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ATM: What it is, and what it isn't. Carey Williamson. University of Calgary. ATM: What it is. Asynchronous Transfer Mode A low-layer networking technology based on fast packet-switching of small fixed size packets called cells

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atm what it is and what it isn t

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

Carey Williamson

University of Calgary

atm what it is
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
atm what it isn t
ATM: What it isn’t
  • 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)
t1 transmission
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

t1 transmission cont d
T1 Transmission (Cont’d)

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

synchronous transfer mode
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
asynchronous transfer mode
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
atm transmission

Slotted transmission scheme

All slots are the same size (53 bytes = 1 ATM cell)

Any user can use any empty slot

No notion of specific slots assigned to specific users

Bandwidth allocation in ATM may reserve a certain

percentage of the total slots for a given call, but which

slots you get is determined at time of transmission

ATM Transmission

Can support arbitrary bit rates

asynchronous transfer mode1
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)
advantages of atm
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
characteristics of atm
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
atm cell
ATM Cell
  • 53 bytes
  • 5 byte header
  • 48 byte payload (data)
  • Virtual Path Identifier (VPI)
  • Virtual Channel Identifier (VCI)
  • Simple control fields
slide13

ATM Cell Format

GFC

VPI

VPI

VCI

VCI

VCI

PT

RES

CLP

HEC

Payload

(48 bytes)

ATM UNI Cell Specification

why atm will win
Why ATM will win
  • ATM is a scalable technology
    • scalable in bandwidth
    • scalable in distance
    • scalable in deployment
atm scalable bandwidth
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
atm scalable distance
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
atm scalable deployment
ATM: Scalable Deployment
  • Emphasis on interoperability, compatibility
  • Incremental evolutionary path to ATM
  • Ethernet => switched Ethernet => ATM hub
  • Start with one switch, N ports, plus NICs
  • Add more ports as needed
  • Add more switches as needed
  • Hierarchical cascading structure
why atm will win1
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