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Another concern with current DSL installations is that most products do not offer security solutions. The RLAN model greatly reduces this risk because the links are isolated at Layer 2, but all connectivity must be provided by the head-end. This includes Internet connectivity. For Internet connections to an ISP, the risk is significantly greater, especially when considering the bandwidth available for an attack and the use of static IP addresses or address pools. A number of significant attacks have already occurred as a result of these issues, and while they should not deter the use of the technology, the risks should be addressed with firewall technology.

A third consideration in DSL is the installation delay compared to other technologies. Vendors are moving towards splitterless hardware so that the telephone company does not have to install a splitter in the home. The splitter divides the traditional phone signals from the data stream and provides a jack for standard telephones—DSL transports data and voice over the same twisted-pair wiring used for standard analog phone service. At present, installations require weeks to complete in order to validate the circuit to the home and install the splitter.

Cable Modems

It would be unfair to present the DSL technologies without providing some space for cable modems. Cable modems operate over the same cable system that provides television services using the same coax cable that is already used in the home. Most installations will provide two cables, one for the television and one for the data converter, but the signaling and system are the same. This is accomplished by allocating a television channel to data services. Bandwidth varies with the installation; however, 2Mbps in each direction is not uncommon.

Detractors of cable modem technology are quick to point out that these installations are shared bandwidth, similar to Ethernet, which results in contention for the wire among neighbors. This design also introduces a security risk in that network analysis is possible, although vendors are working to address this concern. This issue does not exist in DSL, as the local loop connection to the home is switched. Traffic is not integrated until it reaches the central office, and the switch will only forward traffic destined for the end station based on the MAC address. Cable modems are a shared technology—similar to 802.2 Ethernet versus 10-Base-T. Along the same lines, a cable modem is really a broadband Ethernet bridge.

Network designers may wish to consider cable modems as part of a VPN deployment, as the technology will not lend itself to the RLAN-type (Remote LAN-type) designs availed in DSL. Recall that an RLAN requires Layer 2 isolation—a service not offered by cable modem providers at present. This may change in the future if channels can be isolated to specific users. This may be especially true in very remote rural areas, where cable is available and DSL is not.


Remote connectivity has become increasingly important in modern networks as various organizations expand their requirements. These requirements frequently include the need for data to be available at locations out-side of the traditional corporate office. Such sites may include retail sales outlets, employee homes, hotel rooms, and customer locations.

This chapter presented two of the more traditional remote-access technologies—ISDN and X.25. Both have been used heavily to provide point-of-sale access to corporate data, including credit card verification and inventory systems. While deployments are waning in the shadow of low-speed, low-cost Frame Relay and xDSL solutions, designers and administrators will have to work with these older technologies for the foreseeable future.

In addition to the specific remote-access technologies incorporated into the exam objectives, this chapter also addressed the various design models for providing remote connectivity to telecommuters and other remote staff. These included:

  Remote gateway
  Remote control
  Remote node

The chapter also discussed some of the needs frequently addressed in remote access solutions and the technology Cisco recommends to meet these challenges.

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