IS 3413

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IS 3413

• Chapter 13 Frame Relay
• Dr. Jan Clark
• SPRING, 2003

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Definition of Terms

• COS Class of Service sets a priority of data
  delivery, based upon the class. Higher priority
  data get delivered before lower priority data
  (example which should have higher priority
  streaming video or email?)
• QOS Quality of Service involves establishing
  certain parameters for a specific transmission
  e.g. amount of bandwidth required for a given
  priority data transmission, max. amount of
  latency tolerated
• Both are required to deliver real-time voice and
  video traffic

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Frame Relay

• Public WAN packet-switching protocol
• Provides LAN-LAN connectivity
• Relays frames across a network from source to
  destination
• Connection-oriented protocol must first
  establish a connection before two nodes can
  communicate

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LAN-LAN ConnectivityPrior to Frame Relay

• Conventional dial-up circuit switching
• Dedicated leased line using point-to-point
  protocols or X.25 Packet Switching
• (Both have significant problems in todays
  technology)

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Interconnectivity Problems of Leased Line and
X.25 (Fig. 13.1)

• Leased Line If LANs want to communicate with
  multiple other sources simultaneously, need
  multi-port routers and multiple leased lines
  very expensive and difficult to maintain
• X.25 does not support multi channels (BD)
• Works on permanent virtual circuit, or virtual
  call
• Permanent virtual circuit follows same path
  each time acts like leased line, but line is
  shared
• Virtual call virtual circuit dynamically
  allocated (need a call initiation procedure)

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Cost and Complexity of Dedicated Circuits
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Frame Relay Solution

• Provide single connection into a public network,
  rather than multiple connections (Fig. 13.2)
• Reduces cost and complexity of the network (Fig.
  13.3)
• Frame relay circuit costs not based on distance
• Circuits may, or may not, be permanent

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Fig. 13.2 Frame Relay Cloud
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Fig. 13.3 Reduced Cost and Complexity
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Frame Relay Circuits

• Connection-oriented protocol
• Relies on permanent virtual circuit (PVC)
  provide non-dedicated connections through a
  shared medium (bandwidth is shared among multiple
  sites (simplex lines)
• Can also support switched virtual circuit (SVC)

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Frame RelayPermanent Virtual Circuit

• Pre-determined link between source and
  destination
• Bandwidth is shared among multiple sites, not
  dedicated (Fig. 13.4)
• Bandwidth cost based on Committed Information
  Rate (CIR)
• Frames encapsulate data contained in layers 3-7

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CIR

• If Leased line, a fixed amount of bandwidth is
  provided, regardless of use.
• If CIR, frame relay provider guarantees a given
  throughput (bps) during normal workloads - user
  hedges bet and hopes for more
• CIR can be symmetric or asymmetric (as in ADSL)
• If CIR is too small, network becomes congested,
  and frames may be dropped
• If CIR is too high, you are paying for excessive
  bandwidth
• Service provider does not guarantee delivery
  above CIR

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BURST

• A data transmission that is equal to, or exceeds
  the CIR
• Committed Burst Maximum amount of data the
  provider guarantees to deliver within a specified
  time period (CIR/time) In general, CIR should
  not exceed 70 of port speed
• Excessive Burst Maximum amount of uncommitted
  data a provider will attempt to deliver within a
  specified time period (Fig. 13.6)

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Committed plus Excessive Burst
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Setting CIR to Zero

• Allows customer to take extra bandwidth when
  available
• Helps provider establish usage patterns
  normally only done initially, if at all
• Similar to over-selling seats on planes

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Switched Virtual Circuit

• Frame Relay also supports virtual circuits (SVC)
• SVCs also support CIRs
• Circuits between source and destination are
  established when needed (logical dynamic, rather
  than logical permanent)
• Analogous to PSTN - paths change between
  connections

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PVC Advantages and Disadvantages

• Widespread availability
• Less complex network design
• Less expensive equipment
• Permanent connections - always paying for a
  certain amount of bandwidth, regardless of use
• Every time a new connection is required, a new
  permanent circuit must be established

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Frame Relay Basic Operation

• Packet switching every frame carries source and
  destination address
• Data link layer protocol, but does not support
  flow control, error detection, frame sequencing
  or ACK (all performed at end nodes (routers))
• Since frame integrity performed by end nodes, FR
  is fast and efficient
• Statistical multiplexing multiple subscribers
  share same backbone

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Primary components of Frame Relay

• End nodes - interface between a customer's site
  and service provider's network (bridges, routers,
  workstations, etc.)
• Frame relay switches - transmit data from source
  to destination
• Communication links PVCs or SVCs

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FRAD - Frame Relay Access Devices

• Accepts data frames from local network and
  assembles them into frame relay frames
• Encapsulates data contained in layers 3-7.
• Destination FRAD reassembles the data, checks
  integrity, and places it on local backbone for
  delivery to destination node

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Frame Relay Frames (Fig. 13.8)

• No frame size limitation - accommodates Ethernet
  or Token Ring without having to segment
• DE slot - Discard eligibility
• If DE set to 1, considered low priority, and
  first to be discarded when network congested

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Data Link Connection Identifier (DLCI)

• Virtual circuit addresses assigned to PVCs or
  SVCs
• Enable multiple virtual circuits, representing
  logical connections, to be multiplexed
• When a logical connection first established,
  unique DLCI assigned
• DLCI appended to each frame sent between source
  and destination

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Congestion Control and Management

• If no ACK within a given time period, sending
  node assumes frame was discarded
• Discarded frames must be retransmitted,
  increasing network traffic
• Provider responsibilities
• Design networks to provide sufficient bandwidth
• Ensure links are error free
• Prevent customers from setting CIR gt 70 port
  speed
• Prevent any node from monopolizing the system
• Distribute resources in fair and equitable manner

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VOFR - Voice Over Frame Relay (Fig. 13.10)

• Voice-capable FRADs accommodate the nuances
  associated with transmitting voice
• Voice compression - eliminates pauses and
  redundant information, thus reducing required
  bandwidth
• Echo cancellation - eliminate voice echoing and
  distortion
• Delay control techniques - traffic prioritization
  and fragmentation
• CIRs must be increased to accommodate voice
  traffic
• Can also support real time video, with adequate
  bandwidth and CIRs

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VOFR Example (Fig. 13.10)
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Status of Frame Relay

• Originally designed for data rates from 56 kbps
  to T1 (1.54 Mbps)
• Now operates at T3 (44.736 Mbps)
• Can support on-demand connections, multicast
  communications, data compression and CoS/QoS
• More efficient than ATM when transmitting
  Ethernet frames (Ethernet frame 64 bytes, ATM
  frame 53 bytes)