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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: Asymmetric DSL (ADSL) Symmetric DSL (SDSL) High Bit Rate DSL (HDSL) ISDN (like) DSL (IDSL) Consumer DSL (CDSL), also called DSL-lite or G.lite DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop copper lines. DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. Multiple DSL subscriber lines are multiplexed into a single, h...

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 a...

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 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 requ...

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 sign...

2.1.6 WAN link options Circuit switching establishes a dedicated physical connection for voice or data between a sender and receiver. Before communication can start, it is necessary to establish the connection by setting the switches. This is done by the telephone system, using the dialed number. ISDN is used on digital lines as well as on voice-grade lines. To avoid the delays associated with setting up a connection, telephone 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: Plain Old Telephone System (POTS) ISDN Basic Rate Interface (BRI) ISDN Primary Rate Interface (PRI) 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 servic...

2.1.5  Packet and circuit switching  Packet-switched networks were developed to overcome the expense of public circuit-switched networks and to provide a more cost-effective WAN technology. When a subscriber makes a telephone call, the dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the originating caller to that of the called party. Because of the switching operation used to establish the circuit, the telephone system is called a circuit-switched network. If the telephones are replaced with modems, then the switched circuit is able to carry computer data. The internal path taken by the circuit between exchanges is shared by a number of conversations. Time division multiplexing (TDM) is used to give each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber. If the circuit carries computer data, the usage of this fixed c...

2.1.3 WAN Standards WANs use the OSI reference model, but focus mainly on Layer 1 and Layer 2. WAN standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation. WAN standards are defined and managed by a number of recognized authorities. The physical layer protocols describe how to provide electrical, mechanical, operational, and functional connections to the services provided by a communications service provider. Some of the common physical layer standards are listed in Figure , and their connectors illustrated in Figure . The data link layer protocols define how data is encapsulated for transmission to remote sites, and the mechanisms for transferring the resulting frames. A variety of different technologies are used, such as ISDN, Frame Relay or Asynchronous Transfer Mode (ATM). These protocols use the same basic framing mechanism, high-level data link control (HDLC), an ISO...