frame relay most popular psdn today –offers speeds of 64 kbps to about 40 mbps; this covers the...
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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
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
0 7
DLCI (6 bits) C/R AE
DLCI (4 bits) FECN BECN DE AE
DLCI (7 bits) AE
DLCI (7 bits) AE
Bits
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
0 7
DLCI (6 bits) C/R AE
DLCI (4 bits) FECN BECN DE AE
DLCI (7 bits) AE
DLCI (7 bits) AE
Bits
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
0 7
DLCI (6 bits) C/R AE
DLCI FECN BECN DE AE
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 FECN BECN DE 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
Frame Relay Frame Structure• Address Field of Frame Relay Frame
– Command/Response bit useful in applications that have commands and responses
– Not widely used
0 7
DLCI (6 bits) C/R AE
DLCI FECN BECN DE AE
DLCI AE
DLCI AE
Bits