Notes Odom, Chapter 4 Flashcards Set:

Transcription

1 EDTECH 552 (SP11) Susan Ferdon Notes Odom, Chapter 4 Flashcards Set: telco Leased Line CSU/DSU Local Telephone Company Owns the cables and has right of way for leased lines used for WANs. A serial communication circuit between two end points, provided by some service provider, typically a telephone company or a telco. Provides basic connectivity between to points. Work with a service provider to install a circuit connection is always available. Sometimes called leased circuit or Point-to-Point WAN Connection. (p. 76) Channel Service Unit/Data Service Unit A device that understands the Layer 1 details of serial links installed by a telco and how to use a serial cable to communicate with networking equipment such as routers. CO Central Office Telco point of origin for the four-wire cable from the telco to your building. Cabling runs from CO Wan switch CSU router. demarc Demarcation point On network diagrams, marks the physical point separating telco-owned equipment from customer-owned. CPE Customer Premises Equipment Any equipment related to communications that is located at the customer site, as opposed to inside the telephone company s network. Clock Rate The speed at which a serial link encodes bits o nthe transmission medium. Also called bandwidth or link speed. Customer specifies the speed when ordering a circuit and telco installs a circuit that runs at that speed. CSU/DSUs on each end must be configured to match the defined speed. (p. 80)

2 Synchronization Various devices must synchronize their clocks so that they run at the exact same speed. Devices operate at close to the same speeds and listen to the speed of the other device on the other side of the link. One side makes small adjustments to match the other side. Master/Slave DCE (Layer 1) DTE (Layer 1) DS0 DS1 PCM Access Link Back-to-Back Link Synchronization occurs between the two CSU/DSUs on a leased line by having one CSU/DSU (the slave) adjust its clock to match the clock rate of the other CSU/DSU (the master). Networking devices synchronize their clocks several times per second. Data Communications Equipment The device providing clocking on a WAN link, typically a CSU/DSU. DCE cables reverse the transmit and receive pins, like a crossover cable in Ethernet. DCE cable has female connectors. (p. 81) Data Terminal Equipment The device receiving clocking; typically the router. DTE cable sends and transmits from the same pins on both ends, like a straight-through cable in Ethernet. DTE cable has male connectors. (p. 81) Digital Signal Level 0 A 64-kbps line or channel of a faster line inside a telco whose origins are to support a single voice call using the original voice (PCM) codecs. (p. 82) Digital Signal Level 1 A Mbps line from the telco, with 24 DS0 channels of 64 kbps each, plus an 8-kbps management and framing channel. Also called T1. Pulse Code Modulation A technique of encoding analog voice into a 64-kbps datat stream by sampling with 8-bit resolution at a rate of 8000 times per second. In Frame Relay, the physical serial link that connects a Frame Relay DTE device, usually a router, to a Frame Relay switch. The access link uses the same physical layer standards as do point-to-point leased lines. A serial link between two routers, created without CSU/DSUs, by connecting a DTE cable to one router and a DCE cable to the other. Typically used in labs to build serial links without the expense of an actual leased line

3 Clocking Frame Relay HDLC HDLC Framing Packet (Frame) Switching PPP Serial Cable Synchronous from the telco. The process of supplying a signal over a cable, either on a separate pin on a serial cable or as part of the signal transitions in the transmitted signal, so that the receiving device can deep synchronization with the sending device. An international standard data-link protocol that defines the capabilities to create a frame-switched (packetswitched) service, allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay Service. High-Level Data Link Control A bit-oriented synchronous data layer protocol developed by the International Organization for Standardization (ISO). HDLC has three tasks: 1) detect errors, 2) discard frames with errors, 3) identify the type of packet inside the HDLC frame. HDLC header includes Address field and Protocol Type field. HDLC trailer contains FCS (Frame Check Sequence). Cisco proprietary HDLC frame also includes Type field. A generic reference to network services, typically WAN services, in which the service examines the contents of the transmitted data to make some type of forwarding decision. This term is mainly used to contrast with the WAN term circuit switching, in which the provider sets up a Layer 1 circuit between two devices, and the provider makes no attempt to interpret the meaning of the bits. Point-to-Point Protocol A protocol that provides router-to-router and host-tonetwork connections over synchronous point-to-point and asynchronous point-to-point circuits. A type of cable with many different styles of connectors used to connect a router to an external CSU/DSU on a leased-line installation. The imposition of time ordering on a bit stream. Practically, a device will try to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.

4 T1 VC PVC CIR A line from a telco that allows transmission of data at Mbps, with the ability to treat the line as 24 different 64-kbps DS0 channels (plus 8 kbps of overhead). Virtual Circuit The logical path a frame travels between each pair of routers. In packet (frame) switched services like Frame Relay, VC refers to the ability of two DTE devices (typically routers) to send and receive data directly to each other, which supplies the same function as a physical leased line (leased circuit), but doing so without a physical circuit. This term is meant as a contrast with a leased line or leased circuit. Permanent Virtual Circuit Virtual Circuit preconfigured by the Service Provider. In contrast to VCs that are configured on an as-needed basis. Committed Information Rate Guarantee by the service provide in Frame Relay that a particular VC gets at least that much bandwidth. CIR is like the bandwidth or clock rate of a point-to-point circuit except that it is a minimum value more can actually be sent, in most cases. OSI Layer 1 for Point-to-Point WANs The biggest distinction between LANs and WANs relates to how far apart the devices can be and still be capable of sending and receiving data. Called Point-to-Point Leased lines or leased circuits. To create such long links, or circuits [needed for WAN], the actual physical cabling is owned, installed, and managed by a company that has the right of way to run cables under streets.

5 Components: network of cables with WAN switch at each end. Cable from WAN switch to customer s CSU. CSU/DSU receives signal from telco. CSU/DSU connects to router. Cabling Standards o Variety of WAN interface cards for routers synchronous and asynchronous serial interfaces. o Connector types: 60-pin D-shell connects to router, other end matches CSU/DSU interface: EIA/TIA-232, EIA/TIA-449, V.35, X.21, or EIA-530. o Cable from CO to SCU/DSU typically uses RJ-48 which is same size and shape as RJ-45 used for Ethernet. Many Cisco routers support serial interfaces that have an integrated internal CSU/DSU physical line from CO is connected directly to the router. To make the link work, various devices need to synchronize their clocks so that they run at the exact same speed. Choose the type of cable needed, DCE or DTE, based on whether the router is acting like at DCe or DTE. Typically, the router acts as a DTE, so the router must use a DTE cable to connect to the CSU/DSU. (p. 81)

6 WAN Speeds Standards define how fast a point-to-point link can run. When telcos built their first digital networks, they chose a baseline transmission speed of 64kbps because that was the necessary bandwidth for a single voice call. (p. 82). Most telcos offer leased lines in multiples of 64 kbps. DS1 (digital signal level 1) standard defines a single line that supports 24 DS0s, plus an 8-kbps overhead channel. Name(s) of Line Bit Rate DSO 64 kbps *DS1 (T1) Mbps (24 DSOs, plus 8 kbps overhead) DS3 (T3) Mbps (28 DS1s, plus management overhead) *E Mbps (32 DSOs) E Mbps (16 E1s, plus management overhead) J1 (Y1) Mbps (32 DSOs; Japanese standard) *BOLD book says remember this for the exam. OSI Layer 2 for Point-to-Point WANs The two most popular data link layer protocols used on point-to-point links are High-Level Data Link Control (HDLC) and Point-to-point Protocol (PPP). (p. 83) Point-to-point links are fairly simple, so HDLC has only a small amount of work to do. HDLC: o Used when there is a direct connection from one router to another router. o Determines if data passed link without errors o Discards frames if errors occurred o Identifies type of packet inside HDLC frame so receiving device knows packet type o HDLC defines framing: HDLC header includes Address field, Protocol Type field, and trailer with FCS field. o Performs error detection, like Ethernet; no error recovery. o Identifies encapsulated data, like Ethernet. Type field identifies the type of packet inside the frame same values in HDLC Type field as Ethernet Protocol Type field. o Original HDLC standards did not include Type field, Cisco added one so HDLC will not work when connecting a Cisco router to another vendor s router.

7 o Address field now superfluous originally there was more than one address on a circuit and the Address field was needed. Now there is one address on the circuit the router on one end knows there is only one possible recipient of the data the router on the other end. o International Telecommunications Union (ITU) defined HDLC. Point-to-Point Protocol (PPP): o Behaves much like HDLC o Framing looks identical to the Cisco proprietary HDLC framing. o Discards frames if errors occur. o Uses 2-byte Protocol Type field any vendor can communicate with any other vendors products. o When connecting a Cisco router to another vendor s router, PPP is the data link layer protocol of choice. o Defined much later that HDCL specifications many additional features. o Most popular and feature-rich of WAN data link layer protocols. o Used in multi-vendor router point-to-point serial link environments. o Internet Engineering Task Force (IETF) defined PPP. Frame Relay and Packet-Switching Services o The two most commonly used Packet Switching Services are Frame Relay and Asynchronous Transfer Mode (ATM).

8 o Physical WAN connectivity exists, just like leased line. The difference is what that leased line connects to. In Frame Relay, the leased line from the customer connects to a Frame Relay Switch at the telco, as opposed to a WAN Switch (generic term). o Leased Line one customer connects. Frame Relay multiple customers tap into same physical line, each has it s own separate network. o Can connect a large number of routers using a single serial link from each router into the packet-switching service. o Once connected, each router can send packet to all the others, much like a hub or switch. Frame Relay o Multiaccess networks; more than two devices can attach to the network, similar to LANs. o Same Layer 1 features as point-to-point leased lines. o Leased line is installed is installed between each router and a nearby Frame Relay switch; called access links. o Leased line runs from one router to a Frame Relay switch. o Frame relay switch in the telco network examines the data frame sent by the router. Frame relay defines its own data link header and trailer, the header holds a field called Data Link Connection Identifier (DLCI), WAN switches forwards the frame based on the DLCI until it reaches the destination router. o Frame Relay header and trailer are defined by a protocol called Link Access Procedure Frame (LAPF). Frame relay uses Frame Switching (Layer 2) as it switches (forwards) incoming frames to devices one by one based on the DLCI. o DIFFERENCE between Frame Relay and point-to-point: Frame Relay switch actually examines data frames sent by router. o A form of packet switching provider chooses where to send each data packet. o Also called a frame-switching service, packet switching is a more general term. o Frame Relay switches are DCE. o Logical path that frame travels between each pair of routers is called a Frame Relay Virtual Circuit. o Can expand WAN without the need for as much hardware. o Compete with other customer s need for bandwidth but CIR (committed information rate) is guaranteed; don t get own leased line, have to share. Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the

9 network. This capability often can reduce the equipment and network complexity required to connect multiple DTE devices.

10

11 Do I Know This Already Quiz, Chapter 4 - pp TOPIC Q# 1 st Try 2 nd Try 3 rd Try 1 B 2 B OSI Layer 1 for Point-to-- Point WANs 3 A B 4 A OSI Layer 2 for Point-to-- 5 C, D E Point WANs 6? E Frame Relay and Packet- 7 A Switching Serices 8 B Q3: Which of the following typically connects to a V.35 or RS-232 end ofa cable when cabling a leased line? Answer: B CSU/DSU Explanation: Leased line CSU/DSU serial cable router Serial cable has D-60 Shell connection for router end, V.35 or other for CSU/DSU end. Q5: Which of the following functions of OSI Layer 2 is specified by the protocol standard for PPP, but is implemented with a Cisco proprietary header field for HDLC? Answer: D identifying the type of protocol that is inside the frame. Explanation: Only difference between Cisco proprietary and other HDLC is the type field in the header. Q6: Imagine the Router 1 has three point-to-point serial links, one link each to three remote routers. Which of the following is true about the required HDLC addressing at Router 1? Answer: E none of these answers are correct. Explanation: HDLC doesn t set addressing it provides error detection, discards frames with errors, and identifies packet type.