WAN – Introduction to Wide Area Networks


In our days, large corporations have multiple branches around a continent or even around the world. You can imagine that a company that spans multiple territories has a large data network. To interconnect multiple branches, WAN connections are used because they offer both cost effective and speed. You can imagine an enterprise network as multiple LANs interconnected. In this article I will talk about the main elements that are part of WAN connections and it will serve as an introduction for the following articles. As you already know, a Local Area Connection is a network that interconnects devices like computers, servers, printers etc and is usually located in a single geographical area. Wide Area Networks span large geographical areas and basically are a collection of multiple LANs interconnected. By leasing ISPs connections, companies can connect together networks that are situated in different territories and countries around the globe. WANs use serial connections to interconnect smaller networks, because this type of communication channel provides the highest amount of bandwidth available (increased speed). The Internet is actually a very large WAN because multiple networks (ISPs or large enterprises) are interconnected together to form one huge network in which devices can communicate between each other. VPN connections, branch or regional offices all relay on WAN connections. Enterprises grow from little companies to large ones in time so you can imagine that it’s impossible to implement a hierarchical network model from the beginning.

   We will talk about WAN connections and the technologies used in the following articles, but for now you’ll have to know that WANs operate at the last two OSI layers, the Physical and Data Link layers. These layers are responsible for frame encapsulation, flow control and physical delivery of data over the medium. We have talked about these layers a while ago in the networking fundamentals articles. The Data Link layer receives packets from the network layer, encapsulates them into data frames and then sends these frames to the physical layer for further processing. The lowest layer of the OSI model provides all the physical connections, electrical standards and transmission elements needed to transfer data from one point to another. To talk about WAN connections, we will first need to understand all the elements and devices involved in this technology:
DCE device (Data Communication Equipment) – hardware device that transfers data from the service provider to the local network. You probably know by now that DCE is responsible for setting the clock signal in a serial connection.
DTE device (Data Terminal Equipment) – device used to forward data from the local network to the local loop using the DCE. The local loop is the physical connection between the DCE device and the ISP’s network.
Demarcation Point – the point where the local network is separated from the ISP’s network.
CPE (Customer Premises Equipment) – all devices that are located in the local network (routers, switches, modems, storage devices, etc).
These are some of the elements used when talking about WAN connections. Some of the hardware devices that are most common in WANs connections are:
Modem – hardware device that converts analog signals into digital signals and vice versa.
Router – the most common hardware device used in WAN technologies. Routers use CSU/DSU devices or modems to connect to the Provider’s Wide Area Network. Core routers make up the backbone of every computer network. A CSU/DSU device is used to receive frames from the Service Provider and to forward them to the local network.
WAN switch – hardware device that works at the data link layer. It is used to receive and forward frames that are part of WAN technologies such as Frame Relay, HDLC or ATM.
   Some of the well known WAN technologies used today include Circuit Switched, Packet Switched and Point-to-Point. I will talk about point-to-point connections (PPP) and Packet Switched connections (Frame Relay) in a later article because these two are needed for the CCNA exam. In Circuit Switched networks such as ISDN or PSTN, devices must first establish a circuit, from source to destination, before sending data over the medium. Circuit Switched networks use TDM (Time Division Multiplexing) technology in which every node receives a time interval in which it can transmit information over the network. In Packet Switched networks, devices do not have to establish dedicated lines before transmitting. Data is encapsulated into packets which can be routed using different paths. A router can send a piece of data using one path and another piece of the same data using another path. There are two types of Packet switched technologies used today:
Connectionless – packets sent over networks include all the information needed to route them (source and destination addresses).
Connection-oriented – each packet has a predefined route and each one includes a unique identifier for that particular path. In Frame-Relay technology, these identifiers are called DLCIs (Data Link Connection Identifiers). Multiple DLCIs are used to forward packets from one point to another and all together they from a virtual circuit (VC).
   As a conclusion, in our days there are many technologies that can be used in WAN implementations. You can either use a public or a private infrastructure to connect multiple LANs. By using the public Internet, VPN connections must be used to secure, authenticate and forward packets that are part of the same company. In private WANs, Switched or Dedicated lines can be used. As I’ve told you earlier, switched networks can be either Circuit-Switched (ISDN or PSTN) or Packet-Switched (Frame Relay,ATM or x.25). In Leased Lines, dedicated connections (E1,E3,T1,T3 etc.) are used to forward packets from branch offices to the main office.
That’s it for this article folks, I hope you’ve made a general idea of WANs. In the following articles we will continue talking about different WAN technologies used today. Have a wonderful day and enjoy IT training day.
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