Introduction to WANs
In the previous chapters, we have been primarily focused on LAN technologies, however, as an enterprise grows, so does its networking needs. Consider company ABC, they formed in 2010 in New York, however, over the last couple of years they have grown rapidly and they now have three branches, one in Los Angeles, Miami and they recently diversified into London.
LANs would not be viable for communication over the four geographical information, and therefore there is need for WANs to be incorporated. In the next few chapters, we will look at the various WAN technologies that enterprises can use.
In this chapter, we will look at some of the concepts and technologies that are used in the WAN.
Definition of a WAN
A WAN network can be defined as a network that extends and operates over a larger geographical area as compared to a LAN.
Unlike LAN networks, which connect users and intermediary devices within a small area such as a building complex, WAN networks are large and they span over large geographical distances. The administration of the WAN is usually by the service provider and therefore for an enterprise to use the WAN, they have to pay.
The characteristics that mainly differentiate the WANs from the LANs are:
- Geographical scope. WANs can extend over very large geographical distances
- The WAN networks are mainly administered by the service providers such as cable companies, internet service providers among others.
- In the LANs, we primarily use parallel connections between the various devices, whereas in the WAN we mainly use the serial cables since they can span over large distances.
WANs and the OSI Model
The operation of the WAN is usually at the physical and the data link layers of the OSI model. The standards that are used usually describe how the signals are transmitted, and how the frames are addressed, encapsulated and given flow control.
At the physical layer, the WAN describes how electrical signals are transmitted, the types of cables, the speeds and the connections from the ISPs perspective.
At the data link layer, the encapsulation method, flow control, addressing of the frames are described.
WAN physical layer concepts
There are several concepts that describe the operation of WANs at the physical layer. The diagram below shows some of the terms that are used in relation to WAN technologies.
- CPE (Customer Premises Equipment – these are the devices that are used by the subscriber to connect to the service provider.
- DCE (Data Communications Equipment) – this is the device that is used to terminate data to the local loop. This means that it gets data from the DTE devices such as the router and converts it into a form that can be transmitted over the physical medium of the ISP.
- DTE (Data Terminal Equipment) – this are the devices that get the data from the DCE and transmit them to the inside network, typically, a router is usually the DTE device.
- Demarcation point – this is the point in the network where the service provider and the customer have agreed upon as to where responsibility for the WAN connection changes. It can be described as a border between the ISP and the CUSTOMER.
- Local loop – the cables that connect the CPE to the service provider is called the local loop. Typically, this can be a cable that connects the company from the main cabling closet to the main trunk cable.
- Central Office – this is a building that is used by an ISP to provide services to a particular area.
Physical layer protocols
The physical layer standard used in the WAN are shown below. They describe how the DTE and DCE interact, the electrical standards, the cabling types as well as the connectors to be used.
- EIA/TIA- 232 is a protocol that specifies speeds of up to 64Kbps using a 25 pin connector for short distances.
- EIA/TIA- 449/530 is a standard protocol that uses a 36 pin connector and offers speeds of up to 2Mbps, it can also span over larger distances than the EIA/TIA standard.
- EIA/TIA -612/613 is a standard that provides speeds of up to 52Mbps using a 60 pin connector. It is also reffered to as (HSSI) High Speed Serial Interface Protocol.
- V.35 is an ITU standard used between a DCE and DTE device, it offers speeds of up to 2Mbps using a 34 pin connector.
- X.21 protocol is defined by the ITU and it uses a 15 pin connector.
WAN connection options
In this type of connection, there is usually a dedicated circuit between the source and destination network, through the ISP. An example of this is when a person makes a telephone call. The dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the caller to the called party.
ISDN (Integrated Services Digital Network) and PSTN (Public Switched Telephone Network) are good examples of Circuit switched WAN technologies.
In this type of connection, the data is split and transmitted over the common network, the packets are then reassembled at the destination network. With this type of connection, many user nodes can use the same network.
With this connection option, we have two ways to determine the type of link in use.
- Connectionless systems – each packet contains full address information
- Connection oriented – these systems first determine the route to the destination before sending the packets.
Data Link Protocols
There are various Data link layer protocols that are used in the WAN. These define how the data is communicated from the source network to the destination. There are various protocols that can be used. In this course however, we will discuss the protocols listed below.
- Frame relay
WAN technologies in use
There are several technologies that are employed in the WAN, in this course, however, you are not expected to configure them. Most of these technologies are covered in more advanced courses such as CCNP.
- metro Ethernet
In as much as these technologies have not been discussed in this course, it would be wise to research them and know what they entail.