Three choices that an organization generally has for services and facilities in transmitting data

An organization generally has three choices for services and facilities for transmitting data: common carriers, which provide incremental services upon the common carriers facilities; and communications media vendors, which provide facilities that the organization may lease or purchase for its own use.

Some large organizations, such as the federal government, are able to build their own communications networks; however, this option is often prohibitively expensive.

1. Common Carriers:

Common carriers are companies licensed by the government to provide wide area communications services to the public. Wide area networks (WANs) are communications networks that encompass a relatively wide geographical area.

Among the most familiar common carriers are the Bell- system phone companies and firms such as GTE Sprint Communications Corporation, MCI Communications Corporation, and AT&T Communica­tions.

Image Source:daleelteq.com.pk

The Bell-system companies provide in-state and nearby-state com­munications services, and the other firms serve a wider area. Among the less familiar common carriers are the specialized carrier companies, such as those that provide satellite transmission facilities.

2. Switched Lines :

Because points in the public telephone system receive calls from different locations, the phone system is designed to route calls into a huge switch­ing network. Remember the telephone operators in the old-time movies who switched plugs in and out of a big board t connect people?

Today’s switched lines work in much the same way except that most of the switch­ing is automated rather than manual. Users of these lines can “dial-up” computers from their terminals, and the calls are routed, or switched, through paths in public or private phone networks to the proper destinations.

Switched lines are attractive because a person can dial any computer that has a phone number and gain access to it. Also, using the public phone network is often inexpensive because, as with home phones, charges are based directly on the number and length of calls.

Unfortunately, switched lines can transmit data only at slow rates. As a result, switched lines often are impractical for devices such as graphics workstations, which require fast transmission speeds to communicate pictures.

It is also possible to get annoying busy signals on switched lines just as on a home phone. Three common ways of switching data in communications networks are circuit switching, message switching, and packet switching.

3. Circuit Switching :

Circuit switching is the procedure most people encounter when they dial a phone number. You place a call, and you either reach your destination or you don’t. If the party doesn’t answer, you don’t get through.

Message Switching:

Perhaps the most familiar example of message switching is the answering machine. When you place a call, your message reaches its destination whether or not the person is ready to receive it.

Packet Switching:

Packet switching is a method whereby a message is broken up into small packets, each of which can travel along a different path end route to the ultimate destination. The routes along which packets travel to the destina­tion are dynamically determined as they are sent.

Network software selects these routes so as to optimize network traffic flow. As the packets arrive at the destination, they are reassembled into the original message.

Dedicated Lines :

Dedicated lines, or no switched lines, can be leased or purchased from either common carriers or non-government-licensed vendors that sell com­munications media for private use.

Dedicated lines are used to circumvent many of the problems inherent in public-access switching. They provide a constantly available, point-to-point connection between two devices.

Thus, one makes immediate contact with a given computer system every time the “on” switch is activated at the terminal. In addition, because only the owner or lessee has access to the line, dedicated lines can be specially conditioned to transmit data at higher speeds.

Generally the caller is not charged for every call made on the line but pays a flat rate. Thus, users of dedicated lines get to transmit data quickly, in volume, at a fixed cost, and with no threat of busy signals. However, they lack the flexibility to dial up other computer system over these lines.