addition of packet voice and other real-time applications.

DOCSIS Evolution

The first DOCSIS standard was DOCSIS 1.0, which was released in March of 1997. In April of 1999, DOCSIS 1.1 was introduced. It's primary enhancement was the addition of Quality of Service (QoS) to support real-time applications. DOCSIS 2.0 was introduced in December 2001 and primarily focused on enhancing upstream speeds to support symmetric applications. The latest version, DOCSIS 3.0, was published in August of 2006. It's primary enhancements included support for IPv6 and enhanced transmissions speeds to support the increasingly bandwidth hungry applications supported in the Internet.

Although the standards are international in scope, there are variants of the standards in Europe and Japan. These variants primarily address the differences in spectrum allocation within the various countries.

DOCSIS and the OSI Reference Model

If the DOCSIS specifications were aligned with the OSI Reference Model (OSI-RM), they would align with Layer 1 (Physical Layer) and Layer 2 (Data Link Layer). DOCSIS provides great variety in options available at Open Systems Interconnection (OSI) layers 1 and 2, the Physical (PHY) and Media Access Control (MAC) layers.

Physical Layer Issues

Because DOCSIS runs over the HFC network, and has to co-exist with the video channels on that plant, the Physical Layer is somewhat constrained. The downstream channel occupies channels that would otherwise be used for digital video signals. The upstream channels reside in the lowest frequencies of the HFC cable plant (e.g., below 50 MHz). The earliest DOCSIS standards (1.0, 1.1) specified channel widths between 200 kHz and 3.2 MHz for the upstream component. With DOCSIS 2.0, this was widened to 6.4 MHz (although it remains backwards compatible to the earlier versions). Increase in the upstream path was critical to supporting symmetric services.

Channel width, however, is only one part of total carrying capacity. Modulation is another part. Prior to DOCSIS 2.0, modulation was 64-QAM or 256-QAM downstream and QPSK or 16-QAM upstream. DOCSIS 2.0 added 32-QAM, 64-QAM and 128-QAM as upstream options. Which is actually used is a function of the quality of the specific cable plant and can be auto-negotiated by the end points. Although DOCSIS 3.0 provides for higher throughput, it doesn't do so by altering the modulation scheme. Instead, it takes the higher-layer approach of logically bonding multiple data streams at the packet level. In other words, if one channel provides 30 Mbps downstream, two bonded channels should scale to 60 Mbps.

Data Link Layer Issues

It is important to remember the nature of the HFC cable plant. The CMTS sits at the head end of the plant and essentially functions as a router injecting packets into the HFC plant for distribution to the subscribers, and accepting packets from the subscribers nodes for injection into Internet, voice, or transport data services. So there is a one-to-many relationship between the CMTS and the cable modems, and a many-to-one relationship between the cable modems and the CMTS. There is no real need for access control downstream, but there is a need for addressing. On the other hand there is a critical need for access control upstream. DOCSIS provides for both of these at the Media Access Control (MAC) layer.

DOCSIS 1.0 and 1.1 specified the use of Time Division Multiple Access (TDMA). DOCSIS 2.0 also supports TDMA but adds Synchronous Code Division Multiple Access (SCDMA) as an option. In both standards, there is contention resolution mechanism for upstream access control. Because the CMTS controls the facility, DOCSIS can implement a form of contention-based access control that is far less prone to collisions than the CSMA/CD scheme implemented in early (shared bandwidth) Ethernet networks. It is also the MAC Layer that provides mechanisms to support differing QoS in the same data channel.

DOCSIS Throughput

Downstream, all versions of DOCSIS have supported either 30 Mbps per channel (64-QAM) or 43 Mbps (256-QAM). DOCSIS 3.0 achieves higher rates by combining channels at the packet level.

Upstream, however, the standards differ dramatically. Use of QPSK supports upstream data rates in all DOCSIS versions up to about 5 Mbps. Use of 16-QAM boosts that to as much as 10 Mbps, also in all versions of DOCSIS. The introduction of 64-QAM coupled with changes in access control options in DOCSIS 2.0 (supported in DOCSIS 3.0) increases potential throughput to as high as 30 Mbps.

DOSCIS Security

Because the HFC network is intrinsically a shared infrastructure, DOCSIS has to address security concerns. Specifically, it has to ensure that consumer information is kept secure from other subscribers, and it has to protect the service provider from unauthorized access to network services. To do this, DOCSIS defines a security capability in something called the Baseline Privacy Interface (BPI) specifications. BPI was improved in DOCSIS 1.1 and 2.0, and the new version was dubbed Baseline Privacy Interface Plus (BPI+). As part of the DOCSIS release, several enhancements were added and the BPI/BPI+ specification was renamed simply Security (SEC).

To provide for data privacy, BPI/SEC encrypt traffic between the CMTS and the cable modem. BPI and BPI+ specified the use of 56-bit Data Encryption Standard (DES) encryption. SEC added support for 128-bit Advanced Encryption Standard (AES). All three specifications support a key refreshing interval that can be configured by the service provider. The original BPI did not have a key management protocol that authenticated the cable modems. BPI+ and SEC are stronger because they feature authentication based on digital certificates and a public key infrastructure.

External links

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2008q1/docsis.mp3%7Cdownload</mp3> | Data Over Cable Service Interface Specification (DOCSIS)