Cabling the WAN

Cabling the LAN

These notes guide you through the understanding of cabling WAN's.

Introduction

WAN Serial Connections

       WAN Protocols and Connection Methods
       Some WAN Connectors

Routers and Serial Connections

       Normal DTE to ISP's DCE Link
       DTE to DCE WAN Link Simulation

Summary

Introduction

On completion of these notes you should...

• understand that WAN connections are made over serial lines
• have a basic knowledge of the PPP and Frame Relay protocols
• understand common WAN connection methods, such as dial-up, ISDN, DSL and cable
• understand a router acting as a DTE usually connects to a DCE using a serial link
• understand a router can connect to another router using a back-to-back serial link.

Wan Serial Connections

Perhaps you are reading these notes at college, or perhaps at home. Either way, unless you have saved this web page to your hard drive, you will be reading them over an Internet connection. Somehow, the contents of this page traveled from the web server it is stored on across the Internet , to your ISP's server and then onto your PC.

You should be familiar with the patch cables used on local area networks - used to connect devices together so they can communicate. However, to connect to and communicate with devices over a WAN you have to use a serial cable rather than a patch cable. Serial cables transmit data differently to LAN cables and other cables like parallel cables. With serial cables, the data is sent along the cable one bit at a time.

Now, I don't suppose you ran a serial cable all the way from your computer to your ISP's computer - did you? No, thought not!

If you have a dial-up connection, you will have a serial cable that runs from your pc to your modem. Another cable runs from your modem to your telephone socket, that connects over the telephone line to your ISP's network.

On a LAN, the principle is the same, but the device used to connect to the ISP's network is likely to be a device with a higher throughput, such as an ISDN or ADSL line or dedicated high bandwidth lines like T1 or E1. Most WAN links are simply methods of serially connecting two routers through the public telephone network. Some links happen to be faster than others.

The table below shows the maximum length serial cable you should use for the different bandwidths.

  
WAN Protocols and Connection Methods

Now let's look at different ways of connecting to a WAN:-

~~ Point-to-Point Protocol (PPP) ~~

PPP is a protocol for transporting IP traffic over point-to-point links. Many WAN serial connections use PPP as a method for connecting to an ISP. Typically, PPP supports modem dial-up users, as well as ISDN and DSL connections.

~~ Frame Relay ~~

Frame Relay is another kind of protocol used for transporting traffic over serial links. Frame Relay is a commonly used on both dedicated lines such as T1 and also X.25 networks. Frame Relay can allow multiple "virtual circuits " to exist on a single physical connection. It's main advantage is it's speed.

~~ Dial-Up Modem ~~

A modem is a device that converts analog signals to digital and vice-versa. At the sending end, a modem converts digital computer data to analog, so it can be transmitted over an analog telephone line. At the receiving end, a modem converts the analog signal back into digital signals and passes it along to the digital device (e.g. a PC.) The typical bandwidth of a modem today is 56 kbps

A dial-up modem uses the PPP protocol for communications. Typically a dial-up connection over PPP has the following features:

• Dial-on-demand: Activate a PPP connection and dial the phone when packets need to be sent out; bring the connection down after some period of inactivity.
          
• Redial: Activate a PPP connection whenever it goes down, to keep a line up.

~~ Integrated Services Digital Network (ISDN) ~~

ISDN was originally designed to replace the fairly slow dial-up modem over the plain old telephone system (POTS.) An ISDN connection is usually composed of a single delta channel (D channel) and two bearer channels (B channels).

The D channel runs at 16 kbps and is used for signaling and other link control tasks. The B-channels are used for transmitting data and run at 64 kbps each, making a total data bandwidth of 128 kbps. ISDN are dial-on-demand connections and PPP is the protocol typically used to carry data over the B channels.

~~ Digital Subscriber Line (DSL) ~~

Home users are demanding higher speed connections and so DSL broadband connections are becoming more and more popular.

DSL routers can come in two types - routers with usb connections to the PC and routers that connect to the PC's network card using UTP cable. Amazingly , residential DSL connections can achieve T1 or even E1 speeds over existing telephone lines. Speeds from 512 kbps to over 2000 kbps are available.

~~ Cable Modem ~~

Cable connections are also becoming increasingly popular with home users. Such connections use existing coaxial cable TV lines. A coaxial cable line achieves speeds comparable to that of DSL.

~~ T1/ E1 Dedicated Leased Lines ~~

A T1 link is a dedicated digital highspeed circuit - leased from the phone company. A T1 line is contains 24 digital channels, each of which can carry one digitally encoded voice conversation. It provides a bandwidth of 1.544 Mbps. T2 to T4 lines are even faster. For example, the bandwidth of a T4 line is over 274 Mbps.

E1 lines are used in Europe and they are formed from combining 32 digitized phone lines into a single digital data stream. The resulting data stream has a rate of 2.048Mbps.

  
Some WAN Serial Connectors

The table below summarizes the different connectors needed for various WAN connection methods.

Routers and Serial Connection

Routers are essential for routing data between devices on a LAN. A router also forms a LAN to WAN connection, bridging between a physical LAN connection and an synchronous serial WAN link. So, how do you connect a router up to an ISP's connection point and connect to a WAN?

Normal DTE to ISP's DCE Link

Normally, a router usually acts as a DTE, (Data Terminal Equipment.) which connects to an ISP's modem over a serial connection. The ISP's modem is a DCE (Data Communications Equipment) device.

The WAN serial link is strictly synchronous, which means data is transmitted at strict intervals. The DCE usually sends the clocking (timing) signal to the router, which allows it to correctly judge when to transmit.

DTE to DCE WAN Link Simulation

Suppose you want to string two routers back-to-back, perhaps to simulate a WAN environment, then one router would have to act as a normal DTE but the other would have to act as a DCE.

The router acting as the DCE will set the clocking signal. To connect the two routers, two serial cables are used connected back-to-back. For the DTE you use a normal serial cable, ending in a male connector. For the DCE you use a cable ending a female connector. You connect the two serial cables together. Both routers have to be configured appropriately, one to act as a DTE and one as a DCE.

~~Activity~~

Summary

On completing these notes you should have learned the following key points:-

• WAN's use serial communication links. Serial communication means each bit is transmitted one after another.
• Two common WAN protocols are PPP and Frame Relay. Dial-up, ISDN and DSL connections usually run over PPP. Dedicated links, such as T1 can run over Frame Relay.
• To connect a dial-up modem to a telephone socket use phone cable and an RJ-11 connector.
• ISDN has two D channels and one B channel. To connect the ISDN BRI port to an ISP's device, a UTP Category 5 straight-through cable with RJ-45 connectors is used.
• To connect a DSL router to a telephone socket use phone cable and an RJ-11 connector.
• To connect a cable modem to the TV outlet, you use coaxial cable and a BNC connector.
• A router is usually the DTE device. You need a serial cable to connect to a router to an ISP's DCE device.
• To connect one router to another, so one acts as a DTE and the other acts like an ISP's DCE device, you use back-to-back serial cables.