DOCSIS - 3. Architecture - Protocols

3.3 Management

Cable Modems have been designed to be as user friendly as possible without the need for subscribers to have to configure them. When a new cable modem is taken from its box for the first time and plugged in, it has no way of knowing how to connect to the Internet, there are two levels of learning which the Cable Modem must go through for it to successfully be able to start transferring data to and from the Internet. All management of cable modem configuration is done at the Cable Operators end.

At the first level of learning - Initially the Cable Modem needs to learn some Physical Layer information; which frequencies it will be sending and receiving data on with the Cable Operator.

The modem does this by scanning across downstream frequencies (frequencies coming into the modem) with the eventual aim to read special control packets called Upstream Channel Descriptors, these packets are broadcast by the CMTS solely for newly attached Cable Modems. The scanning procedure is done by starting with the relatively low frequencies (some Cable Modems jump to starting scanning at 350Mhz as it is assumed that below this frequency are analog TV signals) and slowly increasing in 6MHz steps to 850Mhz at each step testing whether it is a Digital TV signal or Data by examining the packets for a value called a PID (Program Identifier). Each TV channel would have a unique PID and a special PID of ?1111111111110? (0x1FFE) has been set aside by DOCSIS (Data over Cable Service Interface Specifications) for Cable Modems. This PID is not allowed to be used by TV operators to broadcast standard channels on. Once the correct frequency is reached with the correct PID, the Upstream Channel Descriptor can be read.

3.3.1 Upstream Channel Descriptor

The Upstream Channel Descriptor contains amongst other information the following configuration information:

• Frequency, on which the cable modem is allowed to transmit, ranges between 5 - 42 MHz.
• Initial Rate of Transmission
• Maximum number of bytes that can be transmitted the next time permission to send is granted
• Modulation Techniques (QPSK or QAM-16)

Once the Cable modem knows which upstream and downstream frequencies it can use it then has permission to transmit and must learn one more physical piece of information through a process called ?ranging? in which the cable modem attempts to learn how far it is to the CMTS ? this is needed to synchronise the timeslots on which it is to send. Ranging is done by sending a short packet to the CMTS and measuring the response time [see 2.4.3 Ranging Slots]. [GABE99a]

At the second level of learning - The Cable Operator is responsible for providing two configuration services to the Cable Modems, Assigning IP addresses via DHCP and Assigning Configuration Scripts via TFTP.

3.3.2 DHCP (Dynamic Host Control Protocol) Address Assignment

An Internet protocol responsible for automating the configuration of computers which use TCP/IP. DHCP can be used to automatically assign IP addresses to Cable Modems, to deliver TCP/IP configuration addresses such as the subnet mask and default router and TFTP server and to provide other configuration information such as the addresses for time, news and TFTP servers.

option domain-name "cableoperator.net";
option domain-name-servers 123.123.123.1, 123.123.123.2;
option time-offset 0; 
default-lease-time 21600;
max-lease-time 43200;
option tftp-server-name "123.123.123.10"; #tftp server

subnet 123.123.123.0 netmask 255.255.255.0 {
  option routers 123.123.123.254; #default gateway
  option subnet-mask 255.255.255.0;
  default-lease-time 86400;
  max-lease-time 604800;
  range 123.123.123.50 123.123.123.250;
}

Fig 3.3.2a ? Example of DHCP script showing configuration sent to Cable Modem

3.3.3 TFTP (Trivial File Transfer Protocol) Configuration Assignment

A Simple UDP protocol used by Internet devices to transfer files. Specifically, it is one of the most popular methods used for "remote booting". In the case of Cable Modems it is used to upload/download a configuration script to the Cable Modem called a DOCSIS Config. The script MUST contain the following configuration information:

• Network Access
• Class of Service ? Subscriber connection speed cap. (e.g. 600K)

The following settings are optional:

• Vendor ID
• Firmware Software Upgrade filename & Software Server IP
• CMTS Ethernet MAC Address
• Telephone Settings (if applicable)
• Vendor Specific Configuration (if applicable)
• Downstream Frequency
• Upstream Channel ID [MOTOROLA03]

DownstreamFrequency 123000000;
UpstreamChannelId 1; 
NetworkAccess 1;
ClassOfService {
  ClassID 1; 
  MaxRateDown 512000;
  MaxRateUp 64000;
  PriorityUp 3; 
  GuaranteedUp 32000;
  MaxBurstUp 54314;
  PrivacyEnable 1;
}

Fig 3.3.3a ? Example of DOCSIS Config Script sent to Cable Modem
Source: [SOURCEFORGE]

The learning stages above are only needed for a newly attached Cable Modem. As it can take some time to gather, all the information in the stages above (e.g. Frequencies, Addresses, Configurations, Ranging Information etc.) are stored on NVRAM (Non Volatile RAM) which means that even after a reset or power down the modem will still remember this information.

The Cable modem will finally ask for the current time of day, sets up access to network, assigns IDs and slots and establishes the encryption and decryption keys see [4.1 Authentication].

By allowing all the configuration information to be based at the Cable Operators end, it means that management is made easier with operators storing all configurations centrally and being able to ?push? new DOCSIS Configs onto Cable Modems to adjust Cable Modem operation such as Class of Service, used to slow down or speed up the subscribers? bandwidth. The act of slowing down a Cable Modem through this method is also known as ?Capping?.

3.4 Encapsulation

To make the cable modem data service compatible with the digital TV service, the DOCSIS standard specifies MPEG-2 packets as the framing protocol for downstream data; that is IP packets are placed in the payload section of MPEG-2 frames. Using MPEG-2 means that other types of information also encapsulated with MPEG-2 (e.g., voice and video) can all be sent on the same Radio Frequency carrier as DOCSIS Cable Modem data maximising radio bandwidth.

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