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WAN
Media & Communications Protocols-Thinking Out of the
Box
Here
is an overview of the various transmission media and communication
protocols available for wide area networking and some of
the issues to be considered before choosing the right one.
Solutions
providers considering implementing WAN services for their
large users must make several choices. The first is the
choice of "WAN media" and "Communication
Protocol". This can be a complex decision, as there
are numerous affordable and reliable options.
There
are various WAN media alternatives available for carrying
valuable WAN traffic. The most common choices are standard
telephone lines, digital communications lines, and permanent
leased lines. Other less common options are fiber links,
and various wireless alternatives such as cellular, microwave
and even satellite transmission. We'll look at the most
common alternatives.
WAN
communication protocols are collections of procedures, data
formats and rules used to describe the manner in which data
can be transmitted over a WAN link. The most common of these
protocols are X.25, Frame Relay, PPP and SNA. The communication
protocol is the common language on the WAN. We'll talk about
these protocols in greater depth later.
The
key to choosing the optimal WAN medium and communication
protocol is to understand the specific requirements of the
application(s) it must support. However, the following issues
are to be considered.
What is the application? LAN-LAN connectivity often requires
permanent high bandwidth connections. Remote access requires
an intermediate-speed dial-up connection, while LAN access
to SNA hosts is typically used for mission critical applications
requiring extremely reliable but lower-speed connections.
How
many locations are involved and where are they? Certain
media and communication protocols become more or less cost-effective
depending on how meshed the network is, and how far apart
offices are. Depending on where a branch is located, the
existing communications infrastructure may only permit certain
WAN media and communication protocols to be used.
What
are the traffic patterns? Only certain networks can handle
bursty LAN traffic. Also, the cost of some services varies
with usage, making these alternatives extremely expensive
for high traffic links. Moreover, usage fees may vary according
to the duration for which the link is used; for example,
costs may increase during business hours, which would be
impractical for certain applications
Answers
to the above questions should help narrow the choice of
WAN media and communication protocols.
Leased Lines
Leased
lines continue to be one of the most popular choices for
large corporations, especially in North America. Because
a leased line is a permanent link between two sites, it
can be used for LAN-LAN connections or LAN-SNA host connections.
It is suitable for carrying any communication protocol.
Leased lines offer excellent performance for these applications.
Leased lines support transmission speeds ranging from 19.2
Kbps to T3 (45 Mbps) and pricing is generally bandwidth
and distance sensitive.
Fiber
Links
Fiber links are based on fiber optic cables, which conduct
light very efficiently. The great advantage of fiber cables
over conventional wire cables is the substantial weight
and size savings and the much greater bandwidth offered.
Although fiber is not yet widely used by people in their
homes or offices, it is used for special applications such
as metropolitan area networks. Fiber links can be used to
transport various communication protocols. Fiber is generally
more expensive than the media alternatives discussed so
far.
Wireless
Media (Microwave and Satellite)
Microwave and satellite are closely related because both
are wireless. In both cases, data is transmitted via high
frequency radio signals from origin to a central relay station
or satellite. These media are delay sensitive and best used
for "one to many" applications such as distribution
of financial market information. Because of inconsistent
reliability and high expense, these media are not well suited
to LAN-LAN or LAN-SNA applications. Again, these media can
be used to transport a variety of communication protocols.
Upcoming Technologies ADSL
One promising technology is Asymmetric Digital Subscriber
Line (ADSL). It was developed to allow high bandwidth applications
to be delivered to homes (video on demand, Internet) using
existing unshielded twisted pair wiring. When available,
ADSL will allow high bandwidth in one direction and low
bandwidth in the other. ADSL uses signal encoding, compression,
and modulation techniques and can support various configurations
of data rates depending on the length of wire to be traversed.
A typical ADSL configuration supports a downstream delivery
rate of 6 Mbps and an upstream rate of 64 Kbps up to a distance
of 12,000 feet.
SDSL
Symmetric Digital Subscriber Line (SDSL) is based on the
same technology as ADSL. However, as the name suggests,
it offers equal bandwidth in both directions and is better
suited for applications such as video-conferencing and high-speed
remote LAN access. A typical configuration will support
data rates of 2 Mbps in each direction.
Cable
Modems
Another promising technology is the cable modem that allows
connection of personal computers to high bandwidth cable
TV coax cables. The good news about this technology is the
tremendous amount of bandwidth (up to 30 or 40 MBPS) that
it offers. The bad news is that this technology is not widely
available today and there are no industry standards for
interoperability. Also, unless the local cable company connects
to the public telephone network, users will not be able
to access sites that are not supported by the cable company.
WAN
Communication Protocols
Wide Area Network
communication protocols can seem like a complex subject.
However, two facts can help WAN service users better understand
the technologies.
Any
WAN media can be used to transport any WAN communication
protocol.
Any
WAN communication protocol can be encapsulated in any other
WAN communication protocol.
A
note of caution regarding communication protocols and encapsulation.
If you need to mix and match protocols on your WAN, do so
with care. Be careful to use only open protocols and encapsulation
standards. Here is a brief overview of the most common communication
protocols in use today.
PPP
and MLPPP
PPP is a point to point protocol primarily associated with
the Internet. PPP supports both asynchronous and synchronous
serial lines and provides a point to point connection across
a variety of media. PPP was first proposed as a standard
in 1990 to replace an older de facto standard known as SLIP
(Serial Line Internet Protocol) that requires links to be
established and terminated manually. However, unlike SLIP,
PPP offers significant flexibility. This explains its widespread
use for Internet communication.
Multilink
PPP (MLPPP) is a method for splitting, recombining and sequencing
PPP Datagrams across multiple logical data links. This protocol
was originally designed to exploit multiple B-channels in
ISDN, but is equally applicable to any situation in which
multiple serial PPP links connect two systems.
X.25
Packet-switched protocols have long been an alternative
to point to point protocols such as PPP, with X.25 being
the most mature of these alternatives. Packet-switched connections
are "virtual", in that connected sites are not
physically linked. While point to point protocols are analogous
to a direct pipeline between two sites (point to point),
a packet-switched service is analogous to a delivery service
sending packages between sites via intermediate hubs and
couriers.
For
this reason, a single physical link to the X.25 network
can support multiple concurrent connections to different
sites. X.25 is available almost universally, supporting
transmission speeds ranging from 9.6 KBPS to 512 KBPS, with
guaranteed global delivery. Because X.25 is a mature technology,
it supports numerous access methods. You can connect to
an X.25 network (a network where all traffic is X.25 packets)
through a dial-up telephone link, ISDN, or a dedicated leased
line. X.25 pricing is usage based, but not very distance
sensitive.
Over
long distances, it is far less expensive than using a point
to point protocol. These characteristics make X.25 suitable
for both LAN-LAN and LAN-SNA host connections--especially
for global corporations, which need reliability and universal
availability. X.25 is not, however, suited to high-bandwidth
applications, especially those sensitive to latency introduced
by X.25 networks.
X.25
is also suitable for remote user connectivity. Remote users
worldwide can simply dial into a local X.25 network using
a standard modem. The X.25 service provider's Packet Assembler/Disassembler
(PAD), converts the asynchronous data stream coming from
the modem into X.25 packets. This enables affordable and
reliable remote access to corporate LANs from anywhere in
the world.
Frame
Relay
Frame Relay is a packet-switched protocol, which has replaced
many point to point leased line applications. It was designed
to take advantage of the higher reliability offered by modern
telecommunications systems. Frame Relay can be viewed as
a streamlined version of X.25. It offers greater speed and
reduced latency, at the cost of less error-checking.
Though
Frame Relay availability is steadily growing, it may not
be available in countries with unreliable communications
infrastructures (in which case, X.25, with its advanced
error-checking, will usually be available). Applications
requiring meshed connectivity are especially cost-effective
using Frame Relay, since multiple virtual connections can
exist on a single physical link. An added benefit of Frame
Relay is that it allows bandwidth on demand up to the allowable
clock speed of the line, making it suitable for bursty applications.
Growing
confidence in Frame Relay networks is making it a popular
choice for SNA access (SNA over Frame Relay). It is also
gaining favor for remote access, as service providers begin
to offer dial-up access into Frame Relay networks via POTS
and ISDN. ISDN access into Frame Relay networks will also
enable them to easily back up the permanent Frame Relay
links currently in use. It seems likely that Frame Relay
will eventually enjoy the same degree of flexibility as
X.25 provides today.
Remember,
in a WAN setup the major chunk of the money goes in to the
communication links and WAN hardware does not amount to
even 20 per cent of the total cost. So a user must first
evaluate the wide area networking requirement in detail
and then choose the appropriate WAN media and communication
protocol which best suits his need.
NM
- V
Srivathsan, Technology Consultant, Chennai can be reached
at wan_isdn@yahoo.com
