Overview of WAN Connections Module 1

Transcription

1 Overview of WAN Connections Module 1 Objectives This module introduces the basic elements of WAN connections and describes the role each element plays in creating that connection. After completing this module, you should be able to: Describe the three basic elements of a WAN connection Describe how public carrier networks are used to create a WAN connection Identify the three types of circuits used to create a WAN connection Describe how local loops connect the subscriber s premises to public carrier networks Identify the electrical signaling specifications and related technologies used in public carrier networks Explain the differences and similarities between T-, E-, and J-carrier WAN connections Prerelease 1 1

2 WAN Technologies Introduction Companies that have multiple offices need a cost-effective, efficient means to exchange data between those offices. Many companies have created intranets or extranets, which enable users at different locations to view information and to upload and download information. However, intranets and extranets are only a partial solution to the problem because the sharing of data is limited to what can be posted on the intranet or extranet. Each office must maintain its own database, and users cannot access data stored at other locations. For example, the accounting department at each office must have a separate database, which cannot be shared over an intranet. Security is also an issue because the intranet must be connected to the Internet, in order to serve multiple locations. The various offices connected through the intranet can be protected by firewalls, but firewalls are not impervious to attacks. For many companies, a Wide Area Network (WAN) is a better and more costeffective solution for connecting multiple branch offices to a main office. A WAN allows companies to exchange all types of information, including voice and data. Combining voice and data traffic can reduce operating expenses for many companies. This course focuses on WAN connections created using public carrier networks. Businesses, organizations, and government entities use public carrier networks to create WAN connections for three primary reasons: 1 2 Prerelease

3 Overview of WAN Connections Using public carrier network infrastructure is almost always more cost effective than using privately owned infrastructure. Public carrier networks allow many subscribers to share the costs of installing, managing, and maintaining the infrastructure required to create WAN connections. Using privately owned infrastructure to create long-distance and international WAN connections is impractical, sometimes even impossible, and cost prohibitive. WAN connections that use privately owned infrastructure are generally limited to relatively short distances, and installing them is beyond the capacity of all but the largest organizations. WAN connections created through public carrier networks are substantially similar to WAN connections created using privately owned infrastructure in terms of security and performance. Public carrier networks also provide levels of reliability and redundancy that privately owned infrastructure typically cannot provide. WAN routers connect the LANs at each location, identify the traffic addressed to another LAN, and route the traffic to the next hop. As explained throughout this course, WAN routers support a variety of WAN connection types, including: Dedicated T-, E-, and J-carrier lines Integrated Services Digital Network (ISDN) Digital Subscriber Line (DSL) Prerelease 1 3

4 WAN Technologies A WAN Connection Defined In the most general sense, a WAN is a geographically dispersed telecommunications network. For the purposes of this course, however, a WAN is defined as a network created to connect two or more Local Area Networks (LANs). WAN connections can connect LANs located in the same city or around the world. As the figure shows, a public carrier network is commonly used to create WAN connections between LANs in different parts of the world. Public carrier networks include the Public Switched Telephone Network (PSTN), which serves the United States and Canada, and Public Telephone and Telegraph (PTT) companies, which serve Mexico, Europe, Asia, South America, and other parts of the world. 1 4 Prerelease

5 Overview of WAN Connections Basic Elements of a WAN Connection All WAN connections consist of three basic elements: The physical transmission media. Electrical signaling specifications for generating, transmitting, and receiving signals through various transmission media. Data-link layer protocols that provide logical flow control for moving data between peers in the WAN. (Peers are the devices at either end of a WAN connection.) As the figure shows, physical transmission media and electrical specifications are part of the physical layer (layer one) of the Open Systems Interconnection (OSI) model, and data-link layer protocols are part of the data-link layer (layer two). This module focuses on the physical transmission media, the electrical signaling specifications, and the related OSI layer-one technologies that are used to create WAN connections through public carrier networks. Data-link layer protocols are explained in detail in Module 2: Data-Link Layer Protocols. Prerelease 1 5

6 WAN Technologies Physical Transmission Media and Infrastructure The first basic element of a WAN connection is the physical transmission medium. The most common physical transmission medium used in public carrier networks is twisted-pair copper wire, originally installed for Plain Old Telephone Service (POTS) connections. Twisted pair is currently used in the last mile of 90 percent of all WAN connections. Other physical transmission media include coaxial copper cable, fiber optic cable, and the Earth s atmosphere, which carries signals by such means as infrared and microwave transmissions. The physical transmission media are a large part of what is commonly called infrastructure. Infrastructure also includes telecommunications switching and routing equipment. WAN connections can be created using public carrier network infrastructure, privately owned infrastructure, or a combination of the two. 1 6 Prerelease

7 Overview of WAN Connections Types of WAN Circuits As the figure shows, three types of circuits are used to create WAN connections through public carrier networks: Dedicated circuits Permanent virtual circuits (PVCs) Switched virtual circuits (SVCs) Dedicated Circuits Dedicated circuits are permanent circuits dedicated to a single subscriber. The connection is always active. The subscriber purchases dedicated time slots, or channels, that provide a specific amount of bandwidth that is always available for the subscriber to use. The channels in a dedicated circuit are created using time division multiplexing (TDM), which is discussed later in this module. In addition to providing guaranteed bandwidth at all times, dedicated circuits provide the most secure and reliable WAN connections available. Dedicated circuits are used to create the following point-to-point WAN connections: Carrier lines (which are explained later in this module and in Module 3: Carrier Line WAN Connections) Prerelease 1 7

8 WAN Technologies DSL connections (which are explained in Module 5: DSL WAN Connections) Permanent Virtual Circuits (PVCs) PVCs are also permanent circuits dedicated to a single subscriber. The connection is always active. However, because multiple virtual circuits share a physical circuit, there is no guarantee that any specific amount of bandwidth will be available at any specific time. Sometimes there may not be any bandwidth available on the physical circuit because the physical circuit is saturated. When the physical circuit is saturated, the traffic is temporarily stored at a switching point until bandwidth becomes available. When bandwidth becomes available, the stored traffic is forwarded to its destination. This process is referred to as store-and-forward processing, or packet switching, which is the same processing method used on LANs. PVCs provide an average bandwidth guarantee. The average bandwidth guarantee is accomplished through statistical multiplexing (STM), which underlies packet switching technology. Because PVCs are more cost effective for the public carrier, PVCs are usually less expensive for the subscriber than dedicated circuits. PVCs are commonly used for Frame Relay, which is explained in detail in Module 6: Frame Relay. Switched Virtual Circuits (SVCs) SVCs are identical to PVCs in all respects, except that they are temporary physical circuits. SVCs are activated when a subscriber initiates a connection to transmit data. When all data have been transmitted, the connection is deactivated, and the physical circuit resources are made available to other subscribers. SVCs are used to create dial-up WAN connections, including ISDN WAN connections, which are explained in Module 4: ISDN WAN Connections. 1 8 Prerelease

9 Overview of WAN Connections PSTN (United States and Canada) In the United States and Canada, most WAN connections are created through the PSTN. As the figure shows, the PSTN consists of local exchange carriers (LECs) and interexchange carriers (IXCs). (LECs are also referred to as telcos.) Local Exchange Carriers (LECs) LECs operate the infrastructure that provides access to the PSTN in a limited geographic area. The area served by a LEC is referred to as a local access and transport area (LATA). LECs include incumbent local exchange carriers (ILECs) and competitive local exchange carriers (CLECs). ILECs are the Regional Bell operating companies (RBOCs) that provide service in a specific LATA. For example, SBC is the current ILEC in California. ILECs were created in 1983 when the U.S. government deregulated the telecommunications industry and mandated the breakup of AT&T. Deregulation also led to the creation of CLECs, which provide the same services as ILECs and compete with ILECs in specific geographic areas. For example, Covad Communications is a CLEC that competes with SBC in California. Prerelease 1 9

10 WAN Technologies Interexchange Carriers (IXCs) IXCs aggregate voice and data traffic from numerous LECs. They operate the infrastructure that connects LATAs to the interlatas that move traffic throughout the United States and Canada. AT&T, Sprint, and MCI are all IXCs based in the United States. IXCs are commonly referred to as long-distance carriers. IXCs also provide the infrastructure that enables PSTN subscribers to create WAN connections to PTT networks in Europe, Asia, South America, and other parts of the world Prerelease