Asynchronous Transfer Mode (ATM)

1
Asynchronous Transfer Mode (ATM)

• EE290F
• 22 April 2004
• Shanna Crankshaw

2
ATM definition

• "A transfer mode in which information is
  organized into cells it is asynchronous in the
  sense that the recurrence of cells containing
  information from an individual user is not
  necessarily periodic".

3
What is it really?

• Low-level network layerabove physical layer,
  below AAL (ATM adaptation layer)
• Single transport mechanism for different types of
  traffic (voice, data, video, etc.)
• Streamlined protocol, minimal error and flow
  control capabilities
• Fixed packet size ATM cell
• Simplified processing, management

4
Synchronous Transfer Mode

• Pre-assigned slots, frame boundaries, global
  timing
• Slots identified by position from the start of
  the frame
• BW allocated in units of slots
• Idle slots wasted
• Efficient for Constant Bit Rate traffic

5
Contrast with STM

• Bandwidth on demandSlots assigned on demand,
  users take any empty slot
• Nothing pre-assigned, no global timing
• Slot ? Cell, fixed size of 53 bytes
• Arbitrary bit rates can support T-1 using CBR,
  voice/video using real-time VBR, IP-based traffic
  using ABR and UBR, etc.
• Each cell must be self-identifying (overhead)

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ATM cell contents

• Header 5 bytes
• General Flow Control (GFC) traffic control for
  different QoS, alleviates short-term overloads
• VPI routing field for network
• VCI routing to/from user
• Payload type
• Cell loss priority (CLP)
• Header error control (HEC) can correct single
  bit errors in header
• Information 48 bytes

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Stallings, Data Computer Communications, 6th
ed., Table 11.2

• PT coding
• 000
• 001
• 010
• 011
• 100
• 101
• 110
• 111

______________Interpretation____________ User
data cell, congestion not experienced, SDU
type0 User data cell, congestion not
experienced, SDU type1 User data cell,
congestion experienced, SDU type0 User data
cell, congestion experienced, SDU type1 OAM
segment associated cell OAM end-to-end associated
cell Resource management cell Reserved for future
function
SDU Service Data Unit OAM Operations,
Administration, and Maintenance
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ATM Cells

• Small size, may reduce queuing delay of high
  priority cells
• Fixed size, more efficient switching

NNI
UNI
GFC
VP identifier
VP identifier
VP identifier
5-byte header
VC identifier
VC identifier
PT CLP
PL type CLP
Header error control
Header error control
53 byte cell
Info field, 48 bytes
Info field, 48 bytes
9
VCs are not always VCs

• Virtual Channel
• Transmission is connection-oriented
• VC set up by some signaling protocol before any
  cells can be sent
• Virtual Path Connection (VPC), bundle of VCCs

10
Logical connections

• VPC bundle of VCCs with the same endpoints ?
  all switched together
• Network management of group of connections, not
  many individual ones
• Setup time is for a VP, adding VCs to it
  involves minimal processing

11
Request for VCC originates
Can QoS be satisfied?
Yes
Yes
No
No
Yes
No
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ATM Adaptation Layer (AAL)

• Layer above ATM
• Service dependent
• Mask ATM specifics from user universality
• SAR segmentation and reassembly. Translates
  service data from a non-ATM format into ATM
  cells, then back again at destination
• CS convergence sublayer. Takes care of delay
  jitter, error checking, remove corrupted cells

AAL user
CS
AAL
SAR sublayer
ATM layer
Physical layer
13
ATM Service Categories

• Real-time services
• Constant bit rate uncompressed audio/video info
• Videoconferencing, TV, pay-per-view, VOD, etc.
• rt-Variable bit rate
• Non-real-time services
• nrt-VBR high end system QoS, critical response
  time
• Bank transactions, airline reservations, etc.
• Unspecified bit rate best-effort service
• e.g. text/image messaging, telecommuting
• Available bit rate bursty apps requiring
  reliable end-to-end connection
• e.g. LAN, router-to-router reliability

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ATM advantages

• Universality
• Mixed traffic types, real-time and non-real-time
• Scalability
• LANs, MANs, WANs, WLANs
• Efficient use of network resources Bandwidth on
  demand concept
• Simplified network infrastructure

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ATM challenges

• In-network mux/buffering can lead to cell delay
  or loss ? QoS guarantees
• Many types of traffic
• Large geographic distribution

Traffic modeling, control