Frame Relay What is it

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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
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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
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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

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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

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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

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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
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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
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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
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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
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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
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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
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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

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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 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)

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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

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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

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Frame RelayTopologies

• star
• full mesh

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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

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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

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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

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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

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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??

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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

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Frame Really?

• In my opinion yes
• With caution, but yes