Frame Relay
• Most Popular PSDN Today
– Offers speeds of 64 kbps to about 40 Mbps; This covers
the range of greatest corporate demand
– Most demand is at
the low end of the range
– Priced aggressively
– Both reasons
are critical
Frame Relay
• Low-Cost Service
–
–
–
–
Packet-Switched
Uses virtual circuits to cut costs
Unreliable
Relatively low speeds
• Dedicated Connections
– Always ready to send
and receive
• Like Frame Relay:
ATM
– Packet switched
– Virtual circuits
– Dedicated (Always On) Connections
• Unlike Frame Relay
– Much faster top speed
• 1 Mbps, 25 Mbps, 45 Mbps, 156 kbps, 622 kbps, several Gbps
– May offer quality of service (QoS) guarantees
• Maximum latency for time-critical applications
• Exact cell-by-cell timing
ATM
• Very Expensive
– Complexity because of basic transmission
mechanisms
– Complexity because of quality of service
mechanisms
– High-speed transmission
Frame Relay and ATM
• Most Vendors Offer Both
• To cover speeds from
56 kbps to a few
gigabits per second
Price
• In general, a smooth price-speed
curve across the two services
• At some speed, may offer both
– If so, usually price them the same
FR
ATM
Speed
Frame Relay and ATM
• Both are widely used
• Frame Relay is more popular today because it serves
the range of greatest corporate need (56 kbps to a few
megabits per second) at an attractive price
• As demand for higher-speed links grows, ATM
should become more widely used
– Unless other alternatives to ATM appear, such as 10 Gbps
Ethernet for WANs
Frame Relay Pricing
• Frame Relay Access Device (FRAD)
– Device at each site to connect site network to
Frame Relay WAN
– Paid to PSDN carrier or an equipment
manufacturer
Access
Device
PSDN
Frame Relay Pricing
• Leased Access Line
• Usually paid to telephone company rather than
PSDN
• PSDN often includes a bundled pricing
arrangement, but it must pay the telephone company
if this is the case
• If POPs are not close, access line charges will be
high
Access
Line
PSDN
Frame Relay Pricing
• Port Speed
– At the POP there is a connection port
– Ports come in different speeds
• 65 kbps, 1 Mbps are very common
– Faster ports are more expensive
– Monthly port speed usually is the most expensive
component of PSDN cost
Port at
POP
PSDN
Frame Relay Pricing
• Sometimes, Two Port Speeds are Quoted
– Committed Information Rate (CIR)
• Frames can be sent continuously at this speed
– Available Bit Rate (ABR)
• Higher speed
• Frames sent above the CIR are on standby
• If congestion appears, will be discarded first
Port at
POP
PSDN
Frame Relay Pricing
• Port Speed and Access Line Speed
– Port costs usually are higher than access line charges
– Access line must be as fast as port speed or faster to
avoid wasting expensive port speed
– For example, if the port speed is 1 Mbps, you should
get a T1 (1.544 Mbps) leased access line
Port at
POP
Access Line
PSDN
Frame Relay Pricing
• Permanent Virtual Circuits (PVCs)
– Leased line meshes had a leased line to each site
– PSDNs, usually have PVCs wherever there were
leased lines between sites in the previous leased line
network
• Eases the transition to PSDNs from leased lines
PVC
Leased
Access Line
PVC
Frame Relay Pricing
• Permanent Virtual Circuits (PVCs)
– PVCs to other sites are multiplexed over a site’s
single leased access line
– Pay monthly charge per PVC, based on speed
PVC
Leased
Access Line
PVC
Frame Relay Pricing
• Permanent Virtual Circuits (PVCs)
– Leased access line must be fast enough to
handle all of the PVCs it is multiplexing
– Example: if it multiplexes 15 64 kbps PVCs, the access
line must be 840 kbps (T1 line needed)
PVC
Leased
Access Line
PVC
Frame Relay Pricing
• Other Aspects of Pricing
– Sometimes only flat fees for access lines, ports,
etc, but sometimes also charges based on traffic
volume
– Ongoing (monthly) fees for leased lines, ports,
PVCs, etc.
– Usually also an initial setup or installation
charge
Frame Relay Pricing
• Other Aspects of Pricing
– If offer switched virtual circuits (which are set
up on demand), usually a fee for each set up
– Many vendors offer managed services that have
them manage the Frame Relay network instead
of the customer having to do it
Frame Relay Frame Structure
• Variable Length Frames
– Start flag (01111110) to signal start of frame
– Address field has variable length (2-4 octets)
– Information field to carry data (variable)
– CRC (Cyclical Redundancy Check) field to detect errors
(2 octets)
• If find errors, switch discards the frame
– Stop flag (01111110) to signal end of frame
Frame Relay Frame Structure
• Address Field of Frame Relay Frame
– Variable Length: 2-4 octets
– 4-Octet form shown
– Complex
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
Frame Relay Frame Structure
• Address Extension Bit (AE)
– 0 unless last octet
– 1 if last octet
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
0
DE
0
DLCI
0
DLCI
1
Frame Relay Frame Structure
• Address Field of Frame Relay Frame
– Data Link Control Indicator (DLCI)
– Indicates virtual circuit for switching
– Does not use destination addresses
Bits
0
DLCI (6 bits)
DLCI (4 bits)
FECN BECN
7
C/R
AE
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
Frame Relay Frame Structure
• Address Field of Frame Relay Frame
– If address field is 2 octets long, DLCI is 10 bits long
– If address field is 3 octets long, DLCI is 17 bits long
– If address field is 4 octets long, DLCI is 24 bits long
Bits
0
DLCI (6 bits)
DLCI (4 bits)
FECN BECN
7
C/R
AE
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
Frame Relay Frame Structure
• Address Field of Frame Relay Frame
– Discard Eligible Bit
– If send faster than committed rate, DE bit is set to 1
indicating that it may be discarded first
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
Frame Relay Frame Structure
• Congestion Notification
– If a switch detects congestion, it sets the FECN or
BECN bit
Bits
0
DLCI
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
Frame Relay Frame Structure
• Congestion Control
– Traffic shaping (see Chapter 8)
• The Backward Explicit Congestion Notification
(BECN) bit is set to tell the station that receives
the frame to slow down; This is easy to implement
• The Forward Explicit Congestion Notification
(FECN) bit is more complex; If a station receives
this notification in an incoming frame, it should
tell its communication partner at the other end of
the Frame Relay network to slow down
Frame Relay Frame Structure
• Address Field of Frame Relay Frame
– Command/Response bit useful in applications that have
commands and responses
– Not widely used
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE