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Frame Relay What is it

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Frame Relay (FR) - public network WAN technology based on packet switching ... Max. frame length is 4096 bytes (recommended length is 1600 bytes) ... – PowerPoint PPT presentation

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Title: Frame Relay What is it


1
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
2
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)

VC - Virtual Circuit PVC - Permanent VC
FRAD - Frame Relay Access Device
PVC
A
B
FR switches
3
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 basedANSI 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

4
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

5
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

6
Frame RelayFrame format
Frame header
  • 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)

Address field
Information field
Frame check sequence
Flag
Flag
address
CR
EA
address
FECN
BECN
DE
EA
8 7 6 5 4 3 2
1 8 7 6 5 4 3
2 1
Octet 1
Octet 2
7
Frame Relay Interface types
  • UNI User-Network Interface
  • NNI Network-Network Interface

UNI
NNI
NNI
UNI
PVC segment
Frame Relay network
Frame Relay network
Frame Relay network
user
user
Multi-network PVC
8
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

192kbps
User traffic
EIR
256kbps
CIR
64kbps
time
9
Frame RelayCIR and EIR - how does it work
  • BC TC CIR
  • BE TC EIR

Bits
Clock rate
BCBE
CIR EIR
BC
CIR
Time
T0
Frame 1
Frame 2
Frame 3
Frame 4
Frame 5
T0TC
Within CIR
Within CIR
Marked DE
Marked DE
Discarded
10
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
FRAD
FRAD
BECN
FECN
11
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
C
To A DLCI 121 To B DLCI 243
To A DLCI 182 To C DLCI 121
B
12
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

13
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)

14
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)

15
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

16
Frame RelayIARP
  • 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

17
Frame RelayTopologies
  • star
  • full mesh

18
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 CIREIR is much cheaper

19
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

20
Frame RelayFR 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

21
Frame RelayFR 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

22
Frame RelayHow 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??

23
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
  • CIR0 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

24
Frame Really?
  • In my opinion yes
  • With caution, but yes
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