Introduction to Wide Area Networks

Before you start

Objectives: learn what is WAN, why it is used, and which cabling and signaling methods can be used in WANs.

Prerequisites: no prerequisites.

Key terms: WAN, T1, T3, E1, E3, J1, J3, OC, cabling, speed

 What is WAN

WAN is used to connect two or more networks over a long distance. The first thing we can distinguish between LAN (local area network) and WAN is that WAN usually covers much greater distance. Another difference is the speed. The speed on LAN is usually much greater than on WAN. Also, on WAN we use different hardware devices and different communication protocols.

WAN can be used to connect different sites (different locations) or to connect our LAN to the Internet. For example, let’s say that our organization has offices in different cities or countries and we want to enable communication between them. This is where we will implement WAN to enable communication between those two offices. Also, we can use WAN to connect our organization to the Internet.

WANs can use different standards, and with different standards comes different cabling.

T-Carrier System

The first we should mention is the T-carrier system. It was introduced in 1960, but we can still encounter it today. It is completely digital system. To create T-carrier we use 2 pairs of copper wiring (4 wires). The cabling is usually unshielded twisted pair (UTP), but it can also run using coaxial cabling, fiber-optic cabling and even satellite connections. Two wires are used for transmitting, and two are used for receiving. On T-carrier we can use voice and data communication. T-carrier uses point-to-point connection between the CSU/DSU (Channel Service Unit / Data Service Unit) device in our organization and the ISP (or remote site) on the other side. CSU/DSU is then usually connected to the router in our organization which then interfaces with our network.

There are two specifications with T-carrier system. The first is T1, which is composed of 24 multiplexed channels on 2 pair of copper wiring. Each channel can transfer 64 Kbps, so we have a total bandwidth of 1.544 Mbps. Because T1 uses point-to-point connection, we only have one particular line that we have to worry about. because of that, the data is framed a bit differently than on Ethernet networks. On T1 the DS1 frame is used which is composed of 25 separate fields. The first bit in a frame is called a framing bit. Then we have 24 fields which are 8 bits in length, one field for each one of the channels. So, 24 fields carry data for each channel. That frame is sent 8000 times per second, which gives 1.544 Mbps.

Today, T1 speed is not enough anymore, so this is where T3 specification comes into play. T3 is also part of T-carrier system. The difference between T1 and T3 is that T3 provides more channels. Instead of 24 channels, T3 provides 672 channels, and each channel provides 64 Kbps. The maximum throughput with this configuration is 44.736 Mbps. T3 lines usually run over fiber-optic cable.

E-Carrier System

Another system we need to be familiar with is the E-carrier system. E-carrier is actually similar to T-carrier system. The difference is that E-carrier is implemented in Europe, while T-carrier is specific to USA. E1 specification can transfer data at around 2.048 Mbps, using 32 channels (64 Kbps each). We also have E3 line which transfers 16 E1 signals at the same time, which gives us 32.368 Mbps.  E1/E3 connections also require a CSU/DSU.

J-Carrier System

Another system we need to be familiar with is the J-carrier system. This system is implemented in Japan, and is similar to the T-carrier system. We also have J1 and J3 specifications, J1 being the same as T1. The J3 specifications provides 480 channels, giving a total of 32 Mbps. J1/J3 connections also require a CSU/DSU.

Optical Carrier Specifications

Optical Carrier specifications are used to define communication on fiber optic cabling used with SONET networks (Synchronous Optical Network). It is a series of standards used by telecomunication companies to enable high speed WANs. There are several optical carrier levels that we should be familiar with.

OC levels

The first optical carrier specification is OC 1. With OC 1 level we get 51.84 Mbps. OC 1 is also called a Base rate. There are many more OC levels, each level being multiple of OC 1. For example, we have OC 3 which transfers data at 155.52 Mbps. Another common OC level is OC 12 which provides 622.08 Mbps. Another is OC 24 with 1.244 Gbps. We also have OC 48 with 2.488 Gbps, OC 192 with 10 Gbps, OC 256 with 13.271 Gbps. OC 768 with 39.82 Gbps.

Transfer Methods

In general, there are two different transfer methods that can be used to transfer data in WAN. The first one is called circuit switched network. In this method, if we have two locations that are connected using WAN, the permanent connection is established between them. The permanent circuit is allocated between those two locations and there is only one path which can be used for data flow.

Another method is a packet switched transfer method. This is similar to how LANs work. In this method data is divided into smaller pieces. That data is then transferred using series of routers between two locations. In this case, there might be many different paths for data flow. Also path can change dynamically depending on the load on certain routers.

Circuit switched method is older technology and most newer are packet switched, since it allows multiple host to use network resources at the same time, and in that way share the available bandwidth. In circuit switched network the whole bandwidth is allocated for the organization, even if there is no actual data exchange.