Content Overview As the enterprise grows beyond a single location, it is necessary to interconnect the LANs in the various branches to form a wide-area network (WAN). This module examines some of the options available for these interconnections, the hardware needed to implement them, and the terminology used to discuss them. There are many options currently available today for implementing WAN solutions. They differ in technology, speed, and cost. Familiarity with these technologies is an important part of network design and evaluation. If all data traffic in an enterprise is within a single building, a LAN meets the needs of the organization. Buildings can be interconnected with high-speed data links to form a campus LAN if data must flow between buildings on a single campus. However, a WAN is needed to carry data if it must be transferred between geographically separate locations. Individual remote access to the LAN and connection of the LAN to the Internet are separate study topics, and will not be considered here. Most students will not have the opportunity to design a new WAN, but many will be involved in designing additions and upgrades to existing WANs, and will be able to apply the techniques learned in this module. Students completing this module should be able to:
Content 2.1 WAN Technologies Overview 2.1.1 WAN technology A WAN is a data communications network that operates beyond the geographic scope of a LAN. One primary difference between a WAN and a LAN is that a company or organization must subscribe to an outside WAN service provider in order to use WAN carrier network services. A WAN uses data links provided by carrier services to access the Internet and connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users. WANs generally carry a variety of traffic types, such as voice, data, and video. Telephone and data services are the most commonly used WAN services. Devices on the subscriber premises are called customer premises equipment (CPE). The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office (CO). This cabling is often called the local loop, or "last-mile". A dialed call is connected locally to other local loops, or non-locally through a trunk to a primary center. It then goes to a sectional center and on to a regional or international carrier center as the call travels to its destination. In order for the local loop to carry data, a device such as a modem is needed to prepare the data for transmission. Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE). The customer devices that pass the data to the DCE are called data terminal equipment (DTE). The DCE primarily provides an interface for the DTE into the communication link on the WAN cloud. The DTE/DCE interface uses various physical layer protocols, such as High-Speed Serial Interface (HSSI) and V.35. These protocols establish the codes and electrical parameters the devices use to communicate with each other. WAN links are provided at various speeds measured in bits per second (bps), kilobits per second (kbps or 1000 bps), megabits per second (Mbps or 1000 kbps) or gigabits per second (Gbps or 1000 Mbps). The bps values are generally full duplex. This means that an E1 line can carry 2 Mbps, or a T1 can carry 1.5 Mbps, in each direction simultaneously. Web Links Introduction to WAN Technologies http://www.cisco.com/univercd/cc/td/ doc/ cisintwk/ito_doc/ introwan.htm
Content 2.1 WAN Technologies Overview 2.1.2 WAN devices WANs are groups of LANs connected together with communications links from a service provider. Because the communications links cannot plug directly into the LAN, it is necessary to identify the various pieces of interfacing equipment. LAN-based computers with data to transmit send data to a router that contains both LAN and WAN interfaces. The router will use the Layer 3 address information to deliver the data on the appropriate WAN interface. Routers are active and intelligent network devices and therefore can participate in network management. Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks. Some of these goals are connectivity, reliable performance, management control, and flexibility. The communications link needs signals in an appropriate format. For digital lines, a channel service unit (CSU) and a data service unit (DSU) are required. The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU/DSU may also be built into the interface card in the router. A modem is needed if the local loop is analog rather than digital. Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal. The digital signals are superimposed on an analog voice signal that is modulated for transmission. The modulated signal can be heard as a series of whistles by turning on the internal modem speaker. At the receiving end the analog signals are returned to their digital form, or demodulated. When ISDN is used as the communications link, all equipment attached to the ISDN bus must be ISDN-compatible. Compatibility is generally built into the computer interface for direct dial connections, or the router interface for LAN to WAN connections. Older equipment without an ISDN interface requires an ISDN terminal adapter (TA) for ISDN compatibility. Communication servers concentrate dial-in user communication and remote access to a LAN. They may have a mixture of analog and digital (ISDN) interfaces and support hundreds of simultaneous users. Interactive Media Activity Crossword Puzzle: WAN Devices and Interfaces Upon completing this activity, the student will be able to describe devices and interfaces associated with WAN connections. Web Links WAN http://www.cisco.com/en/US/tech/tk713/ tech_topology_and_network_ serv_and_protocol_suite_ home.html
Content 2.1 WAN Technologies Overview 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 standard, or one of its sub-sets or variants. Interactive Media Activity Crossword Puzzle: WAN Standards Upon completing this activity, the student will