Frame Relay

1
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

2
Frame Relay

• Low-Cost Service
• Packet-Switched
• Uses virtual circuits to cut costs
• Unreliable
• Relatively low speeds
• Dedicated Connections
• Always ready to sendand receive

3
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

4
ATM

• Very Expensive
• Complexity because of basic transmission
  mechanisms
• Complexity because of quality of service
  mechanisms
• High-speed transmission

5
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

ATM
FR
Price
Speed
6
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

7
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
8
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
9
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
10
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
11
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
12
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
13
Frame Relay Pricing

• Permanent Virtual Circuits (PVCs)
• PVCs to other sites are multiplexed over a sites
  single leased access line
• Pay monthly charge per PVC, based on speed

PVC
Leased Access Line
PVC
14
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
15
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

16
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

17
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

18
Frame Relay Frame Structure

• Address Field of Frame Relay Frame
• Variable Length 2-4 octets
• 4-Octet form shown
• Complex

7
Bits
0
DLCI (6 bits)
C/R
AE
DLCI
FECN
BECN
DE
AE
DLCI
AE
DLCI
AE
19
Frame Relay Frame Structure

• Address Extension Bit (AE)
• 0 unless last octet
• 1 if last octet

7
Bits
0
DLCI (6 bits)
C/R
0
DLCI
FECN
BECN
DE
0
DLCI
0
DLCI
1
20
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

7
Bits
0
DLCI (6 bits)
C/R
AE
DLCI (4 bits)
FECN
BECN
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
21
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

7
Bits
0
DLCI (6 bits)
C/R
AE
DLCI (4 bits)
FECN
BECN
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
22
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

7
Bits
0
DLCI (6 bits)
C/R
AE
DLCI
FECN
BECN
DE
AE
DLCI
AE
DLCI
AE
23
Frame Relay Frame Structure

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

7
Bits
0
DLCI
C/R
AE
DLCI
FECN
BECN
DE
AE
DLCI
AE
DLCI
AE
24
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

25
Frame Relay Frame Structure

• Address Field of Frame Relay Frame
• Command/Response bit useful in applications that
  have commands and responses
• Not widely used

7
Bits
0
DLCI (6 bits)
C/R
AE
DLCI
FECN
BECN
DE
AE
DLCI
AE
DLCI
AE