WANs three major sections
The first introduces leased line WANs, a type of WAN link that has been part of enterprise networks since the 1960s.
The second part shows how Ethernet can be used to create WAN services by taking advantage of the longer
cable length possibilities of modern fiber-optic Ethernet standards.
The last part of the chapter takes a survey of common WAN technology used to access the Internet.
Leased Line
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 (fullduplex)
over the leased line, for tens, hundreds, or even thousands of miles.
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 pic 1. The leased line uses two pair of wires, one pair for
each direction of sending data, which allows full-duplex operation.
A leased line provides a Layer 1 service. In other words, it promises to deliver bits between the
devices connected to the leased line. However, the leased line itself does not define a data link
layer protocol to be used on the leased line.
Today, 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 perform a similar 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.
Ethernet as a WAN Technology
For the first several decades of the existence of Ethernet, Ethernet was only appropriate for
LANs. The restrictions on cable lengths and devices might allow a LAN that stretched a kilometer
or two, to support a campus LAN, but that was the limit.
The one Ethernet WAN service goes by two names: Ethernet emulation and Ethernet over MPLS (EoMPLS). Ethernet emulation is a general term, meaning that the service acts like one Ethernet link. EoMPLS refers to Multiprotocol Label Switching (MPLS), which is one technology that can be used inside the SP’s cloud.
The type of EoMPLS service discussed in this book gives the customer an Ethernet link between
two sites. In other words, the EoMPLS service provides a point-to-point connection between two customer devices behavior as if a fiber Ethernet link existed between the two devices.
In this case, the two routers, R1 and R2, connect with an EoMPLS service instead of a serial link. The routers use Ethernet interfaces, and they can send data in both directions at the same time. Physically, each router actually connects to some SP PoP,as shown earlier in pic 3, but logically, the two routers can send Ethernet frames to each other over the link.
WAN
Internet is simply one huge TCP/IP network. In fact, the name “Internet” comes from the core
network layer protocol: Internet Protocol. The Internet includes many LANs, and because the
Internet spans the globe, it of course needs WAN links to connect different sites.
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