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2
.2 WAN Technologies
2.2.1 Analog Dialup
When intermittent, low-volume data transfers are needed, modems
and analog dialed telephone lines provide low capacity and dedicated switched
connections. 
Traditional telephony uses a copper cable, called the local loop,
to connect the telephone handset in the subscriber premises to the public
switched telephone network (PSTN). The signal on the local loop during a call
is a continuously varying electronic signal that is a translation of the
subscriber voice.
The local loop is not suitable for direct transport of binary
computer data, but a modem can send computer data through the voice telephone
network. The modem modulates the binary data into an analog signal at the
source and demodulates the analog signal at the destination to binary data.
The physical characteristics of the local loop and its connection
to the PSTN limit the rate of the signal. The upper limit is around 33 kbps.
The rate can be increased to around 56 kbps if the signal is coming directly
through a digital connection.
For small businesses, this can be adequate for the exchange of
sales figures, prices, routine reports, and email. Using automatic dialup at
night or on weekends for large file transfers and data backup can take
advantage of lower off-peak tariffs (line charges). Tariffs are based on the
distance between the endpoints, time of day, and the duration of the call.
The advantages of modem and analog lines are simplicity, availability,
and low implementation cost. The disadvantages are the low data rates and a
relatively long connection time. The dedicated circuit provided by dialup will
have little delay or jitter for point-to-point traffic, but voice or video
traffic will not operate adequately at relatively low bit rates.
2.2.2 ISDN
The internal connections, or trunks, of the PSTN have changed from
carrying analog frequency-division multiplexed signals, to time-division
multiplexed (TDM) digital signals. An obvious next step is to enable the local
loop to carry digital signals that result in higher capacity switched
connections.
Integrated Services Digital Network (ISDN) turns the local loop
into a TDM digital connection. The connection uses 64 kbps bearer channels (B)
for carrying voice or data and a signaling, delta channel (D) for call set-up
and other purposes.
Basic Rate Interface (BRI) ISDN is intended for the home and small
enterprise and provides two 64 kbps B channels and a 16 kbps D channel. For
larger installations, Primary Rate Interface (PRI) ISDN is available. PRI
delivers twenty-three 64 kbps B channels and one 64 kbps D channel in North America, for a total bit rate of up to 1.544 Mbps.
This includes some additional overhead for synchronization. In Europe, Australia, and
other parts of the world, ISDN PRI provides thirty B channels and one D channel
for a total bit rate of up to 2.048 Mbps, including synchronization overhead. In North America PRI corresponds to a T1
connection. The rate of international PRI corresponds to an E1 connection.
The BRI D channel is underutilized, as it has only two B channels
to control. Some providers allow the D channel to carry data at low bit rates
such as X.25 connections at 9.6 kbps.
For small WANs, the BRI ISDN can provide an ideal connection
mechanism. BRI has a call setup time that is less than a second, and its 64
kbps B channel provide greater capacity than an analog modem link. 
If greater capacity is required, a second B
channel can be activated to provide a total of 128 kbps. Although inadequate
for video, this would permit several simultaneous voice conversations in
addition to data traffic.
Another common application of ISDN is to provide additional
capacity as needed on a leased line connection. The leased line is sized to
carry average traffic loads while ISDN is added during peak demand periods.
ISDN is also used as a backup in the case of a failure of the leased line. ISDN
tariffs are based on a per-B channel basis and are similar to those of analog
voice connections.
With PRI ISDN, multiple B channels can be connected between two end
points. This allows for video conferencing and high bandwidth data connections
with no latency or jitter. Multiple connections can become very expensive over
long distances.
2.1.6
WAN link options
Circuit switching establishes a dedicated physical connection for
voice or data between a sender and receiver. Before communication can start, it
is necessary to establish the connection by setting the switches. This is done
by the telephone system, using the dialed number. ISDN is used on digital lines
as well as on voice-grade lines.
To avoid the delays associated with setting up a connection,
telephone service providers also offer permanent circuits. These dedicated or
leased lines offer higher bandwidth than is available with a switched circuit.
Examples of circuit-switched connections include:
- Plain Old
Telephone System (POTS)
- ISDN Basic Rate
Interface (BRI)
- ISDN Primary
Rate Interface (PRI)
Many WAN users do not make efficient use of the fixed bandwidth
that is available with dedicated, switched, or permanent circuits, because the
data flow fluctuates. Communications providers have data networks available to
more appropriately service these users. In these networks, the data is
transmitted in labeled cells, frames, or packets through a packet-switched
network. Because the internal links between the switches are shared between
many users, the costs of packet switching are lower than those of circuit
switching. Delays (latency) and variability of delay (jitter) are greater in
packet-switched than in circuit-switched networks. This is because the links
are shared and packets must be entirely received at one switch before moving to
the next. Despite the latency and jitter inherent in shared networks, modern
technology allows satisfactory transport of voice and even video communications
on these networks.
Packet-switched networks may establish routes through the switches
for particular end-to-end connections. Routes established when the switches are
started are PVCs. Routes established on demand are SVCs. If the routing is not
pre-established and is worked out by each switch for each packet, the network
is called connectionless.
To connect to a packet-switched network, a subscriber needs a
local loop to the nearest location where the provider makes the service
available. This is called the point-of-presence (POP) of the service. Normally
this will be a dedicated leased line. This line will be much shorter than a
leased line directly connected to the subscriber locations, and often carries
several VCs. Since it is likely that not all the VCs will
require maximum demand simultaneously, the capacity of the leased line can be
smaller than the sum of the individual VCs. Examples of packet or cell switched
connections include:
2.1.5 Packet and circuit switching
Packet-switched networks were developed to overcome the expense of
public circuit-switched networks and to provide a more cost-effective WAN
technology.
When a subscriber makes a telephone call, the dialed number is
used to set switches in the exchanges along the route of the call so that there
is a continuous circuit from the originating caller to that of the called
party. Because of the switching operation used to establish the circuit, the
telephone system is called a circuit-switched network. If the telephones are
replaced with modems, then the switched circuit is able to carry computer data.
The internal path taken by the circuit between exchanges is shared
by a number of conversations. Time division multiplexing (TDM) is used to give
each conversation a share of the connection in turn. TDM assures that a fixed
capacity connection is made available to the subscriber.
If the circuit carries computer data, the usage of this fixed
capacity may not be efficient. For example, if the circuit is used to access
the Internet, there will be a burst of activity on the circuit while a web page
is transferred. This could be followed by no activity while the user reads the
page and then another burst of activity while the next page is transferred.
This variation in usage between none and maximum is typical of computer network
traffic. Because the subscriber has sole use of the fixed capacity allocation,
switched circuits are generally an expensive way of moving data.
An alternative is to allocate the capacity to the traffic only
when it is needed, and share the available capacity between many users. With a
circuit-switched connection, the data bits put on the circuit are automatically
delivered to the far end because the circuit is already established. If the
circuit is to be shared, there must be some mechanism to label the bits so that
the system knows where to deliver them. It is difficult to label individual
bits, therefore they are gathered into groups called cells, frames, or packets.
The packet passes from exchange to exchange for delivery through the provider
network. Networks that implement this system are called packet-switched
networks.
The links that connect the switches in the provider network belong
to an individual subscriber during data transfer, therefore many subscribers
can share the link. Costs can be significantly lower than a dedicated
circuit-switched connection. Data on packet-switched networks are subject to
unpredictable delays when individual packets wait for other subscriber packets
to be transmitted by a switch.
The switches in a packet-switched network determine, from
addressing information in each packet, which link the packet must be sent on
next. There are two approaches to this link determination, connectionless or
connection-oriented. Connectionless systems, such as the Internet, carry full
addressing information in each packet. Each switch must evaluate the address to
determine where to send the packet. Connection-oriented systems predetermine
the route for a packet, and each packet need only carry an identifier. In the
case of Frame Relay, these are called Data Link Control Identifiers (DLCI). The
switch determines the onward route by looking up the identifier in tables held
in memory. The set of entries in the tables identifies a particular route or
circuit through the system. If this circuit is only physically in existence
while a packet is traveling through it, it is called a Virtual Circuit (VC).
The table entries that constitute a VC can be established by sending a
connection request through the network. In this case the resulting circuit is
called a Switched Virtual Circuit (SVC). Data that is to travel on SVCs must
wait until the table entries have been set up. Once established, the SVC may be
in operation for hours, days or weeks. Where a circuit is required to be always
available, a Permanent Virtual Circuit (PVC) will be established. Table entries
are loaded by the switches at boot time so the PVC is always available.
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
2.1.4 WAN Encapsulation
Data from the network layer is passed to the data link layer for
delivery on a physical link, which is normally point-to-point on a WAN
connection. The data link layer builds a frame around the network layer data so
the necessary checks and controls can be applied. Each WAN connection type uses
a Layer 2 protocol to encapsulate traffic while it is crossing the WAN link. To
ensure that the correct encapsulation protocol is used, the Layer 2
encapsulation type used for each router serial interface must be configured.
The choice of encapsulation protocols depends on the WAN technology and the
equipment. Most framing is based on the HDLC standard.
HDLC framing gives reliable delivery of data over unreliable lines
and includes signal mechanisms for flow and error control. The frame always starts and ends with an
8-bit flag field, the bit pattern is 01111110. Because there is a likelihood
that this pattern will occur in the actual data, the sending HDLC system always
inserts a 0 bit after every five 1s in the data field, so in practice the flag
sequence can only occur at the frame ends. The receiving system strips out the
inserted bits. When frames are transmitted consecutively the end flag of the
first frame is used as the start flag of the next frame.
The address field is not needed for WAN links, which are almost
always point-to-point. The address field is still present and may be one or two
bytes long. The control field indicates the frame type, which may be
information, supervisory, or unnumbered:
- Unnumbered
frames carry line setup messages.
- Information
frames carry network layer data.
- Supervisory
frames control the flow of information frames and request data
retransmission in the event of an error.
The control field is normally one byte, but will be two bytes for
extended sliding windows systems. Together the address and control fields are
called the frame header. The encapsulated data follows the control field. Then
a frame check sequence (FCS) uses the cyclic redundancy check (CRC) mechanism
to establish a two or four byte field.
Several data link protocols are used, including sub-sets and proprietary
versions of HDLC. Both PPP and the Cisco version of HDLC have
an extra field in the header to identify the network layer protocol of the
encapsulated data.
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.
WAN Standards
2.1.3
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.
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. 
Module 2: WAN Technologies/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
