Frame Relay Basic

8/14/2019 Frame Relay Basic

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CEENet 97 Workshop

Frame Relay

What is it?? Frame Relay (FR) -public network WAN technology

based on packet switching

FRstandard 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


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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 is1600 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


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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: mainstandardization 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


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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


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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


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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)

FlagAddress

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 FECNBECN DE EA

Octet 1 Octet 2


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Frame Relay

Interface types UNI: User-|Network Interface

NNI: Network-Network Interface

Frame Relay

network

Frame Relay

network

Frame Relay

networkuseruser

UNI UNINNI NNIPVC segment

Multi-network PVC


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Frame Relay

Parameters 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

User

traffic

192kbps

64kbps

EIR

CIR

256kbps

time


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Frame Relay

CIR and EIR - how does it work

BC = TC * CIR

BE = TC * EIR

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5

Within CIR Within CIR Marked DE Marked DE Discarded

Bits

BC+BE

BC

T0T0+TC

Time

CIR

CIR + EIR

Clock rate


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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

BECNFECN

FRAD

FRAD


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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 182To C: DLCI 121


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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


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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)


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Frame Relay

Extensions to the standard Global addressing

Asynchronous status update in LMI

Multicasting - possibility to send frames to multiple endusers (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)


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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


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Frame Relay

IARP FRADs know DLCIs of available PVCs (through LMI),

but dont 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


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Frame Relay

Topologies star

full mesh


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Frame Relay

FR versus leased lineAdvantages:

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


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Frame Relay

FR versus leased linesAdvantages:

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


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Frame Relay

FR versus leased linesAdvantages:

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


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Frame Relay

FR versus leased linesDisadvantages:

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


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Frame Relay

How do you really use it Rent ports at the operators 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

Isnt it simple??


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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


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Frame Really?

In my opinion: yes

With caution, but yes

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Frame Relay Basic

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