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2.2.5 Frame Relay
With increasing demand for higher bandwidth and lower latency
packet switching, communications providers introduced Frame Relay. Although the
network layout appears similar to that for X.25, available data rates are
commonly up to 4 Mbps, with some providers offering even higher rates. 
Frame Relay differs from X.25 in several aspects. Most
importantly, it is a much simpler protocol that works at the data link layer
rather than the network layer.
Frame Relay implements no error or flow control. The simplified
handling of frames leads to reduced latency, and measures taken to avoid frame
build-up at intermediate switches help reduce jitter.
Most Frame Relay connections are PVCs rather than SVCs. The
connection to the network edge is often a leased line but dialup connections
are available from some providers using ISDN lines. The ISDN D channel is used
to set up an SVC on one or more B channels. Frame Relay tariffs are based on
the capacity of the connecting port at the network edge. Additional factors are
the agreed capacity and committed information rate (CIR) of the various PVCs
through the port.
Frame Relay provides permanent shared medium bandwidth connectivity that
carries both voice and data traffic. Frame Relay is ideal for connecting
enterprise LANs. The router on the LAN needs only a single interface, even when
multiple VCs are used. The short-leased line to the Frame Relay network edge
allows cost-effective connections between widely scattered LANs.
2.2.6 ATM
Communications providers saw a need for a permanent shared network
technology that offered very low latency and jitter at much higher bandwidths.
Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond
155 Mbps. As with the other shared technologies, such as X.25 and Frame Relay,
diagrams for ATM WANs look the same.
ATM is a technology that is capable of transferring voice, video,
and data through private and public networks. It is built on a cell-based
architecture rather than on a frame-based architecture. ATM cells are always a
fixed length of 53 bytes. The 53 byte ATM cell contains a 5 byte ATM header
followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited
for carrying voice and video traffic because this traffic is intolerant of
delay. Video and voice traffic do not have to wait for a larger data packet to
be transmitted.
The 53 byte ATM cell is less efficient than the bigger frames and
packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes
of overhead for each 48-byte payload. When the cell is carrying segmented
network layer packets, the overhead will be higher because the ATM switch must
be able to reassemble the packets at the destination. A typical ATM line needs
almost 20% greater bandwidth than Frame Relay to carry the same volume of
network layer data.
ATM offers both PVCs and SVCs, although PVCs are more common with
WANs.
As with other shared technologies, ATM allows multiple virtual circuits
on a single leased line connection to the network edge.
2.2.3 Leased Line
When permanent dedicated connections are required, leased lines
are used with capacities ranging up to 2.5 Gbps.
A point-to-point link provides a pre-established WAN
communications path from the customer premises through the provider network to
a remote destination. Point-to-point lines are usually leased from a carrier
and are called leased lines. Leased lines are available in different
capacities. These dedicated circuits are generally priced
based on bandwidth required and distance between the two connected points.
Point-to-point links are generally more expensive than shared services such as
Frame Relay. The cost of leased-line solutions can become significant when they
are used to connect many sites. There are times when cost of the leased line is
outweighed by the benefits. The dedicated capacity gives no latency or jitter
between the endpoints. Constant availability is essential for some applications
such as electronic commerce.
A router serial port is required for each leased-line connection.
A CSU/DSU and the actual circuit from the service provider are also required.
Leased lines are used extensively for building WANs and give
permanent dedicated capacity. 
They have been the traditional connection of
choice but have a number of disadvantages. WAN traffic is often variable and
leased lines have a fixed capacity. This results in the bandwidth of the line
seldom being exactly what is needed. In addition, each end point would need an
interface on the router which would increase equipment costs. Any changes to
the leased line generally require a site visit by the carrier to change
capacity.
Leased lines provide direct point-to-point connections between
enterprise LANs and connect individual branches to a packet-switched network.
Several connections can be multiplexed over a leased line, resulting in shorter
links and fewer required interfaces.
2.2.4 X.25
In response to the expense of leased lines, telecommunications
providers introduced packet-switched networks using shared lines to reduce
costs. The first of these packet-switched networks was standardized as the X.25
group of protocols. X.25 provides a low bit rate shared variable capacity that
may be either switched or permanent.
X.25 is a network-layer protocol and subscribers are provided with
a network address. Virtual circuits can be established through the network with
call request packets to the target address. The resulting SVC is identified by
a channel number. Data packets labeled with the channel number are delivered to
the corresponding address. Multiple channels can be active on a single
connection.
Subscribers connect to the X.25 network with either leased lines
or dialup connections. X.25 networks can also have pre-established channels
between subscribers that provide a PVC.
X.25 can be very cost effective because tariffs are based on the
amount of data delivered rather than connection time or distance. Data can be
delivered at any rate up to the connection capacity. This provides some
flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps.
In addition, the data packets are subject to the delays typical of shared
networks.
X.25 technology is no longer widely available as a WAN technology
in the US.
Frame Relay has replaced X.25 at many service provider locations.
Typical X.25 applications are point-of-sale card readers. These readers
use X.25 in dialup mode to validate transactions on a central computer. Some
enterprises also use X.25 based value-added networks (VAN) to transfer
Electronic Data Interchange (EDI) invoices, bills of lading, and other
commercial documents. For these applications, the low bandwidth and high
latency are not a concern, because the low cost makes the use of X.25
affordable.
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.
