Title: Frame Relay What is it
1Frame 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
2Frame 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
3Frame 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
4Frame 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
5Frame 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
6Frame 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
7Frame 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
8Frame 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
9Frame RelayCIR and EIR - how does it work
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
10Frame 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
11Frame 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
12Frame 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
13Frame 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)
14Frame 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)
15Frame 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
16Frame 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
17Frame RelayTopologies
18Frame 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
19Frame 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
20Frame 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
21Frame 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
22Frame 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??
23Frame 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
24Frame Really?
- In my opinion yes
- With caution, but yes