WANs are distinguished from LANs by

WANs are distinguished from LANs by the fact that WANs traverse a wider
geographical area. They usually employ point-to-point, dedicated communications
rather than point-to-multipoint communications. They also use different connectivity
devices, depending on the WAN technology in use.
■ A WAN in which each site is connected in a serial fashion to no more than two other
sites is known as a bus topology WAN. This topology often provides the best solution
for organizations with only a few sites and access to dedicated circuits.
■ In a ring topology WAN, each site is connected to two other sites so that the entire
WAN forms a ring pattern. This architecture is similar to the LAN ring topology,
except that most ring topology WANs have the capability to reverse the direction data
travel to avoid a failed site.
■ In the star topology WAN, a single site acts as the central connection point for several other
points. This arrangement allows one connection to fail without affecting other connections.
Therefore, star topology WANs are more fault tolerant than bus or ring WANs.
A mesh topology WAN consists of many directly interconnected sites. In partial-mesh
WANs, only some of the WAN sites are directly interconnected. In full-mesh WANs,
every site is directly connected to every other site. The full-mesh topology is the most
fault tolerant and also the most expensive WAN topology to implement.
■ A tiered topology WAN is one in which sites that are connected in star or ring
formations are interconnected at different levels, with the interconnection points being
organized into layers to form hierarchical groupings.
■ The PSTN (Public Switched Telephone Network) is the network of lines and carrier
equipment that provides telephone service to most homes and businesses. It was originally
composed of analog lines alone, but now also uses digital transmission over fiber-optic or
copper twisted pair cable, microwave, and satellite connections. The local loop portion of
the PSTN is still primarily UTP; it is this portion that limits throughput on the PSTN. The
PSTN provides the foundation for several types of WAN connections, including dial-up
networking, X.25, frame relay, T-carriers, and DSL.
■ FTTP (fiber to the premises) refers to the use of a fiber-optic cable to complete a
carrier’s connection to a subscriber, whether residential or business. FTTP can be
found on PONs (passive optical networks), which are point-to-multipoint networks.
In PONs, a single port on a carrier’s OLT (optical line terminal) is capable of carrying
32 channels, each assigned to a customer. Fiber connects the OLT port with an ONU
(optical network unit) near a group of subscribers. From the ONU, fiber-optic or
copper cabling brings services to multiple customers.
■ X.25 is an analog, packet-switched technology optimized for reliable, long-distance
data transmission. It can support 2-Mbps throughput. X.25 was originally developed
and used for communications between mainframe computers and remote terminals.
■ Frame relay, like X.25, relies on packet switching, but carries digital signals. It does
not analyze frames to check for errors, but simply relays them from node to node, so
frame relay supports higher bandwidth than X.25, offering a maximum of 45-Mbps
throughput.
■ Both X.25 and frame relay are configured as PVCs (permanent virtual circuits), or
point-to-point connections over which data may follow different paths. When leasing
an X.25 or frame relay circuit from a telecommunications carrier, a customer specifies
endpoints and the amount of bandwidth required between them.
■ ISDN (Integrated Services Digital Network) is an international standard for protocols
at the Physical, Data Link, and Transport layers that allows the PSTN to carry digital
signals. ISDN lines may carry voice and data signals simultaneously but require an
ISDN phone to carry voice traffic and an ISDN router and ISDN terminal adapter to
carry data.
■ Two types of ISDN connections are commonly used by consumers in North America:
BRI (Basic Rate Interface) and PRI (Primary Rate Interface). Both use a combination
of bearer channels (B channels) and data channels (D channels). The B channel
transmits and receives data or voice from point to point. The D channel carries
information about the call, such as session initiation and termination signals, caller
identity, call forwarding, and conference calling signals.
■ BRI uses two 64-Kbps circuit-switched B channels and a 16-Kbps D channel. The
maximum throughput for a BRI connection is 128 Kbps. PRI uses 23 B channels andone 64-Kbps D channel. The maximum potential throughput for a PRI connection is
1.544 Mbps. Individual subscribers rarely use PRI, preferring BRI instead, but PRI
may be used by businesses and other organizations that need more throughput.
■ T-carrier technology uses TDM (time division multiplexing) to divide a single channel
into multiple channels for carrying voice, data, video, or other signals. Devices at the
sending end arrange the data streams (multiplex), then devices at the receiving end
filter them back into separate signals (demultiplex).
■ The most common T-carrier implementations are T1 and T3. A T1 circuit can carry
the equivalent of 24 voice channels, giving a maximum data throughput of 1.544
Mbps. A T3 circuit can carry the equivalent of 672 voice channels, giving a maximum
data throughput of 44.736 Mbps.
■ The signal level of a T-carrier refers to its Physical layer electrical signaling
characteristics, as defined by ANSI standards. DS0 is the equivalent of one data or
voice channel. All other signal levels are multiples of DS0.
■ T1 technology can use UTP or STP, preferably the latter. However, twisted pair
wiring cannot adequately carry the high throughput of multiple T1s or T3
transmissions. For T3 transmissions, fiber-optic cable or microwave connections are
necessary.
■ Incoming T-carrier lines terminate in RJ-48 connectors at a smart jack at the
customer’s demarc. Next, signals are processed by a CSU/DSU. The CSU/DSU ensures
connection integrity through error correction and line monitoring and converts the
T-carrier frames into frames the LAN can interpret, and vice versa. It also connects
T-carrier lines with terminating equipment. A CSU/DSU often includes a multiplexer.
■ DSL (digital subscriber line) is a WAN connection method that uses advanced phase or
amplitude modulation in the higher (inaudible) frequencies on a phone line to achieve
throughputs of up to 51.8 Mbps. DSL comes in eight different varieties, each of which is
either asymmetrical or symmetrical. In asymmetrical transmission, more data can be sent
in one direction than in the other direction. In symmetrical transmission, throughput is
equal in both directions. The most popular form of DSL is ADSL.
■ DSL technology creates a dedicated circuit. At the consumer end, a DSL modem
connects computers and telephones to the DSL line. At the carrier end, a DSLAM
(DSL access multiplexer) aggregates multiple incoming DSL lines before connecting
them to the Internet or to larger carriers.
■ Broadband cable is a dedicated service that relies on the cable wiring used for TV
signals. The service can theoretically provide as much as 36-Mbps downstream and
10-Mbps upstream throughput, though actual throughput is much lower.
■ Broadband cable connections require that the customer use a special cable modem to
transmit and receive signals over coaxial cable wiring. In addition, cable companies
must have replaced their coaxial cable plant with hybrid fiber-coax cable to support
bidirectional, digital communications.
■ In some locations, users can access the Internet using BPL (broadband over powerline).
The service is shared among multiple customers, which limits practical throughputs to
no more than 1 Mbps. Each customer accesses the network using a modem plugged
into an electrical outlet. BPL requires users to be within 2 km of a repeater.
ATM (Asynchronous Transfer Mode) is a Data Link layer standard that relies on
fixed packets, called cells, consisting of 48 bytes of data plus a 5-byte header. It’sa
connection-oriented technology based on virtual circuits. Having a reliable connection
enables ATM to guarantee QoS (quality of service) levels for designated transmissions.
■ SONET (Synchronous Optical Network) is a high-bandwidth WAN signaling
technique that specifies framing and multiplexing techniques at the Physical layer of
the OSI model. Its four key strengths are that it can integrate many other WAN
technologies (for example, T-carriers, ISDN, and ATM technology), it offers fast data
transfer rates, it allows for simple link additions and removals, and it provides a high
degree of fault tolerance. Internationally, SONET is known as SDH.
■ SONET depends on fiber-optic transmission media and uses multiplexers to connect
to network devices (such as routers or telephone switches) at the customer’s end. A
typical SONET network takes the form of a dual-ring topology. If one ring breaks,
SONET technology automatically reroutes traffic along a backup ring. This
characteristic, known as self-healing, makes SONET very reliable.