Before you start
Objectives: Learn about different parts of WAN, how we call them, who is responsible for which part of the WAN, and about different methods to send data in WAN.
Prerequisites: no prerequisites.
Key terms: WAN, structure, Packet Switching, Circuit Switching, organization
Organizations often rely on (contract with) Service Providers who have the necessary equipment to connect different network locations over long distances. When organizations do that, we have a typical structure of a WAN. Service Provider will give us access to the WAN and is responsible for moving data between our locations. We can see that structure on the picture below.
As you can see, WAN is typically represented with the cloud icon. The WAN cloud consists of many devices and medium that connect different locations. WAN cloud is maintained by large telecommunication companies. We call this the cloud because we don’t actually know how it is organized and how it works (few people really do). It also isn’t that important to organizations. We only know where we connect to the WAN, and we do that by signing the WAN service by the service provider.
WAN can be organized by using different technologies. This can be Public Switched Telephone Network (PSTN) or over Internet. If we use PSTN, then our service provider is our telephone company over Local Exchange Carrier (LEC). If we connect over Internet, then the WAN service provider is our Internet Service Provider (ISP). The point where local lines connect to the cloud is often called Central Office (CO), so it represents the WAN entry and exit point. The line which connects us with the ISP or LEC is called the Local Loop. Local loop can be twisted pair, fiber optic or other media. The point where the Local Loop enters our organization is called Demarcation point. Service Provider is responsible for everything between the two Demarcation Points. Demarc is sometimes called the Minimum Point of Entry (MPOE) or End User Point Of Termination (EUPOT).
Important Parts of WAN
We are responsible for everything in our location. The equipment that is sometimes used to connect to a WAN is called CSU / DSU (Channel Service Unit / Data Service Unit). The CSU terminates the digital signal from the WAN. The DSU converts the signal into a format that can be read by routers and other devices at our location. CSU / DSU can come as a separate device (often supplied by service provider), and which we then connect to our router. Some routers have integrated CSU / DSU, and in that case we connect our router directly to the line that comes from our service provider.
WAN Transfer Methods
One way of transferring data in the cloud is by using method called Circuit Switching, which establishes a dedicated path from one end to the other, trough the cloud. When we establish a connection, a circuit is established between the two locations, using dedicated devices in the cloud. In this case everything travels the same path in the circuit, over a dedicated connection. The connection stays active, whether we send data or not. The connection may be permanent, or it may be a temporary one.
Another method of moving data in the WAN is called Packet Switching. In this method information that needs to be sent is divided into packets. Each packet is then routed individually trough the WAN cloud, and can take different paths to the destination (the most efficient route). On the destination, packets are then reassembled in the correct order. Packet switching is used in all IP based networks.