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9 WAN Technologies ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Terms you’ll need to understand: Integrated Services Digital Network (ISDN) Point-to-Point Protocol (PPP) Dial on Demand Routing (DDR) High-Level Data Link Control (HDLC) X.25 Frame Relay Leased lines Asynchronous Transfer Mode (ATM) Techniques you’ll need to master: Differentiating among wide area network services, including ISDN, X.25, Frame Relay,ATM, and leased-line protocols Recognizing key WAN terms for ISDN, Frame Relay, X.25, ATM, and leased-line protocols Using common commands to view the status of WAN links 1 2 Chapter 9 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ This chapter examines the methods used to transport data over wide area networks. We will cover common WAN technologies, such as Frame Relay and ATM, and how they are configured on Cisco routers. Also, this chapter covers the following WAN-related CCIE blueprint objectives, as laid out by the Cisco Systems CCIE program: Integrated Services Digital Network (ISDN)—Link Access Procedure on the D channel (LAPD), Basic Rate Interface (BRI) framing, Primary Rate Interface (PRI) framing, signaling, mapping, dialer map, interface types, B/D channels, PPP Multilink X.25—Addressing, routing, Link Access Procedure Balanced (LAPB), error control/recovery, windowing, signaling, mapping, Switched Virtual Circuit (SVC)/Permanent Virtual Circuit (PVC), Protocol Translation Frame Relay—Local Management Interface (LMI), Data Link Connection Identifier (DLCI), permanent virtual circuit (PVC), framing, traffic shaping, Forward Explicit Congestion Notification (FECN), Backward Explicit Congestion Notification (BECN), Committed Information Rate (CIR), Discard Eligibility (DE), mapping, compression Asynchronous Transfer Mode (ATM)—SVC/PVC, ATM Adaptation Layer (AAL), Service Specific Connection Oriented Protocol (SSCOP), User-Network Interface (UNI), Network-Network Interface (NNI), Interim Local Management Interface (ILMI), cell format, quality of service (QoS), RFC 1483, RFC 1577, Private Network-Network Interface (PNNI), Interim-Interswitch Signaling Protocol (IISP), mapping Leased-line protocols—High-Level Data Link Control (HDLC), Point-To-Point Protocol (PPP), async, modems, compression Dial on Demand Routing (DDR)—Short term WAN connections, dial backup Physical layer—Synchronization, Synchronous Optical Network (SONET), T1, E1, encoding As with other chapters in this book, additional information is provided for complete-ness and in preparation for additional subjects as the CCIE program expands. WAN Technologies Wide area networks (WANs) provide connectivity across longer distances than local area networks (LANs), and WANs are typically slower than LANs because they provide connectivity over wider geographical distances.This chapter focuses on the following topics: WAN Technologies 3 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ISDN PPP X.25 Frame Relay ATM Due the to high cost of WANs, Cisco introduced Dial on Demand Routing (DDR) to provide a WAN connectivity only when end user data is sent across to a remote or central location. DDR is used to reduce the ongoing costs of having a dedicated WAN circuit active all the time. We will begin our discussion of WAN technologies by exploring Integrated Ser-vices Digital Network (ISDN) first. ISDN Integrated Services Digital Network (ISDN) is a digital service that enables network users to send and receive data, voice, and video transmissions over a network. ISDN offers a variety of link speeds, ranging from 64Kbps to 2.048Mbps. Therefore, many small- and medium-sized companies find that ISDN is a viable network solution. Basic Rate and Primary Rate Interfaces ISDN can be supplied by a carrier in two main forms—Basic Rate Interface (BRI) and Primary Rate Interface (PRI). An ISDN BRI consists of two 64Kbps services (B channels) and one 16Kbps signaling channel (D channel). An ISDN PRI consists of 23 B or 30 B channels, depending on the country. In North America and Japan, a PRI service consists of 23 B channels. In Europe and Aus-tralia, a PRI service consists of 30 B channels. A signaling channel (or D chan-nel) is used in a PRI service and is a dedicated 64Kbps channel. Note: The effective throughput of a PRI service with 23 channels is 1.472Mbps (23x64Kbps). With 30 B channels, the effective throughput is 1.920Mbps (30x64Kbps). The International Telecommunications Union (ITU) defines the standards for ISDN. ISDN Framing and Frame Format The ISDN Physical layer provides the ability to send outbound traffic and receive inbound traffic by transmitting binary bits over the physical media.The ISDN Data Link layer provides signaling, which ensures that data is sent and received correctly. 4 Chapter 9 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ The signaling protocol used in ISDN is called the Link Access Procedure on the D channel (LAPD). Figure 9.1 shows LAPD’s frame format. The flag field in an LAPD frame (see Figure 9.1) is used to delimit the frame. The frame control status field is a calculation used to determine whether any errors have occurred during a frame’s delivery. If the error-checking calculation result differs from the value in the original frame, an error is generated by the network or router and sent to higher layers for retransmission. Note: The LAPD frame format is similar to the High-Level Data Link Control (HDLC), which is discussed later in this chapter. HDLC is used for point-to-point connections. Layer 3, the Network layer, of the ISDN model is used to provide call establishment, call termination, and information transfer. Before we look at an example of configuring ISDN on a Cisco router using a simple network design, you need to understand leased line protocols so you can apply this knowledge to complex scenarios, such as authentication techniques in Point-to-Point protocols (PPP). Leased Line Protocols A leased line is a service provided by a carrier that maintains a connection between two remote networks separated by some geographical region.These remote sites can range from SOHO (small office home office) to satellite corporate offices. HDLC High-Level Data Link Control (HDLC) is a WAN protocol encapsulation method that allows point-to-point connections between two remote sites.Typically, Field length in bytes 1 2 Flag Address 1 Control Variable Length 1 1 Data FCS Flag SAPI C/R EA TEI EA SAPI – Service Access Point Identifier C/R – Command/Response EA – Extended Access TEI – Terminal Endpoint Identifier (all ones indicate a broadcast) Figure 9.1 LAPD frame format. WAN Technologies 5 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ HDLC is used in a leased-line setup. HDLC is a connectionless protocol that relies on upper layers to recover any frames that have encountered errors across a WAN link. Because HDLC is proprietary, you cannot use HDLC between a Cisco router and another vendor. HDLC is the default encapsulation on Cisco serial interfaces. Cisco routers use HDLC encapsulation, which is proprietary. This proprietary nature is due to the fact that Cisco added an address field in the HDLC frame, which is not present in the HDLC standard, this field is used by Cisco devices to indicate the type of payload (protocol). Cisco routers use the address field in an HDLC frame to indicate a payload type, but other routers or manufacturers that implement the HDLC standard do not use the address field. Point-to-Point Protocol (PPP) PPP was designed to transport user information between two WAN devices (also referred to as point-to-point links). PPP was designed as an improvement over Serial Line Internet Protocol (SLIP). SLIP provided basic IP connectivity. When PPP encapsulation is configured on a Cisco WAN interface, the network administrator can carry protocols such as IP and IPX as well as many others. Cisco routers support PPP over asynchronous lines, High-Speed Serial Interfaces (HSSI), ISDN lines, and synchronous serial ports. PPP has the added function of allowing authentication to take place before any end user data is sent across the link. The following three phases occur in any PPP session: 1. Link Establishment—Link Control Program (LCP) packets are sent to con-figure and test the link. 2. Authentication (optional)—After the link is established, authentication can be used to ensure that link security is maintained. 3. Network layers—In this phase; Network Control Program (NCP) packets deter-mine which protocols will be used across the PPP link. An interesting aspect of PPP is that each protocol (IP, IPX, and so on) supported in this phase is documented in a separate RFC that discusses how it operates over PPP. LCP Link Control Protocol (LCP) is used to establish, configure, and test the link between two devices, such as Cisco routers. LCP provides the necessary negotiations between end devices to activate the link. Once the link is activated, but no data is flowing, the next phase of the PPP session can take place, authentication (if configured) and the NCP. ... - tailieumienphi.vn
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