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

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 Access Device

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 Access Line

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 at POP

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 at POP

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 at POP 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 Leased Access 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 Leased Access 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 kbps PVCs, the access line must be 840 kbps (T1 line needed) PVC Leased Access 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 ( ) 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 ( ) 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/RAE DLCIFECNBECNDEAE DLCIAE DLCIAE Bits

Frame Relay Frame Structure Address Extension Bit (AE) –0 unless last octet –1 if last octet 0 7 DLCI (6 bits)C/R0 DLCIFECNBECNDE0 DLCI0 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/RAE DLCI (4 bits)FECNBECNDEAE 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/RAE DLCI (4 bits)FECNBECNDEAE 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/RAE DLCIFECNBECNDEAE DLCIAE DLCIAE Bits

Frame Relay Frame Structure Congestion Notification –If a switch detects congestion, it sets the FECN or BECN bit 0 7 DLCIC/RAE DLCIFECNBECNDEAE DLCIAE DLCIAE 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/RAE DLCIFECNBECNDEAE DLCIAE DLCIAE Bits