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: Web Links
WAN http://www.cisco.com/en/US/tech/ tk713/ tech_topology_and_
network_serv_and_ protocol_suite_ home.html
Content
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. Interactive Media Activity
Checkbox: WAN Technologies Analog Dialup Upon completing this
activity, the student will be able to identify the
characteristics associated with an analog dialup circuit.
Web Links Dialup Technologies
http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/
dialup.htm
Content 2.2 WAN Technologies
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.
Interactive Media Activity Checkbox: WAN Technologies ISDN
Dialup Upon completing this activity, the student will be able
to identify the characteristics associated with an ISDN dialup
circuit. Web Links Integrated Services Digital Network
(ISDN) http://www.cisco.com/univercd/cc/td/doc/
cisintwk/ito_doc/isdn.htm
Content 2.2 WAN
Technologies 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