Cable Modem DOCSIS

What is DOCSIS?

Data Over Cable Service Interface Specifications (DOCSIS) is a set of standards that govern how an Ethernet packet of information travels through a two-way RF communications path in a Hybrid Fiber Coaxial (HFC) Network. The specifications of DOCSIS define interfaces throughout the HFC network. The DOCSIS standard enables inter-operability of vendors' equipment between the Cable Modem Termination System (CMTS) at the headend and the Cable Modem (CM) at the customer premises (Customer Premises Equipment, or CPE).

Data Over Cable Service Interface Specifications emerged when several organizations including the Institute of Electronic and Electrical Engineering (IEEE) and Phyisical Protocol Working Group worked to develop an international cable modem standard. The Multimedia Cable Network System Partners, Ltd. (MCNS), and Cable Television Laboratories (CableLabs) joined their efforts. In March of 1997, MCNS released its first standard (DOCSIS 1.0) to the manufacturing community.

In 1998, CableLabs began a formal certification program for equipment to set standards and ensure compatibility between cable modems and cable modem termination systems produced by different manufacturers.

Certification Process Overview

The DOCSIS certification and qualification process is managed by a committee of cable operators to which certification results are made available by CableLabs®. Cable Modems receive certifcation; CMTSs receive qualification. Cable operators make their own independent decisions about which modems or CMTSs to purchase and which modem deployments to continue.

Each CM vendor applying for certification receives a written report from the certification board highlighting areas of non-compliance with the DOCSIS specification.

The report is composed of two parts: documentation feedback based on conformance to the specification as detailed in tests (PICS, acceptance test plan, interoperability, and executive summary) performed by vendors at their own facilities, and results of qualitative, quantitative, and stability testing in CableLabs' alpha, beta, and field labs.

For more information regarding the certification or qualification waves, refer to the specific project on the CableLabs webpage or review the certification waves guidelines document.

EuroDOCSIS

tComLabs n.v., located in Ghent, Belgium, acts as the Euro-DOCSIS certification lab and information center.

The mission of the Euro-DOCSIS certification lab is to:

* Provide an interoperability test environment for Euro-DOCSIS products
* Perform certification tests according to the Euro-DOCSIS specification.
* Stimulate the interaction between vendors and operators to define Euro-DOCSIS standards and to discuss related issues.
* Investigate new technologies for Euro-DOCSIS products and develop adopted test procedures.

DOCSIS 1.0 requirements define how data moves up and down the HFC plant and defines the way that different sub-systems interact with one another in a fully-deployed cable data network. Focused on Best Effort, the DOCSIS 1.0 standard was designed as a cheap consumer Web-surfing platform. However, DOCSIS 1.0 does not provide all of the QoS and latency controls required to offer services beyond web surfing, such as toll-quality IP voice services.

The DOCSIS 1.0 specification includes the following features:

* Uniform and consistent service as seen by any subscriber
* Open, non-proprietary operations that permit equipment from multiple vendors to interoperate CMs with low power consumption (4-10 W) that could ultimately be sold in a retail market with no user-configurable parameters
* Asymmetric transport of data with more downstream bandwidth than upstream bandwidth to match the asymmetric data flows of most Internet applications of the time (ex: Web surfing)
* Efficient downstream transport of data encapsulated in MPEG streams with 27-36 Mbps of total user bandwidth carried in a single 6 MHz-wide channel inside of the typical HFC downstream spectrum (88-860 MHz center frequencies)
* Support for either 64QAM (30.341646 Mbps) and 256QAM (42.884296 Mbps) operation in the downstream channel
* Flexible, robust upstream transport of data with 0.32-10.24 Mbps of total bandwidth carried in a single 0.2-3.2 MHz channel inside of the typical HFC upstream spectrum (5-42 MHz center frequencies)
* Simple security measures that provide assurances of privacy for data transported over the shared HFC plant
* Simple network management of equipment via the Simple Network Management Protocol (SNMP)
* Remote software upgrades for improvements

Introduction to DOCSIS 1.1

The second standard, DOCSIS 1.1, was released as an interim specification in March 1999. This standard of DOCSIS ushered in a new class of Cable Modem Termination System (CMTS) equipment known as "Carrier Class" or "Next Generation." Within the cable data network, the principal function of the CMTS is to transparently carry Internet traffic between the MSO headend and subscriber locations. The CMTS modulates the IP signals and sends them to the cable modem. In the reverse direction the CMTS demodulates the cable modem's response and distributes traffic back through the local headend on to the Internet. This new generation of CMTSs introduced additional features outside the scope of DOCSIS 1.1 specifications including scalabililty, reliability, enhanced observability, and wire-speed performance. Many of these features were added to prepare for future telephony-over-IP services that would be offered over the HFC plant.

The second-generation specifications, called DOCSIS 1.1, added key enhancements to the original standard. These enhancements include the following:

* Improved Quality of Service (QoS)
* Multiple service flows per cable modem
* Improved bandwidth efficiency via fragmentation, concatenation, and payload header suppression
* Additional multicast support
* Advanced security and encryption
* Multiple service types
* IP filtering
* Increased counts and measurements

This next-generation standard is designed to be backwards compatible, enabling DOCSIS 1.0 and 1.1 modems to operate in the same spectrum on the same network.

In addition to the service enhancements tied to forthcoming DOCSIS 1.1-based equipment, the specification is also notable for its role in advanced IP services. It is the foundation of both Packet cable, focused on VoIP, and other advanced IP services as well as the home networking initiative known as "CableHome."

DOCSIS 1.1 Interface Categories

As Multiple System Operators (MSOs) seek to maximize financial rewards from investments in Hybrid Fiber Coax (HFC) infrastructure, the ability to deliver multiple Quality of Service (QoS) levels is the key to success.

Standards compatibility and the adoption of next-generation technologies enable operators to provide measurable Service Level Agreements (SLAs) across the cable network and to support real-time applications such as Voice over IP (VOIP), streaming video, and gaming/entertainment applications. Operators can support robust business applications for both small business and residential customers, including Virtual Private Networks (VPNs), corporate video, and remote access. They can create additional revenue streams from Application Service Providers (ASPs) as well as Internet Service Providers (ISPs) and other revenue-sharing partners that deliver customized services or content.
This resulted in the five following interface categories:

1. Data Interfaces

* Cable Modem to CPE Interface (CMCI) the cable-modem-to-customer-premises-equipment (CPE) interface (for example, between the customer's computer and the cable modem).
* Cable Modem Termination System to Network-side Interface (CMTS-NSI), the cable modem termination system network-side interface between the cable modem termination system and the data network.

2. Telephone Return Interface

* Cable Modem Telephone Return Interface (CMTRI) This is the interface between the cable modem and a telephone return path, for use in cases where the return path is not provided or not available via the cable network

3. Operations Support System Interface (OSSI)

These are network element management layer interfaces between the network elements and the high-level OSSs (operations support systems) that support the basic business processes.

4. Radio frequency interfaces. The following RF interfaces are defined:

* Between the cable modem and the cable network.
* Between the CMTS and the cable network, in the downstream direction (traffic toward the
customer)
* Between the CMTS and the cable network, in the upstream direction (traffic from the
customer).

5. Security Interfaces

* BPI: Baseline Privacy Interface

QoS Feature Overview

Requirements

The requirements for QoS include:

* A configuration and registration function for pre-configuring CM-based QoS Service flows and traffic parameters.
* A signaling function for dynamically establishing QoS enabled service flows and traffic parameters
* A traffic-shaping and traffic-policing function for service flow-based traffic management, performed on traffic arriving from the upper layer service interface and outbound to the rf.
* Utilization of MAC scheduling and traffic parameters for upstream service flows.
* Utilization of QoS traffic parameters for downstream service flows.
* Classification of packets arriving from the upper layer service interface to a specific active service flow.
* Grouping of service flow properties into named service classes so upper layer entities and external applications (at both the CM and CMTS) can request service flows with desired QoS parameters in a globally consistent way.

Differentiating between DOCSIS 1.0 and DOCSIS 1.1

Quality of Service (QoS) concepts not included in DOCSIS 1.0 requirements include:

* Packet classification and flow identification
* Service flow QoS scheduling
* Dynamic service establishment
* Fragmentation

What is DOCSIS 2.0?

In response to the growing demand for more upstream bandwidth for applications such as interactive gaming, MP3 file exchanges, Voice over IP telephony (VoIP), and business-to-business applications, CableLabs® has developed DOCSIS 2.0 specifications and requirements for DOCSIS. The new specification builds on the existing DOCSIS 1.1 specification with the addition of advanced capabilities to the upstream MAC and PHY layer protocols.

Benefits of DOCSIS 2.0

* Greater upstream bandwidth by supporting two new advanced modulation schemes
* Improved noise immunity using techniques such as enhanced Forward Error Correction (FEC), interleaving, Advanced Time-Division Multiple Access (ATDMA), and Synchornous Code-Division Mulitple Access (SCDMA). These build on top of the current Time Division Multiple Access (TDMA) in DOCSIS 1.0/1.1.
* Backward compatible with deployed DOCSIS
* Coexist on same channel as deployed DOCSIS
* Interoperable silicon from multiple suppliers

Shortcomings of DOCSIS 2.0

* Does not provide any capacity enhancements for the downstream channel.
* Is not possible with just a simple software upgrade to DOCSIS 1.1 equipment. The equipment requires new DOCSIS 2.0-capable hardware if the new features are to be enabled.

New Modulation Techniques

Time-Division Multiple Access (TDMA), as specified in DOCSIS 1.x, defines the upstream modulation technique used for DOCSIS 1.0 and DOCSIS 1.1. In DOCSIS 2.0, two new modulation techniques have been proposed as improvements over this earlier TDMA scheme. These new modulation techniques are known as:

* Advanced Time-Division Multiple Access (ATDMA)
* Synchronous Code-Division Multiple Access (SCDMA).

DOCSIS 2.0 requires that the CMTS and the CM support all three of these modulation techniques (TDMA, ATDMA, and SCDMA).

The TDMA and ATDMA are similar to one another, but they are quite different from SCDMA. For an example, consider describing TDMA/ATDMA as a communication forum used in conference presentations. In particular, each speaker takes control of the podium for a specific period of time, and they must speak rapidly to communicate their information before relinquishing the podium to the next speaker (who must repeat the process).

SCDMA, on the other hand, is similar to a party where many conversations are occurring in parallel, but the speaker and listener of each conversation are only "tuned in" to their information exchange, and the other conversations merely create background noise. To further build on this analogy, assume that each of the many conversations at the party is being spoken in a different language so that the other conversations are not even understood by the two people in a particular conversation.

Advanced Time-division Multiple Access (ATDMA)

As their names would imply, both TDMA and ATDMA are Time-Division Multiple Access technologies that permit multiple users (CMs) to share the bandwidth within an upstream channel by allowing each of the users to transmit by themselves within a unique burst interval (time slot). This is why TDMA and ATDMA transmissions use the bandwidth in a manner similar to the way in which speakers share the podium in a conference: one speaker at a time.

The ATDMA specification provides many new techniques that will enable MSOs to operate upstream channels with higher throughputs (assuming the noise mitigation techniques described below will permit the higher throughput operation in the presence of the channel noise). Several mechanisms were added to the ATDMA specification to permit this improved operation:

* Three new upstream channel modulation formats were added by ATDMA. These include 8-point QAM (8QAM), 32-point QAM (32QAM), and 64-point QAM (64QAM). The modulation format can be changed for each of the burst intervals.
* A higher symbol rate of 5.12 Msymbol/sec was added by ATDMA.
* The preamble can be transmitted with higher power to permit synchronization to occur more rapidly. This may permit the use of shorter preambles, which will eliminate some of the overhead associated with ATDMA transmission. This results in more bandwidth available for the transmission of user traffic.
* The maximum preamble length was increased to 1536 bits to aid in channel synchronization when using the higher 6.4 MHz channels. (Note: DOCSIS 1.X limited the preamble length to 1024 bits).

Synchronous Code-Division Multiple Access (SCDMA)

While the DOCSIS 2.0 ATDMA scheme and the DOCSIS 1.x TDMA scheme are somewhat similar in basic structure and operation, the DOCSIS 2.0 Synchronous Code-Division Multiple Access (SCDMA) scheme is a member of an entirely different genre of transmission techniques. As stated earlier, SCDMA uses the bandwidth in a manner similar to the way in which people at a party can have many different parallel conversations in different languages and not interfere with one another. SCDMA transmission has been described as a "spread-spectrum" technology or a "spread-time" technology.

Consider a baseline 3.2 MHz-wide upstream channel that is capable of transporting a DOCSIS 1.x TDMA signal using 16QAM. Using TDMA technology, a single cable modem can transmit in the upstream channel in a given burst interval. The transmission produces a sequential stream of 16QAM symbols, and each symbol has a period of 390.625 nsec. The permitted symbol rate in the channel is 2.56 Msymbol/sec, and the resulting bit-rate (with four bits per symbol) is 10.24 Mbps.

Now assume that we want to use the same 3.2 MHz-wide channel to transmit a stream of 16QAM symbols using SCDMA transmission instead of TDMA transmission. From a high-level point-of-view, SCDMA modifies the original symbol-stream using two clever tricks. The first trick is known as symbol spreading. Symbol spreading requires that each symbol be stretched (or spread) in time by a factor of 128, so a single spread symbol would have a period of (390.625 nsec)*(128)=50 usec. The permitted symbol rate for this single SCDMA symbol stream in the channel is 20 ksymbol/sec, and the resulting bit-rate (with four bits per symbol) is 80 kbps (which is 1/128th of the bit-rate for the TDMA symbol stream carried in the same 3.2 MHz-wide channel).

The longer period in SCDMA transmission is known as the "spreading interval." Using Fourier Analysis techniques, it can be shown that these spread symbols consume only 1/128th of the spectral bandwidth that is used by the shorter-period TDMA symbols

Combining DOCSIS 1.0, 1.1, and 2.0

The DOCSIS 2.0 specification recognizes the need for backwards compatibility, coexistence, and interoperability between the different types of equipment that may be used on a single HFC plant. The various combinations of CMTS and cable modem functionalities are illustrated in the table below, and the resulting modes of operation for each of these combinations is also shown.

Combining TDMA, ATDMA, and SCDMA

While the DOCSIS 2.0 specification provides a simple mechanism for allowing DOCSIS 1.0, DOCSIS 1.1, and DOCSIS 2.0 cable modems and CMTSs to interoperate with one another, the co-existence of TDMA, ATDMA, and SCDMA is slightly more complicated.

One serious complication in DOCSIS 2.0 arises from the fact that a CMTS operating with TDMA and ATDMA cable modems must specify time references when granting burst intervals to the cable modems and the transmitting cable modem consumes the entire upstream channel during its burst interval, precluding the use of channel sharing during any particular burst interval.

A CMTS operating with SCDMA cable modems must specify both time references and spreading codes when granting burst intervals to cable modems, and each transmitting cable modem can share the upstream channel with other cable modems during a frame.

As a result of these differences, TDMA/ATDMA cable modems must not be transmitting during frames when SCDMA cable modems are transmitting, and SCDMA cable modems must not be transmitting during burst intervals when TDMA/ATDMA cable modems are transmitting.

To simplify the coordination of the different types of transmission schemes when TDMA/ATDMA cable modems must share a single physical upstream channel with SCDMA cable modems, the DOCSIS 2.0 specification added a new construct known as a logical channel.