Remote Teleworker Solutions – Cable Access Technologies

Having gone through the basics pertaining to branch offices as well as data transmission basics, this section describes broadband cable access technologies, the components used within cable networks and the configuration of Cisco routers when connected to cable access technologies.

Cable modems are network bridge devices that operate at Layer 1 and Layer 2 of the OSI Model and connect home or branch office networks to the Internet through the cable television system. On the network side, cable modems support Ethernet and on the cable side, DOCSIS. However, it should be noted that although cable modems generally follow the DOCSIS standard, implementations commonly can and do vary by provider. DOCSIS is beyond the scope of the current ROUTE exam and will not be described in any further detail in this chapter.

Cable companies use Hybrid Fiber Coaxial cable (HFC) networks to provide fiber and coaxial connections to the customer. The coaxial portion is used to carry the television service and the fiber optical cable is used for the data connection. HFC networks will be described in detail later in this section.  Typical cable throughput varies anywhere between 1Mbps and 6Mbps for downloads and between 128Kbps and 768Kbps for uploads. In theory, however, cable modems have the ability to support up to 30Mbps download speeds. Cable networks are a shared multipoint circuit which means that the actual download and upload speeds depend on the level of activity on the network at any given point in time.

While cable technology and functionality is not a core ROUTE exam requirement, it is still very important to have a basic understanding of cable network components. These components, which are described in the following sections, include the following:

• Cable Modems
• Cable Modem Termination Systems
• Hybrid Fiber Coaxial Networks
• Local Headend

Cable modems (CMs) communicate with routers, called Cable Modem Termination Systems (CMTS), over the HFC plant using the DOCSIS standard. Cable modems are considered the CPE, or Customer Premise Equipment, and they perform the modulation and demodulation of signals received from the computer, etc, to and from the CMTS.

The CMTS is a piece of equipment commonly located in a cable company’s Local Headend (LE). The CMTS provides high speed data services, such as cable internet or Voice over IP, to cable subscribers. The CMTS receives analog data from the CM and modulates it back into a digital format before forwarding it on to the Internet.

Hybrid Fiber Coaxial is used to describe a telecommunications industry term for a broadband network that combines both optical fiber and coaxial cable. The Hybrid Fiber Coaxial (HFC) network is the network that resides between the CM and the CMTS. Within the HFC, one or more fiber nodes are used to convert digital signals from the LE into analog signals which are then sent to the CM at the premise and vice-versa.

The Local Headend (LE) is the cable operator facility where the received signals are processed, formatted, and distributed over to the cable network. The CMTS resides at the LE.  Understandably, these terms may be new and even confusing. Therefore, for additional clarity, Figure 10-6 illustrates these components and how they are integrated in data cable networks:

Fig. 10-6. Cable Access Data Network Components

The cable network components that were described in the previous section are shown in Figure 10-6. The diagram shows broadband connectivity for Small-Office Home-Office (SOHO) users as well as for remote branch offices that connect using broadband technology. For the remote office, the site router will need to be configured to provide additional IP services such as DHCP, NAT, as well as VPN connectivity to the enterprise network.

In most cases, when connecting to the Internet using cable, the cable provide typically provides the cable modem to which the router may then be connected. In some cases, however, your company may select to use their own brand of cable modems based on vendor preference and other factors such as security, manageability and control. It is therefore important to have a fundamental understanding of the tasks involved so that you are able to perform some basic troubleshooting. The following section describes the sequence of steps that a typical cable modem will transition through before it becomes operational. These seven steps are as follows:

1. The cable modem (CM) scans for the downstream (DS) frequency. By default, 53 MHz is the starting frequency for Cisco CMs. The CM locks on to the digital carrier center frequency and looks for the hexadecimal 1FFE MPEG-2 packet identifier (PID), which signifies the DOCSIS standard
2. The CM waits for all upstream channel descriptors (UCDs) used for frequency, modulation profile, channel width, and other information. If the CM receives the wrong UCD, it times out and it tries the next UCD until it finally connects. Some modems might actually listen to an upstream channel change (UCC) command sent by the CMTS on the DS frequency which is used to advise the CM of the UCD it should be using. Current versions of Cisco CMs have essentially three scanning algorithms:

• Scan National Television System Committee (NTSC)
• Scan select European (EuroDOCSIS) frequencies
• Scan for a DOCSIS DS at every frequency divisible by 250 kHz or 1 MHz

1. The Layer 1 and Layer 2 connection between the CM and the CMTS is established.  This allows the CM and the CMTS to become synchronized
2. After the modem and the CMTS are synchronized with levels and timing, the modem obtains its IP address through DHCP. Most CM systems set up a non-routable address space for the modems, such as the 192.168.0.0/24 subnet, which can be allocated to clients, and use a public addressing network for CPE, to allow Internet access
3. The modem obtains the DOCSIS configuration via TFTP. The DOCSIS configuration file is a binary file that contains the parameters required for the modem to come online. This configuration file will vary by provider. The DOCSIS file contains important information such as maximum upstream and downstream speeds, radio frequency information, SNMP management information, and authentication information, amongst other things
4. The modem registers QoS with the CMTS. Quality of Service (QoS) for networks is an industry-wide set of standards and mechanisms for ensuring high-quality performance for critical applications. QoS is very important, especially if the cable provider is also providing the subscriber with Voice over IP (VoIP) services
5. The modem initializes IP services and downloads the configuration file and configures routing and other IP services, such as NAT, so that one or many subscriber devices can access the Internet at the same time. The modem receives the following information from the provider DHCP server:

• The IP Address and Subnet Mask
• The Default Gateway
• The address of the TFTP Server
• The name of the DOCSIS configuration file
• The address of the Time of Day (ToD) server
• The address of the Syslog server
• The DNS domain information

For easier reference, this sequence of steps is summarized and illustrated in Figure 10-7:

Fig. 10-7. Understanding Basic Cable Modem Initialization

Cisco IOS Router Configuration for Cable Internet Connectivity

Given that cable modems have an Ethernet handoff, the configuration of Cisco IOS routers for cable Internet connectivity requires no special configuration on the router itself. Instead, a selected LAN interface on the router is simply physically connected to the cable modem and then configured to receive addressing configuration information via DHCP. Following this, additional standard configuration, such as DHCP and NAT, should be implemented to allow users connected to the internal network access to the Internet.  The steps for configuring a router for cable Internet connectivity are as follows:

1. Configure the router Ethernet interface that is connected to the cable modem to receive addressing information via DHCP using the ip address dhcp interface configuration command. This allows the router to receive the default route and other addressing information, e.g. DNS servers, dynamically from the cable provider

NOTE: In some instances, cable vendors may provide static addressing information for their business class cable customers. If you are assigned static IP addresses, simply configure them on the router interface connected to the modem using the ip address [address] [mask] interface configuration command, followed by the ip route 0.0.0.0 0.0.0.0 [next-hop-address] global configuration command to specify the default route. Because cable is a shared medium, you should avoid configuring a default static route that points out of an interface; e.g. ip route 0.0.0.0 0.0.0.0 fastethernet0/0. This eliminates extensive use of ARP and helps avoid intermittent connectivity issues due to ARP timeouts.

1. Configure IP addressing for the LAN interface that will be connected to the internal network using the ip address [address] [mask] interface configuration command. This interface typically uses networks within the private RFC 1918 range
2. Configure Cisco IOS DHCP server functionality on the router. This allows hosts that are connected to the LAN to receive IP addressing information. This is implemented using the ip dhcp pool [name] global configuration command. When configuring Cisco IOS DHCP server functionality, you can either manually specify DNS servers or use the import all DHCP pool configuration command to allow the information the router will receive from the cable provider, e.g. DNS servers, to be provided to the LAN hosts
3. Configure Network Address Translation/Port Address Translation on the router so that the privately-addressed hosts can access the Internet. Designate inside and outside interfaces using the ip nat insideand ip nat outside interface configuration commands on the internal and external interfaces respectively

The following configuration example shows how to configure a router for basic cable Internet connectivity, assuming that the router is receiving addressing information via DHCP. The configuration also configures the router to import all received DHCP parameters, which will include DNS, WINS server information and the domain prefix. The router is configured to prevent the allocation of the 10.0.0.1 and 10.0.0.2 addresses which are configured on the router itself and the connected LAN switch. And finally, NAT is also configured on the router, allowing hosts on the internal network to access the Internet.

As illustrated above, the configuration itself is very simple and straightforward. However, keep in mind that in some cases corporate policy that dictate that users use corporate DNS servers and domain prefixes. In that event, the import all command should not be used when configuring the DHCP pool and those parameters should be manually entered as illustrated in the following configuration which shows the modified site router DHCP configuration:

NOTE: If the import all command is included, by default, any explicit configuration commands take precedence. Imported options will only be used if no explicit option configuration is included in the Cisco IOS DHCP server functionality configuration. This is often a commonly confused topic when it comes to implementing Cisco IOS DHCP.