Frame2

Frame Relay What is it?? Frame Relay (FR) - public network WAN technology based on packet switching FR standard defines an interface between an end user and a public network. FR is a protocol of 2nd level of OSI model Internal Frame Relay protocol (between switching devices in the cloud) is not standardized (probably it will be some day) Frame Relay cloud end user Frame Relay interface

Frame Relay What is it about? Aim: transport user data between port A and B Data is transmitted as variable length frames Max. frame length is 4096 bytes (recommended length is 1600 bytes) From users point of view: ports A and B are connected with a transparent logical link (virtual circuit - VC) FRAD - Frame Relay Access Device PVC FR switches A B VC - Virtual Circuit PVC - Permanent VC

Frame Relay Standards Frame Relay “independent existence”: In 1990 “Group of Four” (DEC, Northern Telecom, Cisco, Stratacom) presented FR as an independent standard Later this Frame Relay Forum was established: main standardization body for FR 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

Frame Relay Most important features Based on packet (frame) switching Frames of variable length (up to 4096 bytes, typically 1600 bytes) Connection oriented; only permanent connections - PVCs; switched VCs in standard extensions High data rates at user-network interfaces (2Mbps, ultimately up to 45 Mbps) Bandwidth on demand No flow control mechanisms (nearly) No error control (but FCS) or retransmission mechanisms 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

Frame Relay Why was it proposed? Efficiency: increased demand for high throughput networking (X.25 too slow) “ Bursty applications”: LAN connectivity, Internet, not only terminal applications Fibre optic lines: low (very, very low) bit error rates New, smarter software: applications (or higher level protocols like TCP) performing error control, retransmissions; reliable date links delivered by higher levels of OSI model

Frame Relay Frame format begin and end of frame marker (1 byte: 01111110) address field - two bytes: address: DLCI - Data Link Connection Identifier CR: 1 bit, user defined EA: extended address (“1” - there will be next address byte) FECN: Forward Explicit Congestion Notification (see congestion control) BECN: Backward Explicit Congestion Notification DE: Discard Eligibility - this frame can be discarded FCS: Frame Check Sequence (Control Sum) 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

Frame Relay Interface types UNI: User-|Network Interface NNI: Network-Network Interface Frame Relay network Frame Relay network Frame Relay network user user UNI UNI NNI NNI PVC segment Multi-network PVC

Frame Relay Parameters of a UNI interface Physical speed - just clock rate Guaranteed bandwidth parameters CIR: Committed Information Rate B C : Committed Burst Size Extended bandwidth parameters EIR: Extended Information Rate B E : Extended Burst Size T C : Measurement Interval User traffic 192kbps 64kbps EIR CIR 256kbps time

Frame Relay CIR and EIR - how does it work B C = T C * CIR B E = T C * EIR 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

Frame Relay Flow and congestion control There is no explicit flow control in FR; the network informs a user about congestion Congestion: FR frames are discarded from overflowed buffers of switching devices Congestion information: FECN - Forward Explicit Congestion Notification BECN - Backward Explicit Congestion Notification There are recommendations for access devices what to do with FECN and BECN (usually not implemented) Transmission direction BECN FECN FRAD FRAD

Frame Relay Local addressing DLCI (Data Link Connection Identifier) - identification of a virtual circuit DLCI - of local (for a given port) meaning there can be max. 976 VCs on an interface user-network 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 A B C To A: DLCI 121 To B: DLCI 243 To A: DLCI 182 To C: DLCI 121

Frame Relay Global addressing Extension proposed by “Group of Four” 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 Current DLCI format limits number of devices to less than 1000 Another addition to the standard - extended DLCI addresses

Frame Relay Local Management Interface - LMI LMI - a signaling protocol used on an interface: end user - network (UNI) Implementation optional (everybody implements it...) Usage: notification about: creation, deletion, existence of PVCs on a given port notification about status and availability of PVCs periodic checks of integrity of physical connection Planned extensions: dynamic (SVC) channel creation and deletion congestion notification Also planned: LMI for network-network interface (NNI)

Frame Relay Extensions to the standard Global addressing “ Asynchronous status update” in LMI Multicasting - possibility to send frames to multiple end users (FRAD) through a single DLCI identifier Switched Virtual Circuits (SVC) - virtual channels configured dynamically (call setup) for data transmissions and then deleted (as in X.25 or POTS)

Frame Relay Multiprotocol over Frame Relay Standardized in RFC1490 Not only IP, also other protocols, as well as remote bridging over Frame Relay Can be used with LLC, SNAP, IPX, IP Can be used for ARP, RARP, IARP Redefines the data part of the frame and not the address header

Frame Relay IARP FRADs know DLCIs of available PVCs (through LMI), but don’t know IP addresses of other ends IP addresses for given DLCIs are obtained automatically; mapping IP-DLCI is generated - dynamic mapping IARP can be switched of; static maps have to be generated by FRAD user

Frame Relay Topologies star full mesh

Frame Relay FR versus leased line Advantages: Decreases number of ports on user devices important for star topology vital for full mesh topologies ( N(N-1)/2 connections, N(N-1) ports) Backup lines become public operator responsibility and no longer that of an end user; backup connections are switched transparently to the user More bandwidth is available for traffic peaks; CIR can be more expensive than similar leased line; CIR+EIR is much cheaper

Frame Relay FR versus leased lines Advantages: Allows to build virtual LANs over whole countries (because of mesh topology and ARPs); simplifies routing Allows to build private virtual corporate networks; they can be separated from the world at the 2nd level of OSI model - safety A private network can be connected to the Internet in only one point: safety and economy

Frame Relay FR versus leased lines Advantages: Simplicity of the configuration for the end user equipment (not necessarily for the operator…) Example: IP over Frame Relay on Cisco IOS interface serial 0 ip address 194.1.1.1 255.255.255.0 encapsulation frame-relay ietf frame-relay lmi-type ansi

Frame Relay FR versus leased lines Disadvantages: Not for delay sensitive applications like: voice, video (though the former is sometimes transmitted over FR) No guarantee that frames are delivered to the end point; is CIR really CIR? Lots depend on the FR operator; especially overbooking - how many times sum of all CIRs extends physical capacity of operators connections

Frame Relay How do you really use it Rent ports at the operator’s switches (normally together with local leased lines and modems); you have to select clock rates Ask for PVCs between ports you want; it can be your ports, ports on publicly available devices, like border router Configure your FRADs - see Cisco example Isn’t it simple??

Frame Relay Case example: Poland Two big public FR networks: Polish Telecom TPSA (POLPAK-T): at least 1 switch in 50 biggest cities, 2-34Mbps trunks NASK (Academic Operator): switches in some 15 bigger cities Internet connectivity through FR - to border routers CIR=0 PVCs for free Good prices: 256kbps port with PVC to a border router in POLPAK-T - about 350$ a month (all inclusive) PVCs abroad (e.g. direct channel to a router in the US) become to be available; prices better than satellite; not yet tested

Frame Really? In my opinion: yes With caution, but yes

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