The Public Network (pg. 2)


Dedicated Services

Voice, video and data can all be transmitted over different kinds of media over the dedicated lines be it copper, fiber or microwave. With dedicated service, organizations have the use of the line 24 hours a day, 7 days a week, and a flat monthly fee. This is effective if they transmit a high volume of voice, video and data transmissions between locations, since the flat fee is less than paying for multiple pay-per-minute calls to the same location.

The dedicated circuits are not shared, and are for the sole use of the customer who owns or leases the line. If a call is requested to be placed while dedicated private lines are busy, the private switch sends the call over the public switched network. This allows firms to pack a high volume of traffic on dedicated lines, but still have the flexibility to route overflow during peak traffic onto the public network.

Another feature of dedicated lines is that they are leased or built with a fixed capacity or bandwidth.

Most lines are leased with a minimum speed of 56 Kbps due to the small price difference between 56 Kbps and slower lines. Examples of speeds are 9600 bps, 19200 bps, 56 Kbps, 64 Kbps, T-1; 24 channels at 1.54 megabits per second, T1; 2-12 paths in increments of 56 or 64 Kbps per path, T-3; 672 channels at 44 megabits per second, and fractional T-3; 2-18 T-1s in increments of 1.54 megabits. Speeds of greater T-1 are available for large organizations with high bandwidth requirements. Carriers and local telephone companies sell both analog and digital private line.

Most users specify digital due to greater speeds, superior quality, more capacity and greater reliability with fewer errors as compared to analog lines. Speeds greater than 19200 bps are available only in digital formats. Carriers and local telephone companies prefer leasing digital lines and since digital lines fail less frequently than analog lines, maintenance costs are reduced.

There are a number of reasons why organizations prefer dedicated private lines to switched services. One factor is security, especially if transferring proprietary information or financial data. Private lines are not as easily tapped, and organizations even have the option of placing encryption devices on both ends of the dedicated services This scrambles the information leaving the sending location, and unscrambles it before reaching the receiving end. They are also more convenient since numbers connected by private networks are dialed with 4-5 digits instead of the normal 11 digits. On-site dedicated links allow one set of operators to answer calls for multiple locations which saves the cost and manpower of hiring operators for each location.

These links also improve customer service. A call to any company location can be easily transferred to another department without having the customer hang up and dial a different number. Some applications of dedicated services also include video transmission to multiple sites or call transfers to multiple sites within an organization. Employees at multiple sites can also access computer data and communicate via voice, video and mail on joint projects.

There are 4 private line configurations: point-to-point, multi-point, star configuration and mesh configuration. The main problem with these lines is the time required to manage them. A lot of maintenance is involved if a company has a private line with multiple sites that include back-up telecommunication services. Staff time is required for keeping track of equipment inventory associated with dedicated lines. Staff expertise and knowledge is also needed for network maintenance.

Many companies are unwilling to spend money for managing, designing and maintaining of the private facilities so they contract with a carrier for managed value-added services such as frame-relay. These services are known as virtual networks. Virtual networks, such as a frame-relay, are like private networks but without the hassle of maintenance and staff. A carrier manages them, and customers connect to the carrier via a dial-up or dedicated line. Due to the cost of a private network in both staff and lines, a number of organizations have private lines for routes with the highest amount of data traffic, and virtual networks for routes with less voice and data traffic.


Signaling and Network Intelligence

Signaling is the process of sending information between two points of a network to control, route and maintain a telephone call. There are basically 3 types of signals – supervisory signals, alerting signals and addressing signals. Supervisory signals monitor the condition of a telephone and whether it is busy or idle. These signals are also used to determine when the telephone handset is lifted, hung-up or in an idle condition.

The bell tones or strobe lights that alert the end user that a call has arrived are the alerting signals. The addressing signals are dial tones or data pulses that tell the network where to send a call. Signals can be sent over the same channel as voice or data, or on a separate channel. Before 1976, they were sent along the same path, which this was known as in-band signaling. In-band signaling sets up calls slower and is inefficient. Out-of-band signaling, sends signals on a separate channel, and is more efficient, since the other channels carry more data instead of just signaling information. ISDN is an example of a technology that uses out-of-band signaling.

Local telephone companies have 2 types of central offices.

Tandem offices work in tandem (along with) each central office. They connect central offices to each other or to inter-exchange carrier switches, without direct connections to end-users. They carry the calls on trunks and are therefore able to handle a greater volume of calls. In contrast, end-offices provide many POT lines between homes and telephone company switches. The dialing sequence is sent over the same channel as the user data or call. This results in less volume for calls on these lines than on trunks.

Network-based computer intelligence changes the public network from POTs to a network capable of delivering advanced features. This helps to generate huge profits for carriers. Examples of these advanced features are call forwarding, caller ID and voice mail.

A common channel for inter-office signaling or out-of-band signaling was a data communications network laid over the switching networks. Although this started as a way to increase network efficiency, it eventually evolved into the basis for intelligent networks. Signaling System 7 (SS7) is a layered protocol approved by ITU that enables carriers to work harmoniously with each other. Signals are sent between central offices, carriers and end-users. SS7 is based on the common channel signaling architecture. Signals can be sent both ways providing faster speeds. AT&T specified the faster SS7 with signaling links of 64000 bps. This was later approved by the CCITT (translated from French, International Telegraph and Telephone Consultative Committee – now known as the International Telecommunications Union, or ITU).

The implementation of SS7 differed among countries. The US implemented the ANSI (American National Standards Institute) version of SS7 and Europe implemented the ETSI (European Telecommunications Standards Institute) version. SS7 played an important part in lowering costs and increasing the reliability of public networks.

The public network has evolved from one carrying only voice and data calls to a network with increased intelligence, greater capacity and faster recovery from equipment failures.


Take the public network quiz!

Educators Link | Credits | Resources | Copyright Info | Help