The section covers various WAN connections, including private dedicated links,private switched links, and public connection option using the Internet.

Wan Link Connection Options

Many options for implementing WAN solutions currently available. They differ in technology, speed and cost. Familiarity of these technologies is an important part of network design and evaluation:

 

• Private WAN connection options: Private WAN connections include both dedicated and switched communication options:

1. Dedicated communication links: When permanent dedicated connection are required, point-to-point lines are used with various capacities that are limited only by the underlying physical facilities and the willingness of users to pay for these dedicated lines. A point-to-point link provides a preestablished WAN communication path from the customer premises through the provider network to a remote destination. Point-to-point lines usually are leased from a carrier and are also called leased lines.
2. Switched communication links: Switch communication links can be either circuit-switched or packet-switched:

• Circuit-switched communication links: Circuit switching dynamically establishes a dedicated connection for voice or data between a sender and a receiver. Before communication can start, it is necessary to establish the connection through the service provider's network. Examples of Circuit -Switched communication links are analog dialup (PSTN) and ISDN.
• Packet-switched communication links: Many WAN user do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuit, because the data flow fluctuates. Communication providers have data networks available more appropriately service these users. In packet-switched networks, the data is transmitted in label frames, cells or packets. Packet switched communication links include Frame Relay, ATM, X.25 and Metro Ethernet.

•  Public WAN connection options: Public connections use the global internet infrastructure. Until recently, The Internet was not a viable networking option for many businesses because of the significant security risk and lack of adequate performance guarantees in an end-toend Internet connection. With the development of VPN technology, however, the Internet is now an inexpensive and secure option for connecting to teleworkers and remote offices where performance guarantees are not critical. Internet WAN connection links are through broadband services such as DSL, cable modem, and broadband wireless, and they are combined with VPNs technologyto prtovide privacy across the Internet.

Dedicated Connection Link Option

When permanent dedicated connections are required, a point-to-point link is used to provide a preestablished WAN communications path from the customer premises through the provider network to remote destination.

Point-to-point lines usually are leased from carrier and are called leased lines.

Point-to-point links usually are more expensive than shared services such as Frame Relay. The cost of leased line solutions become significant when they are used to connect many sites over increasing distances. However, sometimes the benefits over weigh  the cost of leased lines. The dedicated capacity remove latency and jitter between the endpoints. Constant availability is essential for some application, such as VoIP and video over IP.

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 provide permanent dedicated capacity and are used extensively to build WANs. They have been the traditional connection of choice but have  a number of disadvantages.Leased lines have a fixed capacity: however, WAN traffic is often variable, leaving some of the capacity unused. In addition, each endpoint needs a separate physical interface on the router, which increases equipment costs. Any changes to the leased line generally require a site visit by the carrier.

Circuit-Switched Connection Option

Circuit-switched networks establish a dedicated connection for voice or data between  a sender and receiver. Before any communications can begin, it is necessary to establish the connection through the service provider's network.

Analog Dialup

When intermittent, low-volume data transfers are needed, modems and dialed telephone lines provide low capacity and dedicated switched connections.

Traditional telephony uses a copper cable, called the local loop, to connect the telephone handset in the subscriber premises to the CO. The signal on the local loop during a call  is a continuously varying electronic signal  that is a translation of subscriber analog voice signal.

Traditional local loops can transport binary computer data through the voice telephone network using a modem.  The modem modulates the binary data into an analog signal at the source and demodulates the analog signal into binary data at the destination. The physical characteristics of the local loop and its connection to the PSTN limit the signal's rate to less than 56 kbps.

For small businesses, these relatively low-speed dialup connection are adequate for the exchange of sales figures, prices, routine reports, and e-mail. Using automatic dial at night or on weekends for large file transfer and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs are based between the endpoints, time of day and the cell's duration.

The advantages of the modem and analog lines are simplicity, availability and low implementation cost. The disadvantages are the low data rates and a relatively long connection time. The dedicated circuit has a little delay or jitter for point-to-point traffic, but voice or video traffic does not operate adequately at these low low bit rates.

Integrated Services Digital Network

Integrated Services Digital Network (ISDN) is a circuit-switching technology that enables the local loop of a PSTN to carry digital signals, resulting in higher-capacity switched connections. ISDN changes the internal connections of the PSTN  from carrying analog signal to time division multiplexed (TDM) digital signals. TDM allows two or more signals or bit streams to be transferred as subchannels in one communication channel. A data block  of sub channel 1 is transferred during time slot 1, subchannel 2 during time slot 2 and so on. One TDM frame consist of one time slot per channel. TDM is describe more in the following blog lessons.

ISDN turns the local loop into a TDM  digital connection. This change enables the local loop to carry digital signals that result in higher-capacity switched connections. The connection uses 64-kbps bearer (B) channel to carry voice or data and a signaling, delta channel for call setup and other purposes.

There are two types of ISDN interface:

• Basic Rate Interface (BRI): ISDN is intended for the home and small enterprise and provides two 64-kbps B -channels and a 16-kbps D-channel. The BRI D channel is design for control and often is underused, because it has only two B channels to control. Therefore, some providers allow the D channel to carry  data and low bit rates such ass X.25 connections. at 9.6 kbps.
• Primary Rate Interface (PRI): ISDN  is also available for larger installations. PRI  delivers 23 B channels with 64 kbps and one D channel with 64 kbps.  in north america, for a total bit rate of up to 1.544 Mbps. This includes some additional overhead for synchronization. In Europe , Australia, and other parts of the world, ISDN PRI  provides 30 B  channels and one D channel, for a total bit rate up to 2.048 Mbps., including synchronization overhead. In North America corresponds to an E1 to J1 connection.

For WAN links, which require low bandwidth, the BRI ISDN can provide an ideal connection mechanism. BRI has a call set up time that is less than a second, and the 64 kbps. B channel provides greater capacity than an analog modem link. If greater capacity is require, a second  B channel can be activated to provide a total of  128 kbps. Although this is inadequate for video , it permits several simultaneous voice conversation

in addition to data traffic .

Another common application of ISDN  is to provide additional capacity as needed on a leased-line connection. The leased -line is sized to carry average traffic loads, and ISDN is added  during peak demand periods. ISDN is also used as a back up if the leased-line fails. ISDN tariffs are based on a per-B-channel basis and are similar to those of analog voice connection.

With PRI ISDN, multiple B channels can be connected between two endpoints. This allows for video conferencing and high bandwidth data connections with no latency or jitter. However, multiple connections can be very expensive over long distances.