service providers also offer permanent circuits. These dedicated or leased lines offer higher bandwidth than is available with a switched circuit. Examples of circuit-switched connections include: Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits, because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In these networks, the data is transmitted in labeled cells, frames, or packets through a packet-switched network. Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks. Packet-switched networks may establish routes through the switches for particular end-to-end connections. Routes established when the switches are started are PVCs. Routes established on demand are SVCs. If the routing is not pre-established and is worked out by each switch for each packet, the network is called connectionless. To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available. This is called the point-of-presence (POP) of the service. Normally this will be a dedicated leased line. This line will be much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs. Since it is likely that not all the VCs will require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs. Examples of packet or cell switched connections include: Web Links WAN http://www.cisco.com/en/US/tech/ tk713/ tech_topology_and_ network_serv_and_ protocol_suite_ home.html
Content 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. Interactive Media Activity Checkbox: WAN Technologies Analog Dialup Upon completing this activity, the student will be able to identify the characteristics associated with an analog dialup circuit. Web Links Dialup Technologies http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/ dialup.htm
Content 2.2 WAN Technologies 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 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 thirty B channels and one D channel for a total bit rate of up to 2.048 Mbps, including synchronization overhead. 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. Interactive Media Activity Checkbox: WAN Technologies ISDN Dialup Upon completing this activity, the student will be able to identify the characteristics associated with an ISDN dialup circuit. Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/isdn.htm
Content 2.2 WAN Technologies 2.2.3 Leased line When permanent dedicated connections are required, leased lines are used with capacities ranging up to 2.5 Gbps. A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are called leased lines. Leased lines are available in different capacities. These dedicated circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. The cost of leased-line solutions can become significant when they are used to connect many sites. There are times when cost of the leased line is