What Is Wide Area Network

Published Date: 02 Nov 2017

WAN, which means Wide Area Network, is a computer network that connects smaller networks like local area networks (LAN) that are in vast distances. It simply means WAN is the main point of information exchange of one company that are separated by different locations. WAN is currently considered the fastest and most effective way of sending out computer-based information.

Fig.1 WAN

WAN

The most common and largest WAN that is existing, is the Internet. A lot of information travels within the internet. For the data to travel to its endpoint, a carrier is required. Just like with shipment of goods, there is a road where the data travels and for the data to travel a container is required. In this case, the carrier of the information is packet switching.

There are several types of packet switching: X.25 network, Frame Relay network, ATM (Asynchronous Transfer Mode) network, SMDS (Switched Multimegabit Data Service) and so on. But on this report, only two technologies will be discussed which are X.25 network and Frame Relay network.

X.25 (Packet Switching Public Data Network)

An X.25 Network is a type of packet switching which is connection-oriented. This protocol originated in the 1970s is focused more on how one endpoint connects with the communication devices or the carrier equipment called Data Circuit-terminating Equipment (DCE) while the endpoint, that sends and receives, is called Data Terminal Equipment (DTE).

Fig.2 X.25 Network

x.25.gif

The X.25 protocol was developed recommended by the International Telecommunications Union (ITU), formerly known as International Telegraph and Telephone Consultative Committee (CCITT). ITU developed a set of recommendations, that include the first three layers of ISO-7 layered architecture, on where WAN functions: Layer 1 – The Physical layer, Layer 2 – The Data Link Layer, Layer 3 – The Network Layer.

Fig.3 Three WAN Layers

http://docs.oracle.com/cd/E19069-01/sol.x25.92/806-1234/images/techintro.fig46.epsi.gif

Layer 1 or the Physical Layer covers the electrical, mechanical, functional and procedural signals required to activate, maintain and deactivate the physical circuits on which three standards (V.24, V.35 and X.21) are supported by this layer.

Layer 2 or the Data Link Layer functions as the link of the endpoints. The layer is responsible on making sure that all data from one end of a link is transmitted to the other end completely. It produces packets that are framed and these frames are known as Information frames – with sequential numbers and transfer user data, Supervisory frames – also with sequential numbers and function as the gateway of the Information frames because it will acknowledge and request retransmission or temporary suspension of the Information frames and Unnumbered frames – describes what is the mode of operation.

Layer 3 or the Network Layer’s role is with regard to the end-to-end communications between the devices, also called virtual connection/circuit. This layer can basically manage the calls and data transmitted between systems. Just like in an intersection where cars travel in different directions, there will be a traffic enforcer that controls the flow of the cars between each directions to avoid congestion. Thus, in this illustration, the cars are the data transmitted, directions are the DTE devices and traffic enforcer is the Network Layer.

Fig.4 X.25 Flow and Traffic control

Using a reserved amount of bandwidth, calls are sent by multiple systems on network trunks. This is a mechanism used especially in a high speed packet switching network. DTEs use access nodes to enter the network.

A mentioned in the earlier part of this paper, a virtual connection is the end-to-end connection of DTEs which refers to the logical communication path. This connections has two types: permanent and switched.

Permanent virtual circuit is a less flexible and less usable since it is more expensive than the other one since subscribers are charged with a flat rate per month in addition to the charge per unit of data. It is similar to a leased line when referring to the connection that is always present while Switched virtual circuit is only a temporary connection between devices. Same as with a telephone call, subscribers are charged per minute in addition to a charge per unit of data. Connection will be first established and once data is transferred the connection will be released. Each DTE devices is provided with their own DTE addresses with the same use as a telephone number.

Frame Relay Network

This type of packet switching is faster than X.25 that is why Frame Relay network is also called Fast Packet Switching technology. It can transfer up to 4 KB of variable-length packets at 56kpbs speed using a permanent virtual circuit. It also has only two layers: Physical and Data Link Layers.

In this network, the main point corresponds to each devices connected to a LAN through a separate virtual circuit. It relies on virtual circuit to create a connection across the WAN. The 10-bit address called Data Link Connection Identifier (DLCI) assists the Frame relay in identifying a virtual circuit. Up to 1024 possible DLCIs can be defined and each of DLCI defined is unique not throughout the network but on the local link meaning each will have their permanent route and destination.

Fig.5 Frame Relay Network

Frame Relay.gif

Each PVC for this network is designated with several important parameters. One of these is its Committed Information Rate (CIR) which is the guaranteed number of bits per second that you will receive on a specified period, despite a busy link. It should also transmit the complete data that was, meaning no data should be discarded. Another parameter is Committed Burst Size (Bc), it is the largest number of consecutive bits agreed by the network without discarding any data. Lastly, there is the Excess Burst Size (Be). This is assigned by some network because on this parameter, some data may be discarded upon transmission. Network discards data when the user has already exceeded the data amount agreed to be carried. Data can also be discarded when there is a detected corrupted transmission. This is detected using the Cyclic Redundancy check. Flow control is always done when transmission happens but it cannot be avoided to have network congestion especially when the users of the network transmit data near or more than the network’s capacity thus will also result to data discard.

Fig.6 Frame Relay Network

Because of network congestion that causes to discard data, Frame Relay network, unlike X.25, depends on the higher-layer protocols so that the devices can still recover despite the congestion. The higher layers will identify if the network has discarded data.

ITU has also recommended set of standards for Frame Relay: I.233 – service description and Q.933 – for access signaling.

X.25 versus Frame Relay Network

X.25 Advantages:

- Powerful addressing facilities, routing and relaying of data is done through a series of nodes and networks with the help of Network Layer.

- Congestion and error control, with the help of Supervisory frames, retransmission is done when there is congestion and error checking is done at every node.

- High availability in the face of node and line failures since network can possibly do rerouting

X.25 Disadvantages:

- Delay with the queue

- Communication links is lower in speed

- Data discard

Frame Relay Advantages:

- Cost saving since it does not use leased lines

- Runs on multiple protocol networks

- Controlled user community

- Guaranteed with a minimum throughput

- Error handling capability for data to be transmitted completely

Frame Relay Disadvantages:

- Inherently unsuitable for delay-sensitive traffic, such as voice and video

- Selective operation, only requires high-quality digital circuits

- Not entirely standardized