ATM: What it is, and what it isn't

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ATM What it is, and what it isn't

• Carey Williamson

University of Calgary
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ATM What it is

• Asynchronous Transfer Mode
• A low-layer networking technology based on fast
  packet-switching of small fixed size packets
  called cells
• ATM provides a single transport mechanism for
  integrated services traffic data, voice, video,
  image, graphics...
• All statistically multiplexed at ATM layer

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ATM What it isnt

• Synchronous Transfer Mode (STM)
• STM relies on pre-assigned slots for each user
  within a frame, and global timing information to
  mark frame boundaries
• Example T1 transmission (1.544 Mbps)

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T1 Transmission
Framing bit
Room for 24 calls, with 8 bits from each
8 bit sample for call i
24 x 8 bits 192 bits 1 framing bit 193 bits
125 microseconds
(8000 cycles/sec)
An example of one frame from T1 digital
transmission scheme
193 bits/frame X 8000 frames/sec 1.544 Mbps
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T1 Transmission (Contd)
Frame 1
Frame 2
Frame 3
Call i uses slot i in each frame
8 bits/slot X 1 slot/frame X 8000 frames/sec
64000 bits/sec
64 kbps
Calls can be allocated k slots per frame to give
allocated bandwidth that is k x 64 kbps
Idle slots are wasted
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Synchronous Transfer Mode

• STM relies on positional association slots are
  identified by their relative position from the
  start of the frame (global timing info)
• Each user knows which slot(s) to use
• All slots are the same size (e.g., 8 bits)
• Bandwidth allocated in multiples of slots
• Efficient for Constant Bit Rate traffic
• Inefficient for Variable Bit Rate traffic

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Asynchronous Transfer Mode

• ATM does not use a priori assignment of slots to
  users
• Slots are assigned on demand on an as needed
  basis
• Users can use whichever slots are empty

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ATM Transmission
Can support arbitrary bit rates
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Asynchronous Transfer Mode

• No global timing relationship between slots
  (i.e., cells) of different users (asynchronous)
• Efficient for Variable Bit Rate traffic
• Implication the cell in each slot has to be
  completely self-identifying (i.e., overhead)

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Advantages of ATM

• Better for bursty traffic (i.e., VBR)
• Statistical multiplexing gain
• Better network utilization
• Same mechanism works for all traffic types
• Simple and fast hardware switching

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Characteristics of ATM

• Point to point technology
• Connection-oriented an end-to-end connection
  (called a virtual channel) must be set up using a
  signalling protocol before any data cells can be
  sent on that VC
• Bandwidth on demand
• Statistical multiplexing
• Integrated services

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ATM Cell

• 53 bytes
• 5 byte header
• 48 byte payload (data)
• Virtual Path Identifier (VPI)
• Virtual Channel Identifier (VCI)
• Simple control fields

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ATM Cell Format
GFC
VPI
VPI
VCI
VCI
PT
CLP
VCI
RES
HEC
Payload
(48 bytes)
ATM UNI Cell Specification
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Why ATM will win

• ATM is a scalable technology
• scalable in bandwidth
• scalable in distance
• scalable in deployment

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ATM Scalable Bandwidth

• ATM is not tied to any particular bit rate or
  physical layer network technology
• ATM is simply the abstract concept of fast packet
  switching with small fixed size cells
• Can do low speed ATM (e.g., 1.5 Mbps)
• Can do high speed ATM (e.g., 155 Mbps)
• Primary interest high speed ATM networks

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ATM Scalable Distance

• ATM can be used for LANs
• ATM can be used for MANs
• ATM can be used for WANs
• Initial market ATM LANs, enterprise area
  networks, LAN backbones
• Future wide area network backbone, ATM to the
  desktop, wireless ATM

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ATM Scalable Deployment

• Emphasis on interoperability, compatibility
• Incremental evolutionary path to ATM
• Ethernet gt switched Ethernet gt ATM hub
• Start with one switch, N ports, plus NICs
• Add more ports as needed
• Add more switches as needed
• Hierarchical cascading structure

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Why ATM will win

• ATM is a scalable technology
• scalable in bandwidth
• scalable in distance
• scalable in deployment
• Global standards (ATM Forum, ITU)
• Billions of dollars invested