The big distinction between LANs and WANs relates to how far apart the devices can be
and still be capable of sending and receiving data. WAN connections typically run longer distances than Ethernet.
To create such long links, or circuits, the actual physical cabling is owned, installed, and
managed by a company that has the right of way to run cables under streets. Because a
company that needs to send data over the WAN circuit does not actually own the cable or
line, it is called a leased line.
Companies that can provide leased WAN lines typically started life as the local telephone company, or telco, generic term service provider.
Point-to-point WAN links provide basic connectivity between two points. To get a point-to-
point WAN link, you would work with a service provider to install a circuit. What the phone
company or service provider gives you is similar to what you would have if you made a
phone call between two sites, but you never hung up.
The telco seldom actually runs a 1000-mile cable for you between the two sites. Instead, it has built a large network already and even runs extra cables from the local central office (CO) to your building (a CO is just a building where the telco locates the devices used to create its own network).
Typically, routers connect to a device called an external channel service unit/data service
unit (CSU/DSU). The router cable and typically the CSU/DSU are owned by the telco’s customer, and the wiring to the CO and the gear inside the CO are owned by the telco. So, the telco uses the term demarc, which is short for demarcation point, to refer to the point at which the telco’s responsibility is on one side and the customer’s responsibility is on the other.
The device that provides clocking, typically the CSU/DSU, is considered to be the data communications equipment (DCE). The device receiving clocking, typically the router, is referred to as data terminal equipment (DTE).
*DCE cable needs to supply clock rate.
Customer premises equipment (CPE) refers to devices that are at the customer site,
from the telco’s perspective. For instance, both the CSU/DSU and the router are CPE
devices in this case.
Serial Cabling
WAN Speed
Original mechanism used for converting analog voice to a digital signal is called pulse
code modulation (PCM).
PCM defines that an incoming analog voice signal should be sampled 8000 times per second, and each sample should be represented by an 8-bit code.
So, 64,000 bits were needed to represent 1 second of voice.
OSI Layer 2 Point-to-Point WANs
High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP)
HDLC
HDLC needs to determine if the data passed the link without any errors;
HDLC discards the frame if errors occurred. HDLC defines framing.
HDLC header includes an Address field and a Protocol Type field, with the trailer containing a frame check sequence (FCS) field.
PPP
PPP behaves much like HDLC. The framing looks identical to the
Cisco proprietary HDLC framing. There is an Address field, but the addressing does not
matter. PPP does discard errored frames that do not pass the FCS check.
Frame Relay and Packet Switching
Packet-switching service, physical WAN connectivity exists, similar to a leased line. However, a company can connect a large number of routers to the packet-switching service, using a single serial link from each router into the packet-switching service.
Two types of packet-switching service: Frame Relay and Asynchronous Transfer Mode (ATM).
Frame Relay, has many advantages over point-to-point links, particularly when you connect many sites via a WAN.
Frame Relay is cheaper. Frame Relay networks are multiaccess networks, which means that more than two devices can attach to the network.
A leased line is installed between each router and a nearby Frame Relay switch; these links are called access links.
The difference between Frame Relay and point-to-point links is that the equipment in the
telco actually examines the data frames sent by the router. Frame Relay defines its own
data-link header and trailer. Each Frame Relay header holds an address field called a data-
link connection identifier (DLCI).
Frame Relay switches are called DCE, and the customer equipment—routers, in this case—are called DTE.
DCE refers to the device providing the service, and the term DTE refers to the device
needing the frame-switching service. At the same time, the CSU/DSU provides clocking to
the router.
Frame relay virtual circuit, the logical path that a frame travels between each pair of routers.
Service provider preconfigures all the required details of a VC; these VCs
are called permanent virtual circuits (PVC). VCs share the access link and the Frame Relay network.
Frame Relay is designed with the concept of a committed information rate (CIR). Each VC has a CIR, which is a guarantee by the provider that a particular VC gets at least that much bandwidth.
When R1 needs to forward a packet to R2,
it encapsulates the Layer 3 packet into a Frame Relay header and trailer and then sends
the frame. R1 uses a Frame Relay address called a DLCI in the Frame Relay header,
with the DLCI identifying the correct VC to the provider. This allows the switches to
deliver the frame to R2, ignoring the details of the Layer 3 packet and looking at only
the Frame Relay header and trailer.
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