outweighed by the benefits. The dedicated capacity gives no latency or jitter between the endpoints. Constant availability is essential for some applications such as electronic commerce. 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 are used extensively for building WANs and give permanent dedicated capacity. They have been the traditional connection of choice but have a number of disadvantages. WAN traffic is often variable and leased lines have a fixed capacity. This results in the bandwidth of the line seldom being exactly what is needed. In addition, each end point would need an interface on the router which would increase equipment costs. Any changes to the leased line generally require a site visit by the carrier to change capacity. Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network. Several connections can be multiplexed over a leased line, resulting in shorter links and fewer required interfaces. Interactive Media Activity Checkbox: WAN Technologies Leased Lines Upon completing this activity, the student will be able to identify the characteristics associated with a leased line connection.
Content 2.2 WAN Technologies 2.2.4 X.25 In response to the expense of leased lines, telecommunications providers introduced packet-switched networks using shared lines to reduce costs. The first of these packet-switched networks was standardized as the X.25 group of protocols. X.25 provides a low bit rate shared variable capacity that may be either switched or permanent. X.25 is a network-layer protocol and subscribers are provided with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection. Subscribers connect to the X.25 network with either leased lines or dialup connections. X.25 networks can also have pre-established channels between subscribers that provide a PVC. X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks. X.25 technology is no longer widely available as a WAN technology in the US. Frame Relay has replaced X.25 at many service provider locations. Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. Some enterprises also use X.25 based value-added networks (VAN) to transfer Electronic Data Interchange (EDI) invoices, bills of lading, and other commercial documents. For these applications, the low bandwidth and high latency are not a concern, because the low cost makes the use of X.25 affordable. Interactive Media Activity Checkbox: WAN Technologies X.25 Upon completing this activity, the student will be able to identify the characteristics associated with an X.25 connection. Web Links X.25 Protocol http://www2.rad.com/networks/1996/ x25/x25.htm
Content 2.2 WAN Technologies 2.2.5 Frame Relay With increasing demand for higher bandwidth and lower latency packet switching, communications providers introduced Frame Relay. Although the network layout appears similar to that for X.25, available data rates are commonly up to 4 Mbps, with some providers offering even higher rates. Frame Relay differs from X.25 in several aspects. Most importantly, it is a much simpler protocol that works at the data link layer rather than the network layer. Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter. Most Frame Relay connections are PVCs rather than SVCs. The connection to the network edge is often a leased line but dialup connections are available from some providers using ISDN lines. The ISDN D channel is used to set up an SVC on one or more B channels. Frame Relay tariffs are based on the capacity of the connecting port at the network edge. Additional factors are the agreed capacity and committed information rate (CIR) of the various PVCs through the port. Frame Relay provides permanent shared medium bandwidth connectivity that carries both voice and data traffic. Frame Relay is ideal for connecting enterprise LANs. The router on the LAN needs only a single interface, even when multiple VCs are used. The short-leased line to the FR network edge allows cost-effective connections between widely scattered LANs. Interactive Media Activity Checkbox: WAN Technologies Frame Relay Upon completing this activity, the student will be able to identify the characteristics associated with a Frame Relay circuit. Web Links Frame Relay http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/frame.htm
Content 2.2 WAN Technologies 2.2.6 ATM Communications providers saw a need for a permanent shared network technology that offered very low latency and jitter at much higher bandwidths. Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond 155 Mbps. As with the other shared technologies, such as X.25 and Frame Relay, diagrams for ATM WANs look the same. ATM is a technology that is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The 53 byte ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted. The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead will be higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20% greater bandwidth than Frame Relay to carry the same volume of network layer data. ATM offers both PVCs and SVCs, although PVCs are more common with WANs. As with other shared technologies, ATM allows multiple virtual circuits on a single leased line connection to the network edge. Interactive Media Activity Checkbox: WAN Technologies Asynchronous Transfer Mode (ATM) Upon completing this activity, the student will be able to identify the characteristics associated with ATM connections. Web Links Asynchronous Transfer Mode (ATM) Switching http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/atm.htm
Content 2.2 WAN Technologies 2.2.7 DSL Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typical LANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The term xDSL covers a number of similar yet competing forms of DSL technologies: DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop