Content Overview As the enterprise
grows beyond a single location, it is necessary to interconnect
the LANs in the various branches to form a wide-area network
(WAN). This module examines some of the options available for
these interconnections, the hardware needed to implement them,
and the terminology used to discuss them. There are many
options currently available today for implementing WAN
solutions. They differ in technology, speed, and cost.
Familiarity with these technologies is an important part of
network design and evaluation. If all data traffic in an
enterprise is within a single building, a LAN meets the needs
of the organization. Buildings can be interconnected with
high-speed data links to form a campus LAN if data must flow
between buildings on a single campus. However, a WAN is needed
to carry data if it must be transferred between geographically
separate locations. Individual remote access to the LAN and
connection of the LAN to the Internet are separate study
topics, and will not be considered here. Most students will
not have the opportunity to design a new WAN, but many will be
involved in designing additions and upgrades to existing WANs,
and will be able to apply the techniques learned in this
module. Students completing this module should be able to:
- Differentiate between a LAN and WAN
-
Identify the devices used in a WAN
- List WAN
standards
- Describe WAN encapsulation
-
Classify the various WAN link options
- Differentiate
between packet-switched and circuit-switched WAN technologies
- Compare and contrast current WAN technologies
- Describe equipment involved in the implementation of
various WAN services
- Recommend a WAN service to an
organization based on its needs
- Describe DSL and
cable modem connectivity basics
- Describe a methodical
procedure for designing WANs
- Compare and contrast WAN
topologies
- Compare and contrast WAN design
models
- Recommend a WAN design to an organization
based on its needs
Content 2.1 WAN
Technologies Overview 2.1.1 WAN technology A WAN is a
data communications network that operates beyond the geographic
scope of a LAN. One primary difference between a WAN and a LAN
is that a company or organization must subscribe to an outside
WAN service provider in order 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. WANs
generally carry a variety of traffic types, such as voice,
data, and video. Telephone and data services are the most
commonly used WAN services. Devices on the subscriber premises
are called customer premises equipment (CPE). The subscriber
owns the CPE or leases the CPE from the service provider. A
copper or fiber cable connects the CPE to the service
provider’s nearest exchange or central office (CO). This
cabling is often called the local loop, or
"last-mile". A dialed call is connected locally to
other local loops, or non-locally through a trunk to a primary
center. It then goes to a sectional center and on to a regional
or international carrier center as the call travels to its
destination. In order for the local loop to carry data, a
device such as a modem is needed to prepare the data for
transmission. Devices that put data on the local loop are
called data circuit-terminating equipment, or data
communications equipment (DCE). The customer devices that pass
the data to the DCE are called data terminal equipment (DTE).
The DCE primarily provides an interface for the DTE into the
communication link on the WAN cloud. The DTE/DCE interface uses
various physical layer protocols, such as High-Speed Serial
Interface (HSSI) and V.35. These protocols establish the codes
and electrical parameters the devices use to communicate with
each other. WAN links are provided at various speeds measured
in bits per second (bps), kilobits per second (kbps or 1000
bps), megabits per second (Mbps or 1000 kbps) or gigabits per
second (Gbps or 1000 Mbps). The bps values are generally full
duplex. This means that an E1 line can carry 2 Mbps, or a T1
can carry 1.5 Mbps, in each direction simultaneously. Web
Links Introduction to WAN Technologies
http://www.cisco.com/univercd/cc/td/ doc/ cisintwk/ito_doc/
introwan.htm
Content 2.1 WAN Technologies
Overview 2.1.2 WAN devices WANs are groups of LANs
connected together with communications links from a service
provider. Because the communications links cannot plug directly
into the LAN, it is necessary to identify the various pieces of
interfacing equipment. LAN-based computers with data to
transmit send data to a router that contains both LAN and WAN
interfaces. The router will use the Layer 3 address information
to deliver the data on the appropriate WAN interface. Routers
are active and intelligent network devices and therefore can
participate in network management. Routers manage networks by
providing dynamic control over resources and supporting the
tasks and goals for networks. Some of these goals are
connectivity, reliable performance, management control, and
flexibility. The communications link needs signals in an
appropriate format. For digital lines, a channel service unit
(CSU) and a data service unit (DSU) are required. The two are
often combined into a single piece of equipment, called the
CSU/DSU. The CSU/DSU may also be built into the interface card
in the router. A modem is needed if the local loop is analog
rather than digital. Modems transmit data over voice-grade
telephone lines by modulating and demodulating the signal. The
digital signals are superimposed on an analog voice signal that
is modulated for transmission. The modulated signal can be
heard as a series of whistles by turning on the internal modem
speaker. At the receiving end the analog signals are returned
to their digital form, or demodulated. When ISDN is used as the
communications link, all equipment attached to the ISDN bus
must be ISDN-compatible. Compatibility is generally built into
the computer interface for direct dial connections, or the
router interface for LAN to WAN connections. Older equipment
without an ISDN interface requires an ISDN terminal adapter
(TA) for ISDN compatibility. Communication servers concentrate
dial-in user communication and remote access to a LAN. They may
have a mixture of analog and digital (ISDN) interfaces and
support hundreds of simultaneous users. Interactive Media
Activity Crossword Puzzle: WAN Devices and Interfaces Upon
completing this activity, the student will be able to describe
devices and interfaces associated with WAN connections. Web
Links WAN http://www.cisco.com/en/US/tech/tk713/
tech_topology_and_network_ serv_and_protocol_suite_ home.html
Content 2.1 WAN Technologies Overview
2.1.3 WAN standards WANs use the OSI reference model, but
focus mainly on Layer 1 and Layer 2. WAN standards typically
describe both physical layer delivery methods and data link
layer requirements, including physical addressing, flow
control, and encapsulation. WAN standards are defined and
managed by a number of recognized authorities. The physical
layer protocols describe how to provide electrical, mechanical,
operational, and functional connections to the services
provided by a communications service provider. Some of the
common physical layer standards are listed in Figure , and
their connectors illustrated in Figure . The data link layer
protocols define how data is encapsulated for transmission to
remote sites, and the mechanisms for transferring the resulting
frames. A variety of different technologies are used, such as
ISDN, Frame Relay or Asynchronous Transfer Mode (ATM). These
protocols use the same basic framing mechanism, high-level data
link control (HDLC), an ISO standard, or one of its sub-sets or
variants. Interactive Media Activity Crossword Puzzle:
WAN Standards Upon completing this activity, the student will