Wide Area Networks (WANs) - Transmission Media and Connectivity

While; still interconnecting a number of geographically dispersed smaller networks, today’s Wide Area Networks (WANs) tend to be classified and distinguished by being networks that use routers and publicly accessible communications links. In short the purpose of a WAN is to enable users and computers in one location to communicate with users and computers in other, often very geographically dispersed and separated locations. Without doubt the largest and most well-known WAN is the Internet.

Wide Area Networks (WANs) Transmission Media

WANs typically consist of numerous interconnected switching nodes that facilitate the transmission from any given member device (node) to be routed through various other internetwork connected member devices (nodes) eventually arriving at a specific desired destination device; or in the case of a broadcast or multicast, multiple specific desired destination nodes.

The various intermediary routing/switching nodes are not concerned with the contents of data. Rather, their interest is focused on the provision of a switching or routing facility to move the data from node-to-node until they arrive at their intended destination. Fundamental to routed/switched transmission (transport technologies) is the medium used to convey the message/conversation.

To a large extent; the transmission medium selected, will dictate the architecture, signaling and protocols to be used in building the internetwork (WAN). Performance characteristics such as bandwidth, data transfer rates, data types, data formats, mobility, maximum number of supported users and the types of services (data, voice, video, VoIP etc.) the WAN will deliver are greatly influenced by choice of transmission medium. Although; in some instances, the nature and type of the services that the WAN is intended to deliver will dictate the type of transmission media utilized.

Over time we have seen a number of different transmission media used in building WANs. It is worthy of note that a WAN need not be committed to a single transmission medium. On the contrary; using more than one transmission media for different segments throughout a WAN, allows the network designer greater freedom in adopting and adapting a hierarchal network structure. Thus; most WANs will use various combinations of the following information transmission and transport technologies:

Copper Media - Telephone lines, coaxial cable, Shielded Twisted Pair (STP), Unshielded Twisted Pair (UTP), CAT cable etc

Fiber Optic Media – Single-Mode and Multi-Mode

Wireless - Radio frequency channels, microwave links, satellite channels, Blue Tooth, Wi-Fi, private wireless networks (home, corporate LANs) and publically accessible ad hoc wireless networks “hot spots”

Wide Area Networks (WANs) Transmission Rates

Typically, WAN transmission rates have ranged from 1.2K-bits/sec to 6 M-bit/sec. Although; some connections such as ATM and Leased Lines can reach speeds greater than 156 M-bit/sec.

In more recent times we have seen the advent of ADSL 2+ which has upped the ante even further particularly for the consumer market. Due to the considerable cost benefits and overall fiscal advantages that can be realized from using ADSL2+ technologies business is also implementing ADSL2+ over older services such as leased lines as the preferred Internet connectivity option.

With transmission rates up to 30 Mbps, DSL and cable modem are two high data-transmission rate consumer Internet connections that transmit considerably faster than a dial-up modem (56 kbps). Add to this the fact that they are also generally cheaper than both ISDN and dial-up and you get a very cost-effective solution.

Wide Area Network (WAN) Connectivity Options

Originally WANs were built using expensive leased lines. If WANs such as the Internet were to ever become truly global less costly alternatives to using expensive leased lines when building a WAN had to be found.

Solutions were found including the use of circuit switching or packet switching technologies. Here, network protocols including TCP/IP serve to deliver transport and addressing functions. While protocols such as Packet over SONET/SDH, Multiprotocol Layer Switching (MPLS), Asynchronous Transfer Mode (ATM) and Frame Relay are commonly used by Internet Service Providers (ISPs) to deliver the links that are used in WANs.

Leased Line – Provide secure but comparatively expensive Point-to-Point connectivity between two computers or Local Area Networks (LANs) using protocols such as Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC) and Synchronous Data Link Control (SDLC).

Circuit Switching – A less expensive dedicated circuit path offering bandwidth data transfer rates ranging from 28K-bit/sec to 144K-bit/sec is created between end points. On the downside call setup and connection establishment needs to be renegotiated every time access is desired because the link is not necessarily permanent. The most well known example of circuit switching WAN connectivity is dial-up connections. Point-to-Point Protocol (PPP) and Integrated Service Digital Network (ISDN) are two of the most widely used protocols for circuit switching WAN connectivity.

Packet Switching – Variable length packets are transported over a shared single point-to-point or point-to-multipoint link across a carrier internetwork using Permanent Virtual Circuits (PVC) or Switched Virtual Circuits (SVC). X.25 and Frame Relay are two examples of packet switching protocols used for WAN connectivity.

Cell Relay - Cell Relay is very similar to packet switching, but uses fixed length cells instead of variable length packets. Data is divided into fixed-length cells and then transported across virtual circuits. Unfortunately the overhead can constitute a significant proportion of the total bandwidth. Cell relay protocols such as Asynchronous Transfer Mode (ATM) (up to 155M-bit/sec) are best for simultaneous use of Voice and data.

Virtual Private Network (VPN) – With the recent reductions in Internet connectivity and concurrent increases in bandwidth and transmission rates now offered by ISPs many organizations have opted to use VPN technologies such as those on offer from the likes of Cisco Systems, New Edge Networks, Juniper, Check Point and Vyatta to interconnect their networks. One of VPN’s strong points is encryption and considering the prevalence of cyber-crime today it is no surprise to find that this form of WAN is currently very popular.