The DOCSIS Protocols

As seen in the previous chapter, DOCSIS is really a suite of protocols that cover many of the interfaces between devices. From your computer to the cable modem, the protocol is TCP/IP (OSI Layer 3-4) which is carried within an Ethernet packet (OSI layer 2) which uses the physical interface of 10BaseT (OSI layer 1).

The data from the home is transported between the Cable Modem (CM) and the headend. At the headend, the Cable Modem Terminating System (CMTS) connects into the rest of the network. In this system, the CMTS controls all the cable modems and tells them when to transmit and how much they may transmit.

As shown in Figure 1, there are many CMs and only a single CMTS. In a real system, there would be many CMTSs and each would control a specific area of the network. For a heavy loaded system, there would be more than one CMTS for a geographic area and they would operate at different frequencies.

The major components in a typical Cable TV system are shown in Figure 1.

Figure 1. A typical Cable Data System

The data from the CMTS to the CMs, the downstream direction, is encrypted for your privacy and each CM sees all of the data but only forwards data intended for the computers in your home. The downstream data rate is about 27 Mega bits per second (Mb/s).

Data Formats

The upstream transmission (from the CM to the CMTS) is done at a different frequency than the downstream data. Because of this sharing mechanism, it is possible for the CM to use a single cable to transmit and receive simultaneously. The upstream data rate is set by the cable operator and ranges from 320 Kilobits per second (Kb/s) to 10 Mb/s.

Since there can be many modems sharing an upstream channel, some mechanism must be used to tell each modem when it should send information. This mechanism, called Media Access Control (MAC) is a very important part of the protocol.

As mentioned, the data from your computer to the CM uses TCP/IP inside an Ethernet packet. This same data is also sent to and from the CMTS but put inside another packet.

Figure 2. Downstream and Upstream data transmission

As shown in Figure 2, the downstream data is sent in fixed size units (204 bytes). These units are sent continuously even if there is no data to send so that the system is kept in synchronization.

The upstream data consists of units also. The units are in terms of time instead of bytes and each unit is 6.25 microseconds long and the unit is known as a mini-slot. In order for a CM to transmit data, it requests enough mini-slots to send the number bytes in a packet.

It is the job of the CMTS to decide on what data is to be sent downstream and to decide when to give a CM permission to transmit and how much data the CM should transmit. Because the CMTS has such complete control of the system, it can accurately schedule the accurate delivery of data.

The downstream frame is below shown in Figure 3. The frame size of 204 bytes was carefully chosen so that digital video (MPEG frames) and the data frames (DOCSIS) can be freely mixed on a single TV channel.

Figure 3. The downstream DOCSIS frame

The data being sent downstream may have occasional errors and instead of asking for a retransmission, a special mechanism is used to correct errors. This mechanism, known as a Forward Error Correction (FEC) code is the last 16 bytes of the frame. The payload portion takes the Ethernet frame and breaks it into segments of 184 bytes. Some bits in the Header identify the payload as being the start of the Ethernet frame, within the middle of the Ethernet frame, or the end of the Ethernet frame.

The upstream frame format is shown in Figure 4. Again, the Ethernet frame is put within a MAC frame.

Figure 4. The upstream data frame

The upstream frame is variable length and it uses an FEC to help decrease errors in transmission.

Power-up Sequence

When you turn the power on for the CM, it goes through a sequence of steps before the modem can start sending and receiving data. These steps are:

• Initialization

The CM scans all the TV channels looking for data. It is told which upstream channel to use.




• Ranging

The CM adjust its transmit timing ands adjusts its transmit power levels.




• Establish IP connectivity

Temporary IP address are assigned for the modem (not the computer)




• Receives the current time of day

The modem receives configuration information.




• Registration

CMTS sets up access to network, assigns IDs, slots




• Baseline Privacy

Establish the encryption and decryption keys.

Other Messages

In addition to the description of the messages listed above, there are many more messages that occur in the system. The CM must be managed and isolation of problems in the network need to be done. The Operation Support System Interface (OSSI) is an important part of DOCSIS.

Some additional types of messages occur between your home computer(s) and the network. There are messages to assign the IP address of your computer, setting up groups of computers to receive a broadcast message, and test messages for your computer.

A description of these messages and the messages between the units located in the headend are beyond the scope of this seminar but are included in the suite of DOCSIS specifications

More Information

The DOCSIS specifications are available at: www.cablemodem.com/specifications.html .

Additional DOCSIS seminars:

Overview - An overview of the components of a cable TV system.

The DOCSIS Physical Layer - The downstream/upstream modulation and data rates.

Quality of Service (DOCSIS 1.1) - The changes to DOCSIS 1.0 to implement QoS.

In Summary:

• Downstream frames are a fixed size and video/data may be be intermixed on the TV channel.

• Upstream frames are made up of a collection of mini-slots. (A min-slot is 6.25 microseconds long)

• There are three generations of DOCSIS protocols and these are backward compatible.

• DOCSIS operates using TCP/IP over Ethernet.