ATM Switches

1
ATM Switches

• Cells
• Scalable
• QoS
• Perspective
• Virtual Circuits

2
ATM Switches

• Asynchronous Transfer Mode
• Basic Standards Set by ITU-T
• Partner with ISO in OSI standards
• ATM standards developed within OSI architecture
• ATM Forum Sets Detailed Standards
• Group of mostly ATM vendors
• Moves quickly
• Also tests for interoperability

3
ATM Switches

• Has fixed-length frames are called cells
• Always 48 octets of payload
• Always 5 octets of header
• So always 53 octets total
• Fixed length gives predictability
• Allows switches to process cells very rapidly
  using parallel circuitry

Payload (48 octets)
Header(5 octets)
ATM Cell
4
ATM Switches

• Small cell reduces latency (delay) at each switch
• Some processing must wait for the entire frame
  arrives
• Short frames finish arriving quickly
• Critical for voice

5
ATM Switches

• Highly Scalable
• A few Mbps to a few gigabits per second
• Once critical, but Ethernet speeds are now
  comparable
• Very sophisticated
• Quality of service (QoS)--delivery guarantees for
  maximum latency, exact timing between adjacent
  cells
• Ethernet is only a best-effort service today

6
ATM Switches

• Hardware is very expensive because of complexity
• Retraining and ongoing labor are very expensive
  because of complexity
• ATM has high overhead (extra characters)
• 5 overhead octets for 48 data octets (10
  overhead)
• Actually even worse (see Module E)

7
ATM Switches

• Unfortunately, very expensive
• Has lost the desktop
• It is usually cheaper to use high-capacity
  Ethernet switches with overprovisioning and
  perhaps priority, so that latency does not grow
  to the point where QoS is critical
• In LANs, usually used only where service quality
  is critical, typically when voice is being carried

8
ATM QoS Categories

• ATM Offers Varying Levels of QoS
• Parameters
• Peak cell rate (maximum burst speed)
• Maximum burst size (bits per burst)
• Sustainable cell rate (always allowed)
• Cell Delay Variation Tolerance (CDVT) how exact
  cell-to-cell timing is Critical for voice and
  video
• Cell Loss Ratio Losses during transmission

9
ATM QoS Categories

• ATM Offers Varying Levels of QoS
• For Voice and Video
• ITU-T Class A
• ATM Forum Service Category Constant Bit Rate
  (CBR)
• Low latency
• Low Cell Delay Variation Tolerance
• Strong guarantees for voice and video!

10
ATM QoS Categories

• For IP and LAN Data
• ITU-T Class D
• Several ATM Forum Service Categories
• Developed several categories over Time
• Available bit rate (ABR) weak send if capacity
  is available
• Unspecified bit rate (UBR) weak simpler than
  ABR, but can get almost no share of capacity
• Guaranteed frame rate (GFR) gets roughly fair
  share of capacity during congestion

11
ATM QoS Categories

• For IP and LAN Data
• Several ATM Forum Service Categories
• ABR, UBR, and even GFR give very low status to
  data transmission
• Not even as good as Ethernet priority of service
• Yet costs far more
• So ATM QoS makes little sense if used entirely
  for data
• Has other data transmission benefits, however

12
ATM QoS Categories

• Other Categories
• For Videoconferencing
• May need momentary bandwidth increase if there is
  a burst of motion on the screen
• Needs Low Cell Delay Variation Tolerance
• ATM Class B
• ATM Forum Service Category Variable Bit
  Rate-Real Time (VBR-RT)
• Not widely used or implemented

13
ATM QoS Categories

• Other Categories
• For Connection-Oriented Data
• ATM Class C
• ATM Forum Service Category Variable Bit Rate-Not
  Real Time (VBR-NRT)
• Most data not connection-oriented
• Not widely used, implemented

14
ATM Switches Virtual Circuits

• Often Arranged in a Mesh
• But all traffic between two stations still is
  consigned to a path called a virtual circuit that
  is set up before the first frame transmission

ATM Cell
Virtual Circuit
15
ATM Switches

• Virtual Circuits Mean that there is Only a Single
  Possible Path between Any Two Stations
• Virtual circuits simplify switch operation and so
  lower switch cost

ATM Cell
Virtual Circuit
16
ATM Switches

• Permanent Virtual Circuits (PVCs)
• Designed to operate for weeks, months, or years
• Usually used between permanent sites in a
  corporation
• Simplest and least expensive administratively
  because rarely changed
• Most widely used form of virtual circuit

17
ATM Switches

• Switched Virtual Circuits (SVCs)
• Established just as communication between a pair
  of stations starts
• More flexible than PVCs in what other stations a
  station can reach
• Expensive because each setup costs money
• Until recently, not used much because of
  complexity, added cost

18
ATM Switches

• ATM Frame Header
• Does NOT have a destination address field
• Instead, has two fields that together contain a
  hierarchical virtual circuit number
• Like a route number on a bus--names the route,
  not the destination

Virtual Circuit Number
ATM Header
19
ATM Switches

• Hierarchical Virtual Circuit Number
• Virtual Path Identifier
• Higher-level number Often specifies a site
• Virtual Channel Identifier
• Lower-level number Often specifies a computer at
  a site

Virtual Circuit Number
ATM Header
20
ATM Switches

• ATM Reliability
• Virtual circuit reduces communication to a single
  path
• If a switch or trunk line along the path fails,
  communication stops
• But ATM switches also have addresses, which are
  used to set up a new virtual circuit fairly
  rapidly