CCNA :) Be a Good Network Administrator

2.3  WAN Design  2.3.1 WAN Communication WANS are considered to be a set of data links connecting routers on LANs. User end stations and servers on LANs exchange data. Routers pass data between networks across the data links. Because of cost and legal reasons, a communications provider or a common carrier normally owns the data links that make up a WAN. The links are made available to subscribers for a fee and are used to interconnect LANs or connect to remote networks. WAN data transfer speed (bandwidth) is considerably slower than the 100 Mbps that is common on a LAN. The charges for link provision are the major cost element of a WAN and the design must aim to provide maximum bandwidth at acceptable cost. With user pressure to provide more service access at higher speeds and management pressure to contain cost, determining the optimal WAN configuration is not an easy task. WANs carry a variety of traffic types such as data, voice, and ...

2.2.8 Cable Modem Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from some cable television networks. This allows for greater bandwidth than the conventional telephone local loop. Enhanced cable modems enable two-way, high-speed data transmissions using the same coaxial lines that transmit cable television. Some cable service providers are promising data speeds up to 6.5 times that of T1 leased lines. This speed makes cable an attractive medium for transferring large amounts of digital information quickly, including video clips, audio files, and large amounts of data. Information that would take two minutes to download using ISDN BRI can be downloaded in two seconds through a cable modem connection. Cable modems provide an always-on connection and a simple installation. An always-on cable connection means that connected computers are vulnerable to a security breach at all times and need to be suitably secured with firewalls...

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...