Frame Relay Most Popular PSDN Today –Offers speeds of 64 kbps to about 40 Mbps; This covers the...

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 atthe low end of the range

– Priced aggressively

– Both reasonsare 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

ATM• Like Frame Relay:

– 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

• In general, a smooth price-speed curve across the two services

• At some speed, may offer both– If so, usually price them the same

Speed

Price ATMFR

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

PSDN

AccessDevice

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

PSDN

AccessLine

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

PSDN

Port atPOP

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

PSDN

Port atPOP

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

PSDN

Port atPOP

Access Line

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

PVC

LeasedAccess Line

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

PVC

LeasedAccess Line

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

PVC

LeasedAccess Line

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

0 7

DLCI (6 bits) C/R AE

DLCI FECN BECN DE AE

DLCI AE

DLCI AE

Bits

Frame Relay Frame Structure

• Address Extension Bit (AE)– 0 unless last octet

– 1 if last octet

0 7

DLCI (6 bits) C/R 0

DLCI FECN BECN DE 0

DLCI 0

DLCI 1

Bits


– Data Link Control Indicator (DLCI)

– Indicates virtual circuit for switching

– Does not use destination addresses

0 7


DLCI (4 bits) FECN BECN DE AE

DLCI (7 bits) AE

DLCI (7 bits) AE

Bits


– If address field is 2 octets long, DLCI is 10 bits long



0 7


DLCI (4 bits) FECN BECN DE AE

DLCI (7 bits) AE

DLCI (7 bits) AE

Bits


– Discard Eligible Bit

– If send faster than committed rate, DE bit is set to 1 indicating that it may be discarded first

0 7



DLCI AE

DLCI AE

Bits

Frame Relay Frame Structure

• Congestion Notification– If a switch detects congestion, it sets the FECN or

BECN bit

0 7

DLCI C/R AE


DLCI AE

DLCI AE

Bits

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


– Command/Response bit useful in applications that have commands and responses

– Not widely used

0 7



DLCI AE

DLCI AE

Bits

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