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  1. 1. Frame Relay What is it?? <ul><li>Frame Relay (FR) - public network WAN technology based on packet switching </li></ul><ul><li>FR standard defines an interface between an end user and a public network. FR is a protocol of 2nd level of OSI model </li></ul><ul><li>Internal Frame Relay protocol (between switching devices in the cloud) is not standardized (probably it will be some day) </li></ul>Frame Relay cloud end user Frame Relay interface
  2. 2. Frame Relay What is it about? <ul><li>Aim: transport user data between port A and B </li></ul><ul><li>Data is transmitted as variable length frames Max. frame length is 4096 bytes (recommended length is 1600 bytes) </li></ul><ul><li>From users point of view: ports A and B are connected with a transparent logical link (virtual circuit - VC) </li></ul>FRAD - Frame Relay Access Device PVC FR switches A B VC - Virtual Circuit PVC - Permanent VC
  3. 3. Frame Relay Standards <ul><li>Frame Relay “independent existence”: </li></ul><ul><ul><li>In 1990 “Group of Four” (DEC, Northern Telecom, Cisco, Stratacom) presented FR as an independent standard </li></ul></ul><ul><ul><li>Later this Frame Relay Forum was established: main standardization body for FR </li></ul></ul><ul><li>Standards on which FR is based: ANSI T1.602, ANSI T1.606 (Frame Relaying Bearer Service - Architectural Framework and Service Description, 1990), ANSI T1.607-1990, ANSI T1S1/91-659,ANSI T1.617, ANSI T1.618, CCITT I.122 (Framework for providing Additional Packet Mode Bearer Services, 1988), CCITT Q.922, CCITT Q.933 </li></ul>
  4. 4. Frame Relay Most important features <ul><li>Based on packet (frame) switching </li></ul><ul><li>Frames of variable length (up to 4096 bytes, typically 1600 bytes) </li></ul><ul><li>Connection oriented; only permanent connections - PVCs; switched VCs in standard extensions </li></ul><ul><li>High data rates at user-network interfaces (2Mbps, ultimately up to 45 Mbps) </li></ul><ul><li>Bandwidth on demand </li></ul><ul><li>No flow control mechanisms (nearly) </li></ul><ul><li>No error control (but FCS) or retransmission mechanisms </li></ul><ul><li>All protocol functions implemented at 2nd level (data link) of OSI model No standards for physical interface: can be X.21, V.35, G.703, G.704 </li></ul>
  5. 5. Frame Relay Why was it proposed? <ul><li>Efficiency: increased demand for high throughput networking (X.25 too slow) </li></ul><ul><li>“ Bursty applications”: LAN connectivity, Internet, not only terminal applications </li></ul><ul><li>Fibre optic lines: low (very, very low) bit error rates </li></ul><ul><li>New, smarter software: applications (or higher level protocols like TCP) performing error control, retransmissions; reliable date links delivered by higher levels of OSI model </li></ul>
  6. 6. Frame Relay Frame format <ul><li>begin and end of frame marker (1 byte: 01111110) </li></ul><ul><li>address field - two bytes: </li></ul><ul><ul><li>address: DLCI - Data Link Connection Identifier </li></ul></ul><ul><ul><li>CR: 1 bit, user defined </li></ul></ul><ul><ul><li>EA: extended address (“1” - there will be next address byte) </li></ul></ul><ul><ul><li>FECN: Forward Explicit Congestion Notification (see congestion control) </li></ul></ul><ul><ul><li>BECN: Backward Explicit Congestion Notification </li></ul></ul><ul><ul><li>DE: Discard Eligibility - this frame can be discarded </li></ul></ul><ul><li>FCS: Frame Check Sequence (Control Sum) </li></ul>Flag Address field Information field Frame check sequence Flag Frame header 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 address CR EA address FECN BECN DE EA Octet 1 Octet 2
  7. 7. Frame Relay Interface types <ul><li>UNI: User-|Network Interface </li></ul><ul><li>NNI: Network-Network Interface </li></ul>Frame Relay network Frame Relay network Frame Relay network user user UNI UNI NNI NNI PVC segment Multi-network PVC
  8. 8. Frame Relay Parameters of a UNI interface <ul><li>Physical speed - just clock rate </li></ul><ul><li>Guaranteed bandwidth parameters </li></ul><ul><ul><li>CIR: Committed Information Rate </li></ul></ul><ul><ul><li>B C : Committed Burst Size </li></ul></ul><ul><li>Extended bandwidth parameters </li></ul><ul><ul><li>EIR: Extended Information Rate </li></ul></ul><ul><ul><li>B E : Extended Burst Size </li></ul></ul><ul><li>T C : Measurement Interval </li></ul>User traffic 192kbps 64kbps EIR CIR 256kbps time
  9. 9. Frame Relay CIR and EIR - how does it work <ul><li>B C = T C * CIR </li></ul><ul><li>B E = T C * EIR </li></ul>Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Within CIR Within CIR Marked DE Marked DE Discarded Bits B C +B E B C T 0 T 0 +T C Time CIR CIR + EIR Clock rate
  10. 10. Frame Relay Flow and congestion control <ul><li>There is no explicit flow control in FR; the network informs a user about congestion </li></ul><ul><li>Congestion: FR frames are discarded from overflowed buffers of switching devices </li></ul><ul><li>Congestion information: </li></ul><ul><ul><li>FECN - Forward Explicit Congestion Notification </li></ul></ul><ul><ul><li>BECN - Backward Explicit Congestion Notification </li></ul></ul><ul><li>There are recommendations for access devices what to do with FECN and BECN (usually not implemented) </li></ul>Transmission direction BECN FECN FRAD FRAD
  11. 11. Frame Relay Local addressing <ul><li>DLCI (Data Link Connection Identifier) - identification of a virtual circuit </li></ul><ul><li>DLCI - of local (for a given port) meaning </li></ul><ul><li>there can be max. 976 VCs on an interface user-network </li></ul><ul><li>DLCI values: 0 - LMI channel, 1-15 - reserved, 16-991 - available for VCs, 992-1007 - layer 2 management of FR service, 1008-1022 - reserved, 1023 - in channel layer management </li></ul>A B C To A: DLCI 121 To B: DLCI 243 To A: DLCI 182 To C: DLCI 121
  12. 12. Frame Relay Global addressing <ul><li>Extension proposed by “Group of Four” </li></ul><ul><li>Each end user access device FRAD is assigned a unique DLCI number - a global address Transmission to a given user goes over VC identified by a unique DLCI </li></ul><ul><li>Current DLCI format limits number of devices to less than 1000 </li></ul><ul><li>Another addition to the standard - extended DLCI addresses </li></ul>
  13. 13. Frame Relay Local Management Interface - LMI <ul><li>LMI - a signaling protocol used on an interface: end user - network (UNI) </li></ul><ul><li>Implementation optional (everybody implements it...) </li></ul><ul><li>Usage: </li></ul><ul><ul><li>notification about: creation, deletion, existence of PVCs on a given port </li></ul></ul><ul><ul><li>notification about status and availability of PVCs </li></ul></ul><ul><ul><li>periodic checks of integrity of physical connection </li></ul></ul><ul><li>Planned extensions: </li></ul><ul><ul><li>dynamic (SVC) channel creation and deletion </li></ul></ul><ul><ul><li>congestion notification </li></ul></ul><ul><li>Also planned: LMI for network-network interface (NNI) </li></ul>
  14. 14. Frame Relay Extensions to the standard <ul><li>Global addressing </li></ul><ul><li>“ Asynchronous status update” in LMI </li></ul><ul><li>Multicasting - possibility to send frames to multiple end users (FRAD) through a single DLCI identifier </li></ul><ul><li>Switched Virtual Circuits (SVC) - virtual channels configured dynamically (call setup) for data transmissions and then deleted (as in X.25 or POTS) </li></ul>
  15. 15. Frame Relay Multiprotocol over Frame Relay <ul><li>Standardized in RFC1490 </li></ul><ul><li>Not only IP, also other protocols, as well as remote bridging over Frame Relay </li></ul><ul><li>Can be used with LLC, SNAP, IPX, IP </li></ul><ul><li>Can be used for ARP, RARP, IARP </li></ul><ul><li>Redefines the data part of the frame and not the address header </li></ul>
  16. 16. Frame Relay IARP <ul><li>FRADs know DLCIs of available PVCs (through LMI), but don’t know IP addresses of other ends </li></ul><ul><li>IP addresses for given DLCIs are obtained automatically; mapping IP-DLCI is generated - dynamic mapping </li></ul><ul><li>IARP can be switched of; static maps have to be generated by FRAD user </li></ul>
  17. 17. Frame Relay Topologies <ul><li>star </li></ul><ul><li>full mesh </li></ul>
  18. 18. Frame Relay FR versus leased line <ul><li>Advantages: </li></ul><ul><li>Decreases number of ports on user devices </li></ul><ul><ul><li>important for star topology </li></ul></ul><ul><ul><li>vital for full mesh topologies ( N(N-1)/2 connections, N(N-1) ports) </li></ul></ul><ul><li>Backup lines become public operator responsibility and no longer that of an end user; backup connections are switched transparently to the user </li></ul><ul><li>More bandwidth is available for traffic peaks; CIR can be more expensive than similar leased line; CIR+EIR is much cheaper </li></ul>
  19. 19. Frame Relay FR versus leased lines <ul><li>Advantages: </li></ul><ul><li>Allows to build virtual LANs over whole countries (because of mesh topology and ARPs); simplifies routing </li></ul><ul><li>Allows to build private virtual corporate networks; they can be separated from the world at the 2nd level of OSI model - safety </li></ul><ul><li>A private network can be connected to the Internet in only one point: safety and economy </li></ul>
  20. 20. Frame Relay FR versus leased lines <ul><li>Advantages: </li></ul><ul><li>Simplicity of the configuration for the end user equipment (not necessarily for the operator…) </li></ul><ul><li>Example: IP over Frame Relay on Cisco IOS </li></ul><ul><ul><li>interface serial 0 </li></ul></ul><ul><ul><ul><li>ip address </li></ul></ul></ul><ul><ul><ul><li>encapsulation frame-relay ietf </li></ul></ul></ul><ul><ul><ul><li>frame-relay lmi-type ansi </li></ul></ul></ul>
  21. 21. Frame Relay FR versus leased lines <ul><li>Disadvantages: </li></ul><ul><li>Not for delay sensitive applications like: voice, video (though the former is sometimes transmitted over FR) </li></ul><ul><li>No guarantee that frames are delivered to the end point; is CIR really CIR? </li></ul><ul><li>Lots depend on the FR operator; especially overbooking - how many times sum of all CIRs extends physical capacity of operators connections </li></ul>
  22. 22. Frame Relay How do you really use it <ul><li>Rent ports at the operator’s switches (normally together with local leased lines and modems); you have to select clock rates </li></ul><ul><li>Ask for PVCs between ports you want; it can be your ports, ports on publicly available devices, like border router </li></ul><ul><li>Configure your FRADs - see Cisco example </li></ul><ul><li>Isn’t it simple?? </li></ul>
  23. 23. Frame Relay Case example: Poland <ul><li>Two big public FR networks: </li></ul><ul><ul><li>Polish Telecom TPSA (POLPAK-T): at least 1 switch in 50 biggest cities, 2-34Mbps trunks </li></ul></ul><ul><ul><li>NASK (Academic Operator): switches in some 15 bigger cities </li></ul></ul><ul><li>Internet connectivity through FR - to border routers </li></ul><ul><li>CIR=0 PVCs for free </li></ul><ul><li>Good prices: 256kbps port with PVC to a border router in POLPAK-T - about 350$ a month (all inclusive) </li></ul><ul><li>PVCs abroad (e.g. direct channel to a router in the US) become to be available; prices better than satellite; not yet tested </li></ul>
  24. 24. Frame Really? <ul><li>In my opinion: yes </li></ul><ul><li>With caution, but yes </li></ul>