NET 224: Advanced Routers and Routing

Chapter 11: Wide Area Networking Protocols

Objectives:
 

This chapter introduces concepts about incorporating WANs in a Cisco environment:

  1. WAN services
  2. Implementing WAN protocols
  3. Troubleshooting WANs
  4. Evaluating characteristics of WANs
Concepts:

The text begins by redefining a Wide Area Network. We are told that LANs can cover larger areas than in the past, and bandwidth of WANs has typically been less than LANs, but is not always so. The suggestion is that we can consider a network as having WAN links if the infrastructure of those links is leased by the user, rather than owned by the user. Infrastructure, in this sense, is the cabling, the routers, the switches, and other equipment involved in a link.

Think of making phone call: the caller may own the wiring of a site, the instrument being used, and connectivity devices at that site, but they will not own anything past their own site. The person making a call is "leasing" the use of the infrastructure of one or more companies for that connection. Someone who leases the use of equipment like this can be called a subscriber. (If you have an account with a cable TV service, for example, you are a subscriber on their network.)

The text presents a list of WAN terms that may appear on a certification test:

  • CPE - Customer Premises Equipment is owned by a service subscriber. For instance, I own my cable modem, so it is a CPE device. For someone who leases a cable modem from their service provider, it would not be a CPE device.
  • Demarcation point - This is sometimes called the point of demarcation, or just the demarc. It is the boundary between what the subscriber owns, and what the service provider owns.
    The telephone demarc at my home is a box mounted on an outside wall. I own the wiring from there into my home, and all the devices inside my home. The point is responsibility: who pays for fixing something that breaks or fails. As the text explains, a network demarc is the box at which the service provider's responsibility ends.
  • Telco - a telecommunication service provider. Historically, this role has been filled by telephone companies.
  • Local loop - the infrastructure that connects a demarc to the closest switching point (also called a switching office, or central office) of the telco.
  • Central Office - can also be called a CO, a switching office, or a Point of Presence (POP, which has nothing to do with Post Office Protocol). This is an access point to the service provider's network. As noted above, a subscriber connects to the central office through a local loop.
  • Toll network - a trunk line inside the service provider's network. Remember that a trunk line carries traffic from multiple networks/subnets.
  • DCE - Data Communication Equipment, or Data Channel Equipment, or Data Circuit-terminating Equipment, generally means devices that provide access to communication channels, like modems or CSU/DSUs. Think of this as necessary eqipment to connect to the ISP's data channels.
  • DTE - Data Terminal Equipment generally means computers or terminals. This would typically be CPE equipment.
  • ISP - Internet Service Providers sell or grant access to their networks and the Internet.

In general, you should be able to categorize equipment as belonging to the subscriber or to the service provider.

The text continues with a short list of WAN connection types that begins to drown us in alphabet soup:

  • Dedicated - also called leased lines, or point-to-point lines. These are hotline connections from one subscriber location,through the provider's network, to another subscriber location. Very costly, due to constant up time and wide bandwidth. Can have bandwidth up to T3 level.
  • Circuit-switched - Connections must be established for each session, like dialing a phone call. The text says that, typically, the provider is a phone company, but phone companies are generally the telecommunications providers in all cases. Cheaper, but better suited for short ("bursty") communications.
  • Packet-switched - This is like having VPN service on a larger network. The subscriber gets data transport service over a provider's data network that is shared by other subscribers. Bandwidth may vary from transmission to transmission, depending on the number of other users. Typically uses X.25 or Frame Relay protocols. Bandwidth can be as little as dialup or as much as a T3 line.

Use the command encapsulation ? to see a list of protocols supported by Cisco for WAN connections.

The text seven protocols as the ones most used on Cisco networks. Of these, Frame Relay, HDLC, and PPP are the ones most commonly configured on serial interfaces:

  • Frame Relay - like X.25, but with less overhead for correcting line noise. Has dynamic bandwidth allocation and congestion control (helpful in packet-switched connections),
  • ISDN - Uses existing digital phone lines for data and voice. May used as a backup connection for sites using Frame Relay or T1 lines.
  • LAPB - Link Access Procedure, Balanced. Created to be a connection-oriented service for X.25 connections. Large overhead is caused by timeout and windowing requirements.
  • LAPD - Link Access Procedure, D-Channel. A protocol for the D (signalling) channel of ISDN lines.
  • HDLC - High-level Data-Link Control. A data-link protocol, without a header marking for the network layer protocol being used. Each vendor has its own proprietary version of HDLC: each version works only with that vendor's equipment.
  • PPP - Can be used regardless of the make of the equipment being used.
  • ATM - created for systems that need to send voice, data, and video traffic. ATM packets are called cells, and are 53 bytes long.

The text tells us that WAN connections all use serial connections, passing one bit at a time. Cisco routers use proprietary connectors for serial ports. The connector on the router will either be a 60 pin basic connector, or a smaller smart connector. The connector on the other end of the cable must match the equipment provided by the data carrier. It can be one of several types: EIA/TIA-232, EIA/TIA-449, V.35, X.21, or EIA-530.

The text explains that a router is DTE equipment, which connects to a CSU/DSU, which connects to a demarc. From there, the lesson breaks down a bit. The connection from one CSU/DSU to another is referred to as the DCE network. Everything on the customer side of a CSU/DSU is part of the DTE network.

Students are recommended to go over the material on WAN protocols at the MC MSE site. Note that this collection of notes also includes some remarks on security, material not covered in your text.