WAN or Wide Area Network provides connectivity between offices and remote branches of a company.

There are 2 broad categories of WAN networks, which differ by how different sites can communicate with each other, – multi-access and point-to-point networks.

Multi-access vs Point-to-Point WAN

In the multi-access network, the Service Provider (SP) network connects WAN sites in a full-mesh scheme. Depending on the SP infrastructure, this design can provide better performance because of direct site-to-site connectivity. Examples of multi-access networks include:

• Layer 3 MPLS service
• Metro Ethernet E-LAN service (L2 VPLS)
• Older WAN technologies, such as Frame Relay and ATM
• VPN-based WANs, such as DMVPN and SD-WAN

The current CCNA blueprint does not examine WANs in details. However, to understand IP routing, which is a key component of WAN, you need to understand the basics of the two types of WAN links (Not WAN Networks): serial links and Ethernet WAN links. Routers sit at sites and this can be miles to hundreds of miles apart and these these WAN links connect these routers for communications between the remote sites.

Imagine you on a typical day at the branch office location sitting at your table with your PC connected to your office LAN, either via an Ethernet cable or using a wireless LAN. However, you happen to be checking promotional information on a website whose web server sits at the home office of the company and to make this work, the data must travels over one or more wide-area network (WAN) links.

WAN technologies define the physical (Layer 1) standards and data-link (Layer 2) protocols used to communicate over long distances and two such technologies are

–leased-line WANs and

–Ethernet WANs.

(1)Leased-line WANs

Leased-line WANS have been an option for networks for a long time and are becoming much less common today. The leased line service delivers bits in both directions, at a predetermined speed, using fullduplex logic. In fact, conceptually it acts as if you had a full-duplex crossover Ethernet link between two routers, as shown in the conceptual view.

The leased line uses two pairs of wires, one pair for each direction of sending data, which allows full-duplex operation. 

WAN communications are usually between different geographical locations with long distances between sites. A leased line does not actually exist as a single long cable between the two sites.  The telephone companies creates the leased lines, installs a large network of cables and specialized switching devices to create its own computer network. Although, the customer does not  know all the details of how a telco creates a particular leased line, but you the enterprise engineer need to know about the parts of the link that exist inside the customer’s building at the router and for the purposes of CCNA, you can think of any serial link as a point-to-point connection between two routers.

Leased lines terminologies

Ok, so, the term leased line means the company using the leased line don’t own the line but instead pays a monthly lease fee to use it. Below are some of the names you will find for leased lines used by different companies and the reason for including this is for you not to be confused especially in an exam where you see a T1 line instead of Lased line and you are wondering which line is that, it means the same thing.

Some of the many names for leased lines,

T1 line is a measure of speed, so, saying “We have a T1 line between our two offices” means, you have a 1.544mbps, you can have a T1 DSL modem, T1 Cable Modem, T1 Metro, Frame relay, especially all that is limited to 1.544mbps. Europeans use E1 and theirs is 2.048mbps and both, Very reliable, stable, slow and expensive

NOTE: To create a leased line, physical path must exist between the two routers on the end of the links. The physical cabling must leave the customer buildings where each router sits. However, the telco does not simply install one cable between the two buildings. Instead, it uses what is typically a large and complex network that creates the appearance of a cable between the two routers.

                                    Physical side of WAN

Physical side of WAN

Plan 1

1. So, the old school connection is, the telco run a cable into the building and terminates at a wall jack.
2. RJ45 cable from the telco wall-jack goes into the CSU/DSU as shown.
3. The CSU/DSU takes all the signals from the carrier and converts into serial flavor which then goes to the port behind the CSU/DSU.
4. The the CSU/DSU serial port of the telco device is connected to the clients router through another serial cable.
5. The router then transmits that to the switch of the client for internal use.

OR Plan 2

1. So, the old school connection is, the telco run a cable into the building and terminates at a wall jack.
2. RJ45 cable from the telco wall-jack goes into the back of the client’s router which has a CSU/DSU card inserted eliminating the CSU/DSU device.
3. The client’s router then transmits that through the 10/100 ethernet port to the switch for internal use.

On the OSI model, WAN technologies occupies layer 1 (Physical layer)and layer 2 (Data link Layer); everything else from Layer 3-7 the application layer remains the same. 

While leased line provides a Layer 1 service using the physical cables delivering bits between the router devices connected to the leased line, the leased line itself do not define a data-link layer protocol to be used on the leased line.

Therefore, the two most popular data-link layer protocols used for leased lines between two routers are

• High-Level Data Link Control (HDLC) and

• Point-to-Point Protocol (PPP).

All data-link protocols Role:

To control the correct delivery of data over a physical link of a particular type. For example, the Ethernet data-link protocol uses a destination address field to identify the correct device that should receive the data and an FCS field that allows the receiving device to determine whether the data arrived correctly. HDLC provides similar functions. 

In summary, a leased line with HDLC creates a WAN link between two routers so that they can forward packets for the devices on the attached LANs.

• The leased line itself provides the physical means to transmit the bits, in both directions.
• The HDLC frames provide the means to encapsulate the network layer packet correctly so that it crosses the link between routers.

Pros of Leased lines   

• These lines are simple for the customer, are widely available, are of high quality, and are private.

Cons of Leased Lines

• Compared to newer WAN technologies, higher cost and typically longer lead times to get the service installed.
• By today’s standards, leased-line LANs are slow, with faster speeds in the tens of megabits per second (Mbps).

(2) Ethernet WAN

At some point in time in the history of Ethernet, Ethernet was only appropriate for LANs. The cable length restrictions and devices allowed a LAN that stretched not more than two kilometers to support a campus LAN and that was it. As time passed, Ethernet standards were improved by IEEE in ways that made Ethernet a reasonable WAN technology.

Some Examples,

• the 1000BASE-LX standard uses single-mode fiber cabling, with support for a 5-km cable length;
• the 1000BASE-ZX standard supports an even longer 70-km cable length.

As time went by, and as the IEEE improved cabling distances for fiber Ethernet links, Ethernet became a reasonable WAN technology. Ethernet WAN links use the same data-link protocols as Ethernet LANs, but they use additional features to make the links work over the much longer distances required for WANs. 

This  service provider model has many of the same ideas of how a telco creates a leased line, as shown earlier but now with Ethernet links and devices and not leased lines.  This time around, the Ethernet link (fiber) runs from the customer building and connects to a nearby SP (Service Provider) location called a point of presence (PoP).

Point of presence

Example, a service provider like AT&T will pay the Government and dig up the roads and run fiber cables underground and terminate them at various pop spots they have earmarked and those  Pops are mini AT&T point of presence for companies and residents who wish to enjoy high speed internet, sometimes your speed will vary based on your location to the point of presence.

Examples of point-to-point connections include:

• Metro Ethernet E-Line service (L2 MPLS-based or dedicated physical fiber or DWDM)
• VPN-based site-to-site links

Common names for WAN services

Ethernet WAN: A generic name to differentiate it from an Ethernet LAN.
Ethernet Line Service (E-Line): A term from the Metro Ethernet Forum (MEF) for the kind of point-to-point Ethernet WAN service shown here.

Ethernet emulation: A term emphasizing that the link is not a literal Ethernet link from end to end.

Ethernet over MPLS (EoMPLS): A term that refers to Multiprotocol Label Switching (MPLS), a technology that can be used to create the Ethernet service for the customer.

In summary, I have included these diagrams and amount of information for you to have a good understanding so that when you see the real life devices in your area, you can immediately relate it to your knowledge.

Q&A