Leased Lines WAN (Wide Area Network)

Introduction to Leased Lines WAN (Wide Area Network) this is a part of networking and in this article you will learn Positioning Leased-Line with LANs and Routers, Physical details of Leased-Lines and Building a WAN Link during a Lab.

Imagine that you are the primary network engineer for an enterprise TCP/IP inter network. Your company is building a new building at a site 100 miles away from your corporate headquarters. you’ll of course install a LAN throughout the new building, but you also need to connect that new remote LAN to the rest of the existing enterprise TCP/IP network. to connect the new building’s LAN to the rest of the existing corporate network, you would like some quite a WAN. At a minimum, that WAN must be able to send data from the remote LAN back to the rest of the existing network and vice versa. Leased line WANs do exactly that, forwarding data between two routers. From a basic point of view, a leased line WAN works a lot like an Ethernet crossover cable connecting two routers, but with few distance limitations. Each router can send at any time (full duplex) over the leased line, for tens, hundreds, or even thousands of miles. This section begins by giving some perspective about where leased lines fit with LANs and routers, because one main goal for a WAN is to move data between LANs. the rest of this first section explains the physical details about leased lines, followed with information about data-link protocols.

Positioning Leased Lines with LANs and Routers

The overwhelming majority of end-user devices in an enterprise or small office/home office (SOHO) network connect directly into a LAN. Many PCs use an Ethernet network interface card (NIC) that connects to a switch. More and more, devices use 802.11 wireless LANs, with some devices like phones and tablets supporting only wireless LAN connections.

Now think about a typical company that has many various locations. From a person’s resources perspective, it’d have many employees that work at many locations. From a facilities perspective, the company may need a few large sites, with hundreds or even thousands of individual branch offices, stores, or other small locations. However, from a networking perspective, think of each site as being one or more LANs that need to communicate with each other, and to communicate, those LANs got to be connected to every other employing a WAN. to attach LANs employing a WAN, the inter network uses a router connected to each LAN, with a WAN link between the routers. First, the enterprise’s network engineer would order some quite WAN link. A router at each site connects to both the WAN link and therefore the LAN, as shown in Figure 3-1. Note that a crooked line between the routers is that the common way to represent a leased line when the drawing doesn’t need to show any of the physical details of the line.

The world of WAN technologies includes many different options in addition to the leased line shown within the figure. WAN technology includes a large number of options for physical links, also because the data-link protocols that control those links. By comparison, the wired LAN world basically has one major option today—Ethernet—because Ethernet won the wired LAN battle within the marketplace back within the 1980s and 1990s.

Also Read : Ethernet LAN Overview and Types of Ethernet LAN

Physical Details of Leased Lines

The leased line service delivers bits in both directions, at a predetermined speed, using full-duplex logic. In fact, conceptually it acts as if you had a full-duplex crossover Ethernet link between two routers, as shown in Figure 3-2. The leased line uses two pairs of wires, one pair for each direction of sending data, which allows full-duplex operation.

Of course, leased lines have many differences compared to an Ethernet crossover cable. to make such possibly long links, or circuits, a leased line doesn’t actually exist as one long cable between the two sites. Instead, the telco installs a large network of cables and specialized switching devices to make its own network . The telco network creates a service that acts like a crossover cable between two points, but the physical reality is hidden from the customer. Leased lines come with their own set of terminology also . First, the term leased line refers to the very fact that the corporate using the leased line doesn’t own the line, but instead pays a monthly lease fee to use it. However, many of us today use the generic term service provider to ask a corporation that gives any sort of WAN connectivity, including Internet services. Given their long history, leased lines have had many names. Table 3-2 lists some of those names, mainly so that during a networking job, you’ve got an opportunity to translate from the terms each person uses with a basic description on the meaning of the name.

Leased-Line Cabling

To create a leased line, some physical path must exist between the two routers on the ends of the link. The physical cabling must leave the buildings where each router sits. However, the telco doesn’t simply install one cable between the two buildings. Instead, it uses what’s typically a large and complex network that makes the appearance of a cable between the two routers.

Figure 3-3 gives a touch insight into the cabling that would exist inside the telco for a short leased line. Telcos put their equipment in buildings called central offices (CO). The telco installs cables from the CO to most every other building within the city, expecting to sell services to the people in those buildings one day. The telco would then configure its switches to use some of the capacity on each cable to send data in both directions, creating the equivalent of a crossover cable between the two routers.

Although what happens inside the telco is completely hidden from the telco customer, enterprise engineers do need to know about the parts of the link that exist inside the customer’s building at the router . First, each site has customer premises equipment (CPE), which includes the router, serial interface card, and CSU/DSU. Each router uses a serial interface card that acts somewhat like an Ethernet NIC, sending and receiving data over the physical link. The physical link requires a function called a channel service unit/data service unit (CSU/DSU). The CSU/DSU can either be integrated into the serial interface card within the router or sit outside the router as an external device. Figure 3-4 shows the CPE devices, along side the cabling.

The cabling includes a short serial cable (only if an external CSU/DSU is used) plus the cable installed by the telco for the leased line itself. The serial cable connects the router serial interface to the external CSU/DSU. (Many cable options exist; the cable just must match the connector of the serial interface on one end and therefore the CSU/DSU on the opposite end.) The four-wire cable from the telco plugs in to the CSU/DSU, typically using an RJ-48 connector that has the same size and shape as an RJ-45 connector (as shown in Figure 2-7 in Chapter 2, “Fundamentals of Ethernet LANs”). Telcos offer a wide variety of speeds for leased lines. However, you can’t pick the exact speed you want; instead, you want to pick from an extended list of predefined speeds. Slower-speed links run at multiples of 64 kbps (kilobits per second), while faster links run at multiples of about 1.5 Mbps (megabits per second).

Building a WAN Link during a Lab

On a practical note, to organize for the CCENT and CCNA Routing and Switching exams, you’ll prefer to buy some used router and switch hardware for hands-on practice. If you do, you’ll create the equivalent of a leased line without a true leased line from a telco, and without CSU/DSUs, just using a cabling trick. This short topic tells you adequate information to make a WAN link in your home lab.

First, the serial cables normally used between a router and an external CSU/DSU are called data terminal equipment (DTE) cables. to make a physical WAN link during a lab, you would like two serial cables: one serial DTE cable, plus an identical but slightly different matching data communications equipment (DCE) cable. The DCE cable has a female connector, while the DTE cable has a male connector, which allows the two cables to be attached directly. The DCE cable also does the equivalent task of an Ethernet crossover cable by swapping the transmit and receive wire pairs, as shown in Figure 3-5.

The figure shows the cable details at the top, with the wiring details inside the cable at the bottom. especially , at the bottom of the figure, note that the DCE cable swaps the transmit and receive pairs, whereas the DTE serial cable doesn’t , acting as a straight through cable.

Finally, to form the link work, the router with the DCE cable installed must do one function normally done by the CSU/DSU. The CSU/DSU normally provides a function called clocking, during which it tells the router exactly when to send each bit through signaling over the serial cable. A router serial interface can provide clocking, and therefore the more recent router software versions automatically supply clocking when the router senses a DCE cable is plugged into the serial port. regardless of whether a router has an older or newer software version, you’ll want to understand how to configure serial clocking using the clock rate command. The section “Bandwidth and Clock Rate on Serial Interfaces,” in Chapter 17, “Operating Cisco Routers,” shows a sample configuration.

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