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Chapter 3

Chapter 3. Frame Relay. Frame Relay. FR is a WAN Technology. Uses ( VC ) Virtual Circuits to establish connections across the WAN. DLCIs are used to identify Virtual Circuits . FR can divide a single Physical WAN Interface into multiple Subinterfaces.

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Chapter 3

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  1. Chapter 3 Frame Relay

  2. Frame Relay FR is a WAN Technology. • Uses (VC) Virtual Circuits to establish connections across the WAN. • DLCIs are used to identify Virtual Circuits. • FR can divide a single Physical WAN Interface into multiple Subinterfaces.

  3. Frame Relay is an International Telecommunications Standards Sector (ITU-T) and American National Standard Institute (ANSI) standard.

  4. Frame Relay is a packet-switched connection oriented WAN service

  5. Frame Relay is often used to interconnect LANs

  6. The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC)

  7. Switched virtual circuits (SVCs) are established dynamically by sending signaling messages to the network.

  8. Permanent virtual circuits (PVCs) are preconfigured by the carrier

  9. Frame Relay is configured on a serial interface and the default encapsulation type is the Cisco proprietary version of HDLC

  10. By default, a Frame Relay network provides non-broadcast multi-access (NBMA) connectivity between remote sites

  11. To enable the forwarding of broadcast routing updates in a hub-and-spoke Frame Relay topology, configure the hub router with logically assigned interfaces, called subinterfaces

  12. Subinterfaces are logical subdivisions of a physical interface

  13. Frame Relay Overview • CCITT and American National Standards Institute (ANSI) are standards that define the process for sending data over a public data network (PDN). • A data-link technology streamlined to provide high performance and efficiency. • Operates - Physical and Data Link Layers • Relies on TCP for error correction. • The receiving device drops all error frames with no notification to the sender • Uses - Link Access Procedure for Frame Relay (LAPF)

  14. Frame Relay Overview • Defines the interconnection process • CPE (customer premises equipment) • DCE (data communications equipment ) the service provider’s local access switching equipment • DTE (data terminal equipment) at the CPE • Computing equipment that is not on a LAN may also send data across a Frame Relay network. • The computing equipment will use a FRAD (Frame Relay access device) as its DTE.

  15. Frame Relay Overview • The SP’s switching equipment maintains table mapping connection identifiers to outbound ports. • When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the associated outbound port. • The complete path to the destination must be established prior to sending the first frame.

  16. Frame Relay Overview • Very cost-effective as: • FR allows a single interface to support multiple PVCs (private virtual circuits). • Less equipment required by the customer • Only pay for the average bandwidth rather than the maximum bandwidth requirement.

  17. DLCI (Data-link connection identifier) • A number that identifies the logical circuit between the source and destination device. • The FR switch maps the DLCIs between each pair of routers to create a PVC (private virtual circuit). LMI (Local Management Interface) • A signaling standard between the CPEdevice and the FR switch - responsible for managing the connection and maintaining status between the devices.

  18. LMIs may include: • support for a keepalive mechanism\ • a multicast mechanism that can provide the network server with its local DLCI • multicast addressing, providing the ability to give DLCIs global (whole Frame Relay network) significance, rather than just local significance • a status mechanism, providing an ongoing status on the DLCIs known to the switch

  19. LMI Types • Cisco--LMI type defined jointly by Cisco, StrataCom, Northern Telecom, and DEC • DLCI 1023 is reserved for cisco type) • Ansi-- • defined by ANSI standard T1.617 AnnexD • ITU-T--Q.933 q933a Annex A

  20. 4 3 2 1

  21. CIR (Committed information rate) • The rate, in bits per second, that the Frame Relay switch service agrees to transfer data. • Bc (Committed Burst) • Tc The maximum number of bits that the SP agrees to transfer during anyCommitted Rate Measurement Interval.

  22. Excess Burst • The maximum number of uncommitted bits that the Frame Relay switch will attempt to transfer beyond the CIR. • Depends on the service offerings available by your vendor, typically limited to the port speed of the local access loop.

  23. FECN (Forward explicit congestion notification) • When a FR switch recognizes congestion in the network, it sends a FECN packet to the destination device indicating that congestion has occurred. BECN (Backward explicit congestion notification) • When a FR switch recognizes congestion in the network, it sends a BECN packet to the source router instructing it to reduce the packet rate

  24. Discard Eligibility (DE) Indicator • When the router detects network congestion, the FR switch will first drop packets having the DE bit set. • The DE bit is set on for oversubscribed traffic - the traffic that was received after the CIR was met.

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  26. DLCIs • Frame Relay virtual circuits are identified by DLCIs (data link connection identifiers). • DLCI values are typically assigned by the Frame Relay service provider • ONLY local significance, the values themselves are not unique in the Frame Relay WAN. • Two DTE devices connected by a virtual circuit might use a different DLCI value to refer to the same connection.

  27. Beginning of the frame Ending of the frame

  28. Following the leading flags field are two bytes of address information. • Ten bits of these two bytes make up the actual circuit ID, the DLCI. This is the heart of the Frame Relay header. It identifies the logical connection that is multiplexed into a physical channel.

  29. Three of the remaining bits provide congestion control. • The forward explicit congestion notification (FECN) a bit is set in the Frame to tell the receiving DTE that congestion is experienced in the path from source to destination. • The backward explicit congestion notification (BECN) bit is set in the Frame traveling in the opposite direction from frames encountering a congested path.

  30. TheDE (discard eligibility) bit is set by the DTE to tell the network that a frame has lower importance than other frames and should be discarded before any other frames -- if the network becomes short on resources. • DE is a very simple priority mechanism. • This bit is usually set only when the network is congested.

  31. Data – Variable-length field that contains encapsulated upper-layer data. FCS – Frame Check Sequence (FCS), used to ensure the integrity of transmitted data.

  32. Assume two PVCs: • one between Atlanta and Los Angeles, • one between San Jose and Pittsburgh. Los Angeles uses DLCI 12 to refer to its PVC with Atlanta, while Atlanta refers to the same PVC as DLCI 82. San Jose uses DLCI 12 to refer to its PVC with Pittsburgh. Pittsburgh refers to the same PVC as DLCI 62.

  33. Reports the status of PVCs

  34. LMI Purpose • PVC status – monitors the operational status of the various PVCs • Transmits keepalive packets: • to insure that the PVC stays up • to prevent inactivity, which would shut down the PVC.

  35. LMI • The router must be programmed to choose which LMI type encapsulation will be used. • Options are (frame-relay lmi-type ?): ansi | cisco | q933i • cisco is the default

  36. Inverse ARP The Inverse ARP mechanism allows the router to automatically build the Frame Relay map. • The router learns the DLCIs that are in use from the switch during the initial LMI exchange. • The router then sends an Inverse ARP request to each DLCI for each protocol configured on the interface (if the protocol is supported). • The return information from the Inverse ARP is then used to build the Frame Relay map.

  37. Frame Relay Mapping • The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI. • The resolution is done through a data structure called a Frame Relay map. • This data structure may be: • statically configured in the router, or • the Inverse ARP feature can be used for automatic setup of the map.

  38. Frame Relay Operation - Switching • The Frame Relay switching table consists of four entries: • two for incoming port and DLCI • two for outgoing port and DLCI • The DLCI could, therefore, be remapped as it passes through each switch • The fact that the port reference can be changed is why the DLCI is “locally significant."

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