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So what is it that makes something a
wide area network (WAN)
instead of a local area
network (LAN)? Distance is the first thing that comes to mind, but these days, wireless
LANs can cover some serious turf! So is it bandwidth? Here again, in many places really
big pipes can be had for a price, so that’s not it either. Well, what then? Perhaps one of
the best ways to tell a WAN from a LAN is that you generally own a LAN infrastructure
but you lease a WAN infrastructure from a service provider.
I’ve already talked about a data link that you usually own (Ethernet), but now we’re going to
take a look at the data links you most often don’t own but instead lease from a service provider.
The key to understanding WAN technologies is to be familiar with the different WAN
terms and connection types often used by service providers to join your networks together.
Introduction to Wide Area Networks
505
There are many new WANs available today, but again, this chapter is focusing
on the Cisco ICND1 exam (CCENT) objectives only.
Defining WAN Terms
Before ordering a WAN service type, it would be a good idea to understand the following
terms, which are commonly used by service providers:
Customer premises equipment (CPE)
Customer premises equipment (CPE)
is equipment
that’s owned by the subscriber and located on the subscriber’s premises.
Demarcation point
The
demarcation location
is the spot where the service provider’s responsibility
ends and the CPE begins. It’s generally a device in a telecommunications closet owned
and installed by the telecommunications company (telco). The customer is responsible for cable
(extended demarc) from this box to the CPE, which is usually a connection to a CSU/DSU or
ISDN interface.
Local loop
The
local loop
connects the demarc to the closest switching office, called a
central office.
Central office (CO)
This point connects the customers to the provider’s switching network.
A
central office (CO)
is sometimes referred to as a
point of presence (POP)
.
Toll network
The
toll network
is a trunk line inside a WAN provider’s network. This network
is a collection of switches and facilities owned by the ISP.
It is important to familiarize yourself with these terms, as they are crucial to understanding
WAN technologies.
WAN Connection Types
A WAN can use a number of different connection types, and this section will provide you with an
introduction to the various types of WAN connections you’ll find on the market today. Figure 10.1
shows the different WAN connection types that can be used to connect your LANs (DTE) together
over a DCE network.
The following list explains the WAN connection types:
Leased lines
Typically, these are referred to as a
point-to-point connection
or dedicated connection.
A
leased line
is a pre-established WAN communications path from the CPE through
the DCE switch to the CPE of the remote site, allowing DTE networks to communicate at any
time with no setup procedures before transmitting data. When cost is no object, it’s really the
best choice. It uses synchronous serial lines up to 45Mbps. HDLC and PPP encapsulations are
frequently used on leased lines, and I’ll go over them with you in detail in a bit.
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Chapter 10
Introduction to Wide Area Networks
FIGURE 1 0 . 1
WAN connection types
Circuit switching
When you hear the term
circuit switching
, think phone call. The big
advantage is cost—you pay only for the time you actually use. No data can transfer before an
end-to-end connection is established.
Circuit switching
uses dial-up modems or ISDN and is
used for low-bandwidth data transfers.
Packet switching
This is a WAN switching method that allows you to share bandwidth with
other companies to save money.
Packet switching
can be thought of as a network that’s designed
to look like a leased line yet the cost is more like the cost of circuit switching. There is a downside:
If you need to transfer data constantly, forget about this option. Just get yourself a leased line.
Packet switching will work well only if your data transfers are bursty in nature. Frame Relay and
X.25 are packet-switching technologies. Speeds can range from 56Kbps to T3 (45Mbps).
WAN Support
Cisco supports pretty much every WAN service available, and you can see this with the
encapsulation ?
command from any serial interface (your output may vary depending
on the IOS version you are running):
Router#
config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#
int s0/0
Router(config-if)#
encapsulation ?
atm-dxi ATM-DXI encapsulation
Dedicated
Circuit-switched
Packet-switched
Synchronous serial
Asynchronous serial, ISDN
Synchronous serial
Telephone
company
Service
provider
Introduction to Wide Area Networks
507
bstun Block Serial tunneling (BSTUN)
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
sdlc SDLC
sdlc-primary SDLC (primary)
sdlc-secondary SDLC (secondary)
smds Switched Megabit Data Service (SMDS)
stun Serial tunneling (STUN)
x25 X.25
You cannot configure Ethernet or Token Ring encapsulation on a serial interface.
In this section, I will define the most prominent WAN protocols used today—Frame Relay,
ISDN, LAPB, LAPD, HDLC, PPP, and ATM. Usually, though, the only WAN protocols configured
on a serial interface these days (or for the exam) are HDLC, PPP, and Frame Relay.
Frame Relay
A packet-switched technology that emerged in the early 1990s,
Frame Relay
is
a Data Link and Physical layer specification that provides high performance. Frame Relay
is a successor to X.25, except that much of the technology in X.25 used to compensate for
physical errors (noisy lines) has been eliminated. Frame Relay can be more cost-effective than
point-to-point links and can typically run at speeds of 64Kbps up to 45Mbps (T3). Frame
Relay provides features for dynamic bandwidth allocation and congestion control.
ISDN
Integrated Services Digital Network (ISDN)
is a set of digital services that transmit
voice and data over existing phone lines. ISDN can offer a cost-effective solution for remote
users who need a higher-speed connection than analog dial-up links offer. ISDN is also a good
choice as a backup link for other types of links, such as Frame Relay or a T1 connection.
LAPB
Link Access Procedure, Balanced (LAPB)
was created to be a connection-oriented
protocol at the Data Link layer for use with X.25. It can also be used as a simple data link
transport. LAPB causes a tremendous amount of overhead because of its strict time-out and
windowing techniques.
LAPD
Link Access Procedure, D-Channel (LAPD)
is used with ISDN at the Data Link layer
(layer 2) as a protocol for the D (signaling) channel. LAPD was derived from the Link Access Procedure,
Balanced (LAPB) protocol and is designed primarily to satisfy the signaling requirements
of ISDN basic access.
HDLC
High-Level Data-Link Control (HDLC)
was derived from Synchronous Data
Link Control (SDLC), which was created by IBM as a Data Link connection protocol.
HDLC is a protocol at the Data Link layer, and it has very little overhead compared to
LAPB. HDLC wasn’t intended to encapsulate multiple Network layer protocols across the
same link. The HDLC header carries no identification of the type of protocol being carried
inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own
way of identifying the Network layer protocol, which means that each vendor’s HDLC is
proprietary for its equipment.
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PPP
Point-to-Point Protocol (PPP)
is an industry-standard protocol. Because all multiprotocol
versions of HDLC are proprietary, PPP can be used to create point-to-point links
between different vendors’ equipment. It uses a Network Control Protocol field in the Data
Link header to identify the Network layer protocol. It allows authentication and multilink
connections and can be run over asynchronous and synchronous links.
ATM
Asynchronous Transfer Mode (ATM) was created for time-sensitive traffic, providing
simultaneous transmission of voice, video, and data. ATM uses cells instead of packets that
are a fixed 53 bytes long. It also can use isochronous clocking (external clocking) to help the
data move faster.
PPP and ATM can be configured on an asynchronous serial connection.
HDLC and Frame Relay cannot.
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