copper lines. DSL technology allows the local loop
line to be used for normal telephone voice connection and an
always-on connection for instant network connectivity. Multiple
DSL subscriber lines are multiplexed into a single, high
capacity link by the use of a DSL Access Multiplexer (DSLAM) at
the provider location. DSLAMs incorporate TDM technology to
aggregate many subscriber lines into a less cumbersome single
medium, generally a T3/DS3 connection. Current DSL technologies
are using sophisticated coding and modulation techniques to
achieve data rates up to 8.192 Mbps. The voice channel of a
standard consumer telephone covers the frequency range of 330
Hz to 3.3 KHz. A frequency range, or window, of 4 KHz is
regarded as the requirements for any voice transmission on the
local loop. DSL technologies place upload (upstream) and
download (downstream) data transmissions at frequencies above
this 4 KHz window. This technique is what allows both voice and
data transmissions to occur simultaneously on a DSL service.
The two basic types of DSL technologies are asymmetric (ADSL)
and symmetric (SDSL). All forms of DSL service are categorized
as ADSL or SDSL and there are several varieties of each type.
Asymmetric service provides higher download or downstream
bandwidth to the user than upload bandwidth. Symmetric service
provides the same capacity in both directions. The different
varieties of DSL provide different bandwidths, with
capabilities exceeding those of a T1 or E1 leased line. The
transfer rates are dependent on the actual length of the local
loop and the type and condition of its cabling. For
satisfactory service, the loop must be less than 5.5 kilometers
(3.5 miles). DSL availability is far from universal, and there
are a wide variety of types, standards, and emerging standards.
It is not a popular choice for enterprise computer departments
to support home workers. Generally, a subscriber cannot choose
to connect to the enterprise network directly, but must first
connect to an Internet service provider (ISP). From here, an IP
connection is made through the Internet to the enterprise.
Thus, security risks are incurred. To address security
concerns, DSL services provide capabilities for using Virtual
Private Network (VPN) connections to a VPN server, which is
typically located at the corporate site. Interactive Media
Activity Checkbox: WAN Technologies Digital Subscriber Line
(DSL) Upon completing this activity, the student will be able
to identify the characteristics associated with DSL
connections.
Content 2.2 WAN Technologies
2.2.8 Cable modem Coaxial cable is widely used in urban
areas to distribute television signals. Network access is
available from some cable television networks. This allows for
greater bandwidth than the conventional telephone local loop.
Enhanced cable modems enable two-way, high-speed data
transmissions using the same coaxial lines that transmit cable
television. Some cable service providers are promising data
speeds up to 6.5 times that of T1 leased lines. This speed
makes cable an attractive medium for transferring large amounts
of digital information quickly, including video clips, audio
files, and large amounts of data. Information that would take
two minutes to download using ISDN BRI can be downloaded in two
seconds through a cable modem connection. Cable modems provide
an always-on connection and a simple installation. An always-on
cable connection means that connected computers are vulnerable
to a security breach at all times and need to be suitably
secured with firewalls. To address security concerns, cable
modem services provide capabilities for using Virtual Private
Network (VPN) connections to a VPN server, which is typically
located at the corporate site. A cable modem is capable of
delivering up to 30 to 40 Mbps of data on one 6 MHz cable
channel. This is almost 500 times faster than a 56 Kbps modem.
With a cable modem, a subscriber can continue to receive cable
television service while simultaneously receiving data to a
personal computer. This is accomplished with the help of a
simple one-to-two splitter. Cable modem subscribers must use
the ISP associated with the service provider. All the local
subscribers share the same cable bandwidth. As more users join
the service, available bandwidth may be below the expected
rate. - Interactive Media Activity Checkbox: WAN
Technologies Cable Modems Upon completing this activity, the
student will be able to identify the characteristics associated
with cable modems.
Content 2.3 WAN Design
2.3.1 WAN communication WANS are considered to be a set
of data links connecting routers on LANs. User end stations and
servers on LANs exchange data. Routers pass data between
networks across the data links.Because of cost and legal
reasons, a communications provider or a common carrier normally
owns the data links that make up a WAN. The links are made
available to subscribers for a fee and are used to interconnect
LANs or connect to remote networks. WAN data transfer speed
(bandwidth) is considerably slower than the 100 Mbps that is
common on a LAN. The charges for link provision are the major
cost element of a WAN and the design must aim to provide
maximum bandwidth at acceptable cost. With user pressure to
provide more service access at higher speeds and management
pressure to contain cost, determining the optimal WAN
configuration is not an easy task. WANs carry a variety of
traffic types such as data, voice, and video. The design
selected must provide adequate capacity and transit times to
meet the requirements of the enterprise. Among other
specifications, the design must consider the topology of the
connections between the various sites, the nature of those
connections, and bandwidth capacity. Older WANs often consisted
of data links directly connecting remote mainframe computers.
Today’s WANs, though, connect geographically separated LANs.
End-user stations, servers, and routers communicate across
LANs, and the WAN data links terminate at local routers. By
exchanging Layer 3 address information about directly connected
LANs, routers determine the most appropriate path through the
network for the required data streams. Routers can also provide
quality of service (QoS) management, which allots priorities to
the different traffic streams. Because the WAN is merely a set
of interconnections between LAN based routers, there are no
services on the WAN. WAN technologies function at the lower
three layers of the OSI reference model. Routers determine the
destination of the data from the network layer headers and
transfer the packets to the appropriate data link connection
for delivery on the physical connection. Web Links
Introduction to WAN Technologies
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/introwan.htm
Content
2.3 WAN Design 2.3.2 Steps in WAN design
Designing a WAN can be a challenging task, but approaching the
design in a systematic manner can lead to superior performance
at a reduced cost. Many WANs have evolved over time, therefore
many of the guidelines discussed here may not have been
considered. Every time a modification to an existing WAN is
considered, the steps in this module should be followed. WAN
modifications may arise from changes such as an expansion in
the enterprise the WAN serves, or accommodation of new work
practices and business methods. Enterprises install WAN
connectivity because there is a need to move data in a timely
manner between external branches. The WAN is there to support
the enterprise requirements. Meeting these requirements incurs
costs, such as equipment provisioning and management of the
data links. In designing the WAN, it is necessary to know what
data traffic must be carried, its origin, and its destination.
WANs carry a variety of traffic types with varying requirements
for bandwidth, latency, and jitter. For each pair of end points
and for each traffic type, information is needed on the various
traffic characteristics. Determining this may involve extensive
studies of and consultation with the network users. The design