Frame2

CEENet 97 Workshop M.Krzyzanowski, IdS, Warsaw Univ.

Frame RelayWhat 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 RelayWhat is it about?

• Aim: transport user data between port A and B

• Data is transmitted as variable length framesMax. 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 RelayAccess Device

PVC

FR switches

A B

VC - Virtual CircuitPVC - Permanent VC


Frame RelayStandards

• 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 RelayMost 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 modelNo standards for physical interface: can be X.21, V.35, G.703, G.704


Frame RelayWhy 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 RelayFrame 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)

FlagAddressfield

Informationfield

Frame checksequence Flag

Frame header

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

address CR EA address FECNBECN DE EA

Octet 1 Octet 2


Frame Relay Interface types

• UNI: User-|Network Interface

• NNI: Network-Network Interface

Frame Relaynetwork

Frame Relaynetwork

Frame Relaynetworkuser user

UNI UNINNI NNIPVC segment

Multi-network PVC


Frame RelayParameters of a UNI interface

• Physical speed - just clock rate

• Guaranteed bandwidth parameters

– CIR: Committed Information Rate

– BC: Committed Burst Size

• Extended bandwidth parameters

– EIR: Extended Information Rate

– BE: Extended Burst Size

• TC: Measurement Interval

Usertraffic

192kbps

64kbps

EIR

CIR

256kbps

time


Frame RelayCIR and EIR - how does it work

• BC = TC * CIR

• BE = TC * EIR

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5Within CIR Within CIR Marked DE Marked DE Discarded

Bits

BC+BE

BC

T0

T0+TC

Time

CIR

CIR + EIR

Clock rate


Frame RelayFlow 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 FECNFRAD

FRAD


Frame RelayLocal 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 121To B: DLCI 243

To A: DLCI 182To C: DLCI 121


Frame RelayGlobal addressing

• Extension proposed by “Group of Four”

• Each end user access device FRAD is assigned a unique DLCI number - a global addressTransmission 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 RelayExtensions 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 RelayMultiprotocol 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 RelayIARP

• 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 RelayTopologies

• star

• full mesh


Frame RelayFR 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 RelayFR 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



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



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 RelayHow 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 RelayCase 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

Frame2

Documents

2nd level

frame relay

warsaw univ

frame relaywhat

osi model

ansi t1

ceenet 97

end user