2.2 WAN Technologies
2.2.1 Analog Dialup
When intermittent, low-volume data transfers are needed, modems and analog 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 public switched telephone network (PSTN). The signal on the local loop during a call is a continuously varying electronic signal that is a translation of the subscriber voice.
The local loop is not suitable for direct transport of binary computer data, but a modem can send computer data through the voice telephone network. The modem modulates the binary data into an analog signal at the source and demodulates the analog signal at the destination to binary data.
The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal. The upper limit is around 33 kbps. The rate can be increased to around 56 kbps if the signal is coming directly through a digital connection.
For small businesses, this can be adequate for the exchange of sales figures, prices, routine reports, and email. Using automatic dialup at night or on weekends for large file transfers and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs are based on the distance between the endpoints, time of day, and the duration of the call.
The advantages of 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 provided by dialup will have little delay or jitter for point-to-point traffic, but voice or video traffic will not operate adequately at relatively low bit rates.
2.2.2 ISDN
The internal connections, or trunks, of the PSTN have changed from carrying analog frequency-division multiplexed signals, to time-division multiplexed (TDM) digital signals. An obvious next step is to enable the local loop to carry digital signals that result in higher capacity switched connections.
Integrated Services Digital Network (ISDN) turns the local loop into a TDM digital connection. The connection uses 64 kbps bearer channels (B) for carrying voice or data and a signaling, delta channel (D) for call set-up and other purposes.
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. For larger installations, Primary Rate Interface (PRI) ISDN is available. PRI delivers twenty-three 64 kbps B channels and one 64 kbps D channel inNorth America , for a total bit rate of up to 1.544 Mbps.
This includes some additional overhead for synchronization. In Europe, Australia
The BRI D channel is underutilized, as it has only two B channels to control. Some providers allow the D channel to carry data at low bit rates such as X.25 connections at 9.6 kbps.
For small WANs, the BRI ISDN can provide an ideal connection mechanism. BRI has a call setup time that is less than a second, and its 64 kbps B channel provide greater capacity than an analog modem link.
If greater capacity is required, a second B
channel can be activated to provide a total of 128 kbps. Although inadequate
for video, this would permit several simultaneous voice conversations 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 while ISDN is added during peak demand periods. ISDN is also used as a backup in the case of a failure of the leased line. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two end points. This allows for video conferencing and high bandwidth data connections with no latency or jitter. Multiple connections can become very expensive over long distances.
2.2.1 Analog Dialup
When intermittent, low-volume data transfers are needed, modems and analog 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 public switched telephone network (PSTN). The signal on the local loop during a call is a continuously varying electronic signal that is a translation of the subscriber voice.
The local loop is not suitable for direct transport of binary computer data, but a modem can send computer data through the voice telephone network. The modem modulates the binary data into an analog signal at the source and demodulates the analog signal at the destination to binary data.
The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal. The upper limit is around 33 kbps. The rate can be increased to around 56 kbps if the signal is coming directly through a digital connection.
For small businesses, this can be adequate for the exchange of sales figures, prices, routine reports, and email. Using automatic dialup at night or on weekends for large file transfers and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs are based on the distance between the endpoints, time of day, and the duration of the call.
The advantages of 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 provided by dialup will have little delay or jitter for point-to-point traffic, but voice or video traffic will not operate adequately at relatively low bit rates.
2.2.2 ISDN
The internal connections, or trunks, of the PSTN have changed from carrying analog frequency-division multiplexed signals, to time-division multiplexed (TDM) digital signals. An obvious next step is to enable the local loop to carry digital signals that result in higher capacity switched connections.
Integrated Services Digital Network (ISDN) turns the local loop into a TDM digital connection. The connection uses 64 kbps bearer channels (B) for carrying voice or data and a signaling, delta channel (D) for call set-up and other purposes.
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. For larger installations, Primary Rate Interface (PRI) ISDN is available. PRI delivers twenty-three 64 kbps B channels and one 64 kbps D channel in
In North America PRI corresponds to a T1
connection. The rate of international PRI corresponds to an E1 connection. The BRI D channel is underutilized, as it has only two B channels to control. Some providers allow the D channel to carry data at low bit rates such as X.25 connections at 9.6 kbps.
For small WANs, the BRI ISDN can provide an ideal connection mechanism. BRI has a call setup time that is less than a second, and its 64 kbps B channel provide greater capacity than an analog modem link.
If greater capacity is required, a second B
channel can be activated to provide a total of 128 kbps. Although inadequate
for video, this would permit several simultaneous voice conversations 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 while ISDN is added during peak demand periods. ISDN is also used as a backup in the case of a failure of the leased line. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two end points. This allows for video conferencing and high bandwidth data connections with no latency or jitter. Multiple connections can become very expensive over long distances.
Comments
Post a Comment