Home > Articles > Networking

The Cable Access Link

Overview of the Cable Access Network

Before we examine in detail the workings of DOCSIS modems and their corresponding CMTSes, we will look briefly, at a high level, how a DOCSIS access network operates.

DOCSIS cable modems located in homes are clients of Cable Modem Termination Systems, which are located at the other end of the coax/fiber access link, at the MSO's headend. In order that the CMTS is properly able to manage the access network, on which there may be several hundred cable modems all competing for the available upstream and downstream bandwidth, DOCSIS cable modems are required to obtain and obey instructions from the CMTS. This ensures that the resources are allocated fairly and efficiently among the active cable modems on the network.

Initialization

When a cable modem is first connected to the cable and powered up, a complex series of transactions takes place between the modem and its CMTS. The modem is at all times unaware (at least directly) of the presence of other modems on the network; the modem communicates only with the CMTS. This is true even if two modems on the same access network wish to communicate—all traffic passes through the CMTS.

The cable modem initialization sequence is as follows.

  1. Locate a downstream channel and synchronize operation with the CMTS.

  2. Obtain upstream transmit parameters from the CMTS.

  3. Perform a ranging operation.

  4. Confirm that IP connectivity exists.

  5. Synchronize time of day with the CMTS.

  6. Transfer operational parameters between CM and CMTS.

  7. Register.

  8. Initialize Baseline Privacy Plus.

In order to perform this sequence in a reliable and secure manner, two items are placed in the cable modem at the time of manufacture. These items are in nonvolatile memory and should never be changed.

  • A unique 48-bit MAC address (equivalent to the MAC address in an ordinary Ethernet network interface card)

  • An X.509 digital certificate, which is used to authenticate the modem to the CMTS. Typically, this certificate is signed by the modem manufacturer, whose public key the service provider has obtained through other means and that is loaded into the CMTS software so that it can authenticate modems as they attempt to initialize and obtain service.

Downstream Synchronization

The cable modem begins to scan the 6 MHz downstream video channels, looking for a CMTS signal. If the modem has previously been used and is simply restarting after a temporary failure—for example, power-down—the modem first tries to lock on to a CMTS signal in the most recently used downstream channel. It continues to scan until it finds a signal that it can properly detect and with which it can properly synchronize.

Obtaining Upstream Parameters

The CMTS periodically transmits messages called Upstream Channel Descriptors (UCDs) on all downstream channels. Nominally, UCDs are broadcast every two seconds. UCDs describe the correct parameters that a modem must use to transmit on the various upstream channels to which the CMTS is currently listening.

When the modem receives a UCD containing parameters for a channel that it can use, it stores this information and uses it to determine the transmit parameters for future upstream transmissions.

As well as UCDs, the CMTS periodically transmits SYNC messages (nominally every 200 milliseconds). These contain information about the CMTS's notion of time and allow the modem to synchronize properly with the CMTS and the other modems on the network.

Ranging

A number of operational parameters within the modem may need to be adjusted slightly in order to guarantee that all modems on the access link are operating cooperatively. For example, the transmit power level or the center frequency of the upstream channel might need to be adjusted slightly if the modem is out of alignment. In addition, since cable modems are not all at the same distance from the CMTS, it is insufficient for a CM merely to synchronize its clock with the CMTS. It must also have some notion of the transmission delay between itself and the CMTS, otherwise the transmissions from two modems, one told to transmit at time t and another told to transmit at time t', might overlap.2

In order to make these adjustments, the cable modem must actively exchange information with the CMTS. It does this through a process known as ranging. Cable modem transmissions are sent in one of two modes: contention or noncontention. In the cable system, time is divided into short intervals known as minislots, which are a small multiple of 6.25 microseconds in length. (The precise duration of a minislot depends on the modulation scheme in use. Basically, a minislot is usually the time taken to transmit 16 octets.) Noncontention minislots are allocated by the CMTS in such a way that only one CM is permitted to transmit within the minislot. Transmissions occurring in noncontention minislots have a high probability of being received correctly at the CMTS, since it is guaranteed that there will be no other signal on the line in the same upstream channel at the same time. Contention minislots (which are typically about 25% of the available total) are unallocated, and any CM is permitted to transmit during them. These transmissions may have a low probability of being received correctly if there are many active devices on the access network.

The CMTS manages the ratio of contention to noncontention minislots, just as it manages exactly which modem may transmit during a noncontention minislot. In fact, at the risk of digressing from the point at hand, calculating optimum ratio of noncontention to contention minislots is an interesting problem in network bandwidth management, since it depends on the kind of data that is passing across the network. If most of the traffic flows at relatively constant rates (for example, when the network is handling principally telephony traffic), then there are fewer ad hoc requests for upstream bandwidth, and the need for contention minislots decreases. This in turn allows the CMTS to allocate more bandwidth to noncontention minislots, and thus even more telephony-like traffic may be permitted to flow. If, on the other hand, the traffic is "bursty", such as occurs with Web browsing, then the number of contention minislots typically needs to be increased and the usable bandwidth of the system decreases. A good working average of noncontention:contention mode slots for "typical" traffic is roughly 3:1.

Except for informational messages, transmissions sent in contention mode usually demand an explicit response from the CMTS. If the expected response is not received, the CM will usually retransmit the transmission in another contention-mode minislot, and will continue to do so until a response is received.

Ranging requests are sent in contention mode and so may need to be repeated a number of times before the CM receives the information it desires from the CMTS. In response to a ranging request, the CMTS will instruct the CM to adjust parameters such as clock skew, carrier frequency and transmit power so that they are within acceptable limits.

In addition to the ranging performed during initialization, the CMTS provides specific opportunities for each attached CM to perform subsequent ranging operations to ensure that slight adjustments to the operational parameters may be made as necessary, so that the entire system stays acceptably synchronized.

Establishing IP Connectivity

Once the low-level transmission parameters are properly set, the CM should be able to communicate correctly with the CMTS (and, through it, to the MSO's network on the far side of the CMTS). It now begins communication by transmitting a Dynamic Host Configuration Protocol (DHCP) "discover" request. In response, a DHCP server provides the modem with an assigned IP address, as well as the address of another DHCP server (possibly the same one) that can provide the modem with more parameters. The initial DHCP response also contains name of a file that contains further, network-specific configuration parameters for the CM. The CM issues a DHCP request to the second server and obtains whatever additional parameters are needed to establish IP connectivity with the network. Note that it does not yet download the configuration file.

Synchronizing Time of Day

As well as a low-level shared notion of time (for the correct synchronization of packet transmissions), the CM and the CMTS need to share a common notion of the approximate time of day, which may be used for logging abnormal events and for key management by the security system (which will typically require that keys be changed periodically).

One of the parameters obtained from the DHCP server is the address of a Time Server (which may be the DHCP server itself). The modem connects to this server on port 37 and obtains the time, using the Time Protocol specified in RFC 868.

Transferring Operational Parameters

The CM now downloads the configuration file whose name was provided by the original DHCP server. This download uses the Trivial File Transfer Protocol specified in RFC 1350. The operational parameters overwrite any default values configured into the modem during manufacture.

A large number of parameters may (but need not) be present in the configuration file. These parameters provide values used by the low-level system, such as upstream and downstream channel frequencies and data rates, as well the addresses of various network servers, timer values, and so on. If explicit values are not provided, the modem adopts sensible default values provided at the time of manufacture.

The configuration file may direct the modem to use an upstream or downstream channel different from the one it is already using, in which case the modem switches to the new channel(s) and performs a new Ranging request.

Registering

Once the modem has obtained and processed the configuration file, it informs its CMTS of the values of its operational parameters in a Registration Request message. The CMTS assigns Service IDs (SIDs), which will be used to identify the various classes of service flowing through this particular modem and informs the modem of the SID values that have been assigned to it.

Initializing Baseline Privacy Plus

A security association between a cable modem and its CMTS allows information to flow between the two without fear that the data can be read or manipulated by a third party. This is an important requirement on a cable access network, since there is at least a theoretical possibility that a neighbor may be eavesdropping on the CM-CMTS communication.

In order to create a security association, the modem now initializes its Baseline Privacy Plus (BPI+) configuration, which effectively secures the link from casual eavesdroppers. (BPI+, however, uses only 56-bit DES to secure the link, which is insufficient to deter a determined attempt to decrypt the traffic.) Once BPI+ is correctly initialized, the modem is a fully fledged member of the network, operating completely under the control of the CMTS.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020