We have came a long way in CCNA Prep, but until now we have not discussed concepts of the wide area network (WAN). I have seen that many people do not feel comfortable with WAN technologies because they don’t have a basic understanding of WAN. So let’s begin with some basic questions, such as “What is WAN?”
“WAN technology is a packet switching (data communication) or circuit switching (traditional telephony) network that operates beyond the geographic scope of a local area network.” In this article, our main area of focus will be on packet switching technologies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. The one thing to remember about traditional telephony is that, in a public switched telephone network (PSTN), circuit switching technology like integrated services digital network (ISDN) is used for wide area network connections. ISDN was the main backbone of carrying voice traffic for more than 100 years and is still used in legacy telecommunication networks. If you study Cisco Voice technology, you will get a chance to know more about it.
WANs are different from LANs in several ways. While a LAN connects computers, peripherals, and other devices in a single building or other small geographic area, a WAN allows the transmission of data across greater geographic distances. In addition, an enterprise must subscribe to a WAN service provider to use WAN carrier network services. A LAN is typically owned by the company or organization that uses it for running the network.
WANs use facilities provided by a service provider or carrier, such as a telephone or cable company, to connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users. WANs generally carry a variety of traffic types, such as voice, data, and video.
Here are the three major characteristics of WANs:
- WANs generally connect devices that are separated by a broader geographical distance than can be served by a LAN or MAN.
- WANs use the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers.
- WANs use serial connections of various types to provide access to bandwidth over large geographic areas.
Why Is a WAN Necessary for an Enterprise Network?
LAN technologies provide both speed and cost efficiency for the transmission of data in organizations over relatively small geographic distances. However, there are other business needs that require communication among remote sites, including the following:
- People in the regional or branch offices of an organization need to be able to communicate and share data with the central site.
- Organizations often want to share information with other organizations across large distances. For example, software manufacturers routinely communicate product and promotion information to distributors that sell their products to end users.
- Employees who travel on company business frequently need to access information that resides on their corporate networks.
In addition, home computer users need to send and receive data across increasingly greater distances.
Here are some examples of daily usage of WAN technology:
It is now common in many households for consumers to communicate with banks, stores, and a variety of providers of goods and services via computers.
Students do research for classes by accessing library indexes and publications located in other parts of their country and in other parts of the world.
Since it is obviously not feasible to connect computers across a country or around the world in the same way that computers are connected in a LAN with cables, different technologies have evolved to support this need. Increasingly, the Internet is being used as an inexpensive alternative to using an enterprise WAN for some applications. New technologies are available to businesses to provide security and privacy for their Internet communications and transactions.
A Point-to-Point WAN Link
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay.
Figure 1 illustrates a typical point-to-point link through a WAN.
WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.
A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model.
Figure 2 illustrates two routers at remote ends of a WAN that are connected by WAN switches
An access server acts as a concentration point for dial-in and dial-out connections.
Figure 3 illustrates an access server concentrating dial-out connections into a WAN.
A modem is a device that interprets digital and analogue signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analogue communication facilities. At the destination, these analogue signals are restored to their digital form.
Figure 4 illustrates a simple modem-to-modem connection through a WAN.
A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a router to a digital circuit, such as a T1. The CSU/DSU also provides signal timing for communication between these devices.
Figure 5 illustrates the placement of the CSU/DSU in a WAN implementation.
ISDN Terminal Adapter
An ISDN terminal adapter is a device used to connect ISDN basic rate interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analogue to digital signals.
WAN Physical Layer Terminology
One primary difference between a WAN and a LAN is that a company or organization must subscribe to an outside WAN service provider to use WAN carrier network services. A WAN uses data links provided by carrier services to access the Internet and connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users.
The WAN access physical layer describes the physical connection between the company network and the service provider network. The figure illustrates the terminology commonly used to describe physical WAN connections, including:
Customer premises equipment (CPE)—The devices and inside wiring located at the premises of the subscriber and connected with a telecommunication channel of a carrier. The subscriber either owns the CPE or leases the CPE from the service provider. A subscriber, in this context, is a company that arranges for WAN services from a service provider or carrier.
Data communications equipment (DCE)—Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. The DCE primarily provides an interface to connect subscribers to a communication link on the WAN cloud.
- Data terminal equipment (DTE)—The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE.
- Local loop—The copper or fiber telephone cable that connects the CPE at the subscriber site to the CO of the service provider. The local loop is also sometimes called the “last mile.”
So let’s talk briefly about common packet-switching technologies that are used for enterprise WAN networks. They include MPLS, Frame Relay, ATM, and legacy X.25. The latest and most popular WAN technology day is multiprotocol label switching (MPLS), which is not in the CCNA syllabus, so we will discuss this in other articles.
X.25 was the first popular packet switched technology used in WAN; it is a legacy network layer protocol that provides subscribers with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection.
Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. For these applications, the low bandwidth and high latency are not a concern, and the low cost makes X.25 affordable. X.25 link speeds vary from 2400 b/s up to 2 Mb/s. However, public networks are usually low capacity, with speeds rarely exceeding above 64 kb/s.
X.25 networks are now in dramatic decline, being replaced by newer Layer 2 technologies such as Frame Relay, ATM, and ADSL. However, they are still in use in many portions of the developing world, where there is limited access to newer technologies.
Although the network layout appears similar to X.25, Frame Relay differs from X.25 in several ways. Most important, it is a much simpler protocol that works at the data link layer rather than the network layer. Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter. Frame Relay offers data rates up to 4 Mb/s, with some providers offering even higher rates. Frame Relay VCs are uniquely identified by a DLCI, which ensures bidirectional communication from one DTE device to another. Most Frame Relay connections are PVCs rather than SVCs.
Frame Relay provides permanent, shared, medium-bandwidth connectivity that carries both voice and data traffic. Frame Relay is ideal for connecting enterprise LANs. The router on the LAN needs only a single interface, even when multiple VCs are used. The short-leased line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.
If you want to know more about Frame Relay, then please go through with my previous articles about the topic”
Asynchronous transfer mode (ATM) technology is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5-byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.
The 53-byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. Accessing the WAN, Version 4.0, the cell is carrying segmented network layer packets; the overhead is higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20% greater bandwidth than Frame Relay to carry the same volume of network layer data.
ATM was designed to be extremely scalable and can support link speeds of T1/E1 to OC-12 (622Mb/s) and higher. ATM offers both PVCs and SVCs, although PVCs are more common with WANs. And, as with other shared technologies, ATM allows multiple VCs on a single leased-line connection to the network edge.
I know this article is based on theory but I tried to make it interesting with giving information as much as I can. Hopefully it has cleared up all of the confusion regarding WAN. If you still feel any doubts, as a writer I always feel happy if I get any feedback from my readers.
Guide to Cisco Certified Network Associate certification by Todd Lamle, Sybex press.
Guide to Cisco Certified Network Associate by Richard Deal.
31 Days Before Your CCNA Exam: A Day-By-Day Review Guide for the CCNA Exam written by Allan Johnson.
Cisco WAN Quick Start by Ronald W. McCarty