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FRAME RELAY

FRAME RELAY. What is Frame Relay?. high-performance WAN protocol operates at the physical and data link layers Originally designed for use across ISDN interfaces An example of packet-switched technology described as a streamlined version of X.25. Introduction.

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FRAME RELAY

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  1. FRAME RELAY

  2. What is Frame Relay? • high-performance WAN protocol • operates at the physical and data link layers • Originally designed for use across ISDN interfaces • An example of packet-switched technology • described as a streamlined version of X.25

  3. Introduction • Frame Relay (FR) is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. • FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces. • Today, it is used over a variety of other network interfaces as well. • FR is an example of a packet-switched technology. • Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth.

  4. Frame Relay Devices • Devices attached to a Frame Relay WAN fall into the following two general categories: • Data terminal equipment (DTE) • DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. • Example of DTE devices are terminals, personal computers, routers, and bridges. • Data circuit-terminating equipment (DCE) • DCEs are carrier-owned internetworking devices. • The purpose of DCE equipments is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN.

  5. Frame Relay Devices (cont.)

  6. Frame Relay vs. X.25 • Frame Relay is a Layer 2 protocol suite, X.25 provides services at Layer 3 • Frame Relay offers higher performance and greater transmission efficiency than X.25

  7. Frame Relay Devices Figure 1 Frame Relay Devices

  8. Circuit-Switching • Long-haul telecom network designed for voice • Network resources dedicated to one call • Shortcomings when used for data: • Inefficient (high idle time) • Constant data rate

  9. Packet-Switching • Data transmitted in short blocks, or packets • Packet length < 1000 octets • Each packet contains user data plus control info (routing) • Store and forward

  10. Advantages with compared to Circuit-Switching • Greater line efficiency (many packets can go over shared link) • Data rate conversions • Non-blocking under heavy traffic (but increased delays). When traffic becomes heavy on a circuit-switching network, some calls are blocked. • Priorities can be used.

  11. Disadvantages relative to Circuit-Switching • Packets incur additional delay with every node they pass through • Jitter: variation in packet delay • Data overhead in every packet for routing information, etc • Processing overhead for every packet at every node traversed

  12. Switching Technique • Large messages broken up into smaller packets • Datagram • Each packet sent independently of the others • No call setup • More reliable (can route around failed nodes or congestion) • Virtual circuit • Fixed route established before any packets sent • No need for routing decision for each packet at each node

  13. Frame Relay Virtual Circuits • provides connection-oriented data link layer communication • a logical connection between two data terminal equipment across a Frame Relay packet-switched network • provide a bi-directional communications path from one DTE device to another

  14. Frame Relay Virtual Circuits • Switched virtual circuits (SVCs) • temporary connections requires sporadic data transfer between DTE devices across the Frame Relay network • Call Setup • Data Transfer • Idle • Call Termination

  15. Frame Relay Virtual Circuits • Permanent Virtual Circuits (PVCs) • used for frequent and consistent data transfers between DTE devices across the Frame Relay network • Data Transfer • Idle

  16. Congestion Control Mechanism • Forward-explicit congestion notification (FECN) • Backward-explicit congestion notification (BECN)

  17. Forward-explicit congestion notification (FECN) • initiated when a DTE device sends Frame Relay frames into the network • When the framesreach the destination DTE device, the frameexperienced congestion in the path from source to destination • flow-control may be initiated, or the indication may be ignored

  18. Backward-explicit congestion notification (BECN) • DCE devices set thevalue of the BECN bit to 1 in frames traveling in the opposite direction, informs the receiving DTE device that a particular path through the network iscongested • flow-control may be initiated, or the indication may be ignored

  19. Frame Relay Discard Eligibility (DE) • (DE) bit is used to indicate that a frame has lower importance than otherframes • When the network becomes congested, DCEdevices will discardframes with the DE bit

  20. Frame Relay Error Checking • common error-checking mechanism known as the cyclic redundancy check(CRC) • CRC compares two calculated values to determine whether errors occurred during thetransmission

  21. Frame Relay Network Implementation • consists of a number of DTE devicesconnected toremote ports on multiplexer equipment via traditional point-to-point services

  22. Public Carrier-Provided Networks • Frame Relay switching equipment is locatedin the central offices of a telecommunications carrier • The DCE equipment also is owned by the telecommunications provider • The majority of today’s Frame Relay networks are public carrier-provided networks

  23. Private Enterprise Networks • the administration and maintenance of the network are the responsibilitiesof the enterprise • All the equipment, including the switching equipment, isowned by the customer

  24. Frame Relay Frames Figure 3 Frame Relay Frame

  25. Frame Relay Frames • Flags indicate the beginning and end of the frame • Three primary components make up the FrameRelay frame • the header and address area • the user-data portion • the frame-check sequence(FCS)

  26. Frame Relay Frames • The address area (2 bytes) • 10 bits represents the actualcircuit identifier • 6 bits of fields related to congestionmanagement

  27. Frame Relay Frame Formats • Standard Frame Relay Frame • LMI Frame Format

  28. Standard Frame Relay Frame • Flags • Delimits the beginning and end of the frame • The value of this field is always the same (7E or 01111110)

  29. Standard Frame Relay Frame • Address – contains the following information • DLCI: The 10-bit DLCI is the essence of the Frame Relay header, values have local significance only, devices at opposite endscan usedifferent DLCI valuesfor the same virtual connection

  30. Standard Frame Relay Frame • Address • Extended Address (EA):used to indicate whether the byte in which the EA valueis 1 is the last addressing field, the eighth bit of eachbyte of the Address field is used to indicate the EA

  31. Standard Frame Relay Frame • Address • Congestion Control: consists of the three bits; FECN, BECN, and DE bits

  32. Standard Frame Relay Frame • Data – Contains encapsulated upper-layer data • serves totransport the higher-layer protocol packet (PDU) through a Frame Relay network

  33. Standard Frame Relay Frame • Frame Check Sequence • Ensures the integrity of transmitted data

  34. LMI Frame Format Figure 4 Nine fields comprise the Frame Relay that conforms to the LMI format

  35. LMI Frame Format • Flag - Delimits the beginning and end of the frame • LMI DLCI - Identifies the frame as an LMI frame instead of a basic Frame Relay frame • Unnumbered Information Indicator - Sets the poll/final bit to zero

  36. LMI Frame Format • Protocol Discriminator - Always contains a value indicating that the frame is an LMI frame • Call Reference - Always contains zeros. This field currently is not used for any purpose • Message Type • Status-inquiry message: Allows a user device to inquire about the status of the network • Status message: Responds to status-inquiry messages. Status messages include keep-alivesand PVC status messages

  37. LMI Frame Format • Information Elements—Contains a variable number of individual information elements (IEs) • IE Identifier: Uniquely identifies the IE • IE Length: Indicates the length of the IE • Data: Consists of one or more bytes containing encapsulated upper-layer data • Frame Check Sequence (FCS) - Ensures the integrity of transmitted data

  38. Frame Relay Inverse ARP and LMI Operation (cont.) Frame Relay Cloud DLCI=400 DLCI=100 172.168.5.7 172.168.5.5 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Frame Relay Map 172.168.5.7 DLCI 100 Active 5

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