Network Devices

LANs and WANs

LANs were first used between PCs when users needed to connect with other PCs in the same building to share resources. A LAN is a high-speed, yet relatively inexpensive, network that allows connected computers to communicate. LANs have limited reach (hence the term local-area network), typically less than a few hundred meters, so they can only connect devices that are in the same room or building, or possibly within the same campus.

A LAN is an always-on connectionin other words, you don't have to dial up or otherwise connect to it when you want to send some data. LANs also belong to the organization in which they are deployed, so no incremental cost is typically associated with sending data.

A variety of LAN technologies are available, some of which are shown in the center of Figure B-4 and briefly described here:

• Ethernet and IEEE 802.3, running at 10 megabits per second (Mbps), use a carrier sense multiple access collision detect (CSMA/CD) technology. When a CSMA/CD device has data to send, it listens to see whether any of the other devices on the wire (multiple access) are transmitting (carrier sense). If no other device is transmitting, this device starts to send its data, listening all the time in case another device erroneously starts to send data (collision detect).

• Fast Ethernet (at 100 Mbps), covered by the IEEE 802.3u specification, also uses the CSMA/CD technology.

• Gigabit Ethernet (running at 1 gigabit per second [Gbps]) is covered by the IEEE 802.3z and 802.3ab specifications and uses the CSMA/CD technology.

• Wireless LAN (WLAN) standards, defined by the IEEE 802.11 specifications, are capable of speeds up to 54 Mbps under the 802.11g specification.

Figure B-4. A Variety of LAN and WAN Standards

WANs interconnect devices (which are usually connected to LANs) that are located over a relatively broad geographical area (hence the term wide-area network). Compared to a LAN, a typical WAN is slower, requires a connection request when you want to send data, and usually belongs to another organization (called the service provider). You pay the service provider a fee (known as a tariff) for the use of the WAN; this fee could be a fixed monthly amount, or it could be variable based on usage and/or distance.

Just as you find many types of LANs, many types of WANs are also available, some of which are illustrated on the right side of Figure B-4. Like LANs, WANs function at the lower two layers of the OSI model (X.25 and Integrated Services Digital Network [ISDN] also function at Layer 3). The service you use depends on many factors, including what is available where you are and, of course, the cost of the service.

Some of the common WAN technologies include the following:

• Packet-switched network A network that shares the service provider's facilities. The service provider creates permanent virtual circuits (PVCs) and switched virtual circuits (SVCs) that deliver data between subscribers' sites. X.25 and Frame Relay are examples of packet-switched networks.

• Leased line A point-to-point connection that is reserved for transmission. Common data link layer protocols used in this case are Point-to-Point Protocol (PPP) and High-Level Data Link Control (HDLC).

• Circuit-switched network A physical path that is reserved for the duration of the connection between two points. ISDN Basic Rate Interface (BRI) is an example of this type of network.

Two other technologies, digital subscriber line (DSL) and cable, connect residential and business premises to service providers' premises; they are described as follows:

• DSL Utilizes unused bandwidth on traditional copper telephone lines to deliver traffic at higher speeds than traditional modems allow. The most common DSL implementation is asymmetric DSL (ADSL). ADSL allows regular telephone traffic to simultaneously share the line with high-speed data traffic so that only one telephone line is required to support both high-speed Internet and normal telephone services.

• Cable Utilizes unused bandwidth on cable television networks to deliver data at higher speeds than traditional modems allow.