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. Interactive Media
Activity Checkbox: WAN Technologies Leased Lines Upon
completing this activity, the student will be able to identify
the characteristics associated with a leased line connection.
Content 2.2 WAN Technologies 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. Interactive Media Activity Checkbox: WAN
Technologies X.25 Upon completing this activity, the student
will be able to identify the characteristics associated with an
X.25 connection. Web Links X.25 Protocol
http://www2.rad.com/networks/1996/ x25/x25.htm
Content
2.2 WAN Technologies 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 FR network edge allows
cost-effective connections between widely scattered LANs.
Interactive Media Activity Checkbox: WAN Technologies Frame
Relay Upon completing this activity, the student will be able
to identify the characteristics associated with a Frame Relay
circuit. Web Links Frame Relay
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/frame.htm
Content 2.2 WAN
Technologies 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.
Interactive Media Activity Checkbox: WAN Technologies
Asynchronous Transfer Mode (ATM) Upon completing this activity,
the student will be able to identify the characteristics
associated with ATM connections. Web Links Asynchronous
Transfer Mode (ATM) Switching http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/atm.htm
Content 2.2 WAN
Technologies 2.2.7 DSL Digital Subscriber Line (DSL)
technology is a broadband technology that uses existing
twisted-pair telephone lines to transport high-bandwidth data
to service subscribers. DSL service is considered broadband, as
opposed to the baseband service for typical LANs. Broadband
refers to a technique which uses multiple frequencies within
the same physical medium to transmit data. The term xDSL covers
a number of similar yet competing forms of DSL technologies:
- Asymmetric DSL (ADSL)
- Symmetric DSL
(SDSL)
- High Bit Rate DSL (HDSL)
- ISDN (like)
DSL (IDSL)
- Rate Adaptive DSL (RADSL)
- Consumer
DSL (CDSL), also called DSL-lite or G.lite
DSL
technology allows the service provider to offer high-speed
network services to customers, utilizing installed local loop