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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 Netw
ork 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

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