Introduction to ATM
Asynchronous Transfer Mode - ATM, attempts to combine the best of both worlds -- the guaranteed delivery of circuit-switched networks and the robustness and efficiency of packet-switching networks. 1
What is ATM?
Not Automated Teller Machines! Asynchronous Transfer Mode Proposed Telecommunication Standard for Broadband ISDN
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CCITT Definition
A transfer mode in which the information is organised into cells; it is asynchronous in the sense that the recurrence of cells containing information from a particular user is not necessarily periodic (from CCITT Recommendation I.113, Section 2.2) periodic: appears at equal time intervals as in T1
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ATM vs. STM
If ATM stands for Asynchronous Transfer Mode Then there should also be STM or Synchronous Transfer Mode Why is STM bad? ATM is a networking technology so let us see what are networks
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Without Networks?
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What is a Network?
Networks provide for communication between devices without direct connections. Networks are indicated by clouds that hide the internal components and workings of the network. Networks provide for each device only needing a connection to the Network.
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Kinds of Networks
By Communication Technique
Switched Networks Broadcast Networks e.g. LANs
By Geographical Coverage
Wide Area Network
Cover large geographical areas, often crossing public right-of-ways Usually consist of several interconnected switching points Small scope, usually a building or cluster Typically owned by the same organization that owns the equipment 7
Local Area Network
A Switched Network
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Circuit-Switching
Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes Dominant in both voice and data communications today
e.g. PSTN is a circuit-switched network
Relatively inefficient (100% dedication even without 100% utilization)
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Circuit-Switching Stages
Circuit establishment Transfer of information
point-to-point from endpoints to node internal switching/multiplexing among nodes
Circuit disconnect
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Circuit Establishment
Station requests connection from node Node determines best route, sends message to next link Each subsequent node continues the establishment of a path Once nodes have established connection, test message is sent to determine if receiver is ready/able to accept message
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Information Transfer
Point-to-point transfer from source to node Internal switching and multiplexed transfer from node to node Point-to-point transfer from node to receiver Usually a full-duplex connection throughout
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Circuit Disconnect
When transfer is complete, one station initiates termination Signals must be propagated to all nodes used in transit in order to free up resources
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Circuit Switching
Circuit is established for the complete duration based on TDM Also referred to as STM (Synchronous Transfer Mode) Very inflexible, since once the duration of time slot is determined, the relative bit rate is fixed
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Circuit Switching Application
Circuit switching is well suited for analog voice communications as in the telephone network. Circuit switching turns out to be rather inefficient for data networks due to its resource allocation nature. Circuit Switching is ill-suited to data communication because data traffic is BAD
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Circuit Switching for Data
When Circuit Switching networks started to be used for data communications it became clear that:
Data traffic is bursty so most of the time allocated resources would be unutilized. There is asymmetry in the data rate required for each communicating party.
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Packet Switching
User information is encapsulated in packets Packets contain additional information used inside the network for routing, error correction, flow control, etc Designed in the sixties and seventies, when only poor to medium quality transmission links were available Packets have variable length and hence require complex buffer management inside the network 17
Packet Switching Operation
Data are transmitted in short packets. Typically an upper bound on packet size is 1000 octets. If a station has a longer message to send it breaks it up into a series of small packets. Each packet now contains part of the user's data and some control information. The control information should at least contain:
Destination Address Source Address
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Packet Switching Operation
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Packet Switching Operation
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Virtual Circuits and Datagrams
There are two approaches to packet switching network design:
Datagram Packet Switching Virtual Circuit Packet Switching
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Datagram Packet Switching
In datagram approach each packet is treated independently with no reference to packets that have gone before. No connection is set up. More processing time per packet per node Robust in the face of link or node failures.
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Virtual Circuit Packet Switching
In the Virtual Circuit approach a preplanned route is established before any packets are sent. There is a call set up before the exchange of data. All packets follow the same route and therefore arrive in sequence. More set up time Less routing or processing time Susceptible to data loss in the face of link or node failure 24
Packet Switching Datagram Approach
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Packet Switching Virtual Circuit Approach
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Effect of Packet Size
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Comparison with Circuit Switching
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ATM Network Concepts
OSI no longer a complete, or good, match to ATM layers Connections oriented
virtual paths and virtual circuits
ATM has general requirements for signaling, network performance, traffic control, OAMP
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Layered Architectures
X.25 packet-switched network Router-based networking Switching vs routing Frame relay network ATM network
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X.25 Packet-Switched Network
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Router-based Networking
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Switching vs Routing
Switching path set up at connection time simple table look up table maintainance via signaling no out of sequence delivery lost path may lose connection much faster than pure routing link decision made ahead of time, and resources allocated then Routing can work as connectionless complex routing algorithm table maintainance via protocol out of sequence delivery likely robust: no connections lost significant processing delay output link decision based on packet header contents - at every node
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Frame relaying
Eliminates as much as possible of the overhead of packet switched networks No hop-by-hop flow control and error control Can be viewed as streamlined version of X.25 Packet Switching
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Frame Relay Network
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ATM Network
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Switching Techniques Compared
Fixed Bit Rate
Simplicity Variable Bit Rate
Complexity
Circuit Switching
Asynchronous Transfer Mode
Frame Relaying
Packet Switching
ATM, attempts to combine the best of both worlds -- the guaranteed delivery of circuitswitched networks and the robustness and efficiency of packet-switching networks.
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What’s Wrong with Other Networking Technologies
Either for LAN or WAN Either variable messages sizes or fixed data rate Developed for one class of traffic i.e. either data or voice or video
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Why ATM?
Integrates Voice, Video and Data Uses short fixed length packets called cells Best effort delivery system Bandwidth on demand Connection Oriented technology - Every cell with the same source and destination travels over the same route Potential to remove performance bottlenecks in today's LANs and WANs
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Modern Networking Requirements
The requirements of modern networking involve:
Handling multiple types of traffic (voice, video, data), all with individual characteristics that make very different demands (sometimes downright opposed to each other) of the communications channel A fair and equitable way of charging for transport services, to provide the user with economically priced access, and the carrier with a profitable return on investment Reliability and flexibility of the communications links Ensuring accessibility to network capacity for both existing and future equipment and services with minimal disruption in existing operations 41
Voice
Its generation is asynchronous (a speaker may speak anytime) Its transmission must be synchronous (once the message starts, it must flow continuously as it is spoken) The bandwidth required for a voice conversation in digital communication is relatively small and constant (64K) The signals may contain a high degree of error and the information can still be retrieved correctly (after all, at each end there is an intelligent human being that can always ask "Huh, whaddya say?") 42
Video
The generation is synchronous (continuous) Its transmission is synchronous (you wouldn't like to see first a half a head, then a pair of feet, then the rest of the image of a person) The bandwidth required is variable and it could range from under 64 Kbps to several Mbps in the same session. (Humans require 25-30 images/second and sometimes, with a sudden change in scenery and a lot of excitement before the camera, there is a tremendous amount of information to be sent in an awfully short time; some other times only very small changes between consecutive screens need be transmitted) Error control should be tight - otherwise the wrong information on the monitor may trigger severe wrongful actions (security misinformation, wrong reaction of robots, etc.)
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Data
Its generation could be either asynchronous (text) or synchronous (telemetry) Its transmission in general can be asynchronous (data typically can wait patiently in buffers), so no special timing relationship between the transmitter and the receiver is required The amount of bandwidth varies enormously from a few bits per second to billions of bits per second The information is extremely error-sensitive, so extreme caution must be exercised in transmission and error control must be very tight.
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Traffic Demands on Network
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ATM vs Others
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ATM Cell Relay: The Underlying Technology
Cell Features Small Benefit Low latency to support real-time services like audio and video
Fixed Length
Fast hardware switching and scalability
Usable in all networks (LAN and
Standardized WAN)
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Virtual Circuit (VC) Concept
Circuit Switching
Path established prior to data transfer No buffering needed within switch
Packet Switching
Efficient use of switch capacity Buffering needed within switch Congestion may cause packet loss
Path established during connection set-up Cells flow along the same path VC numbers assigned on per link basis
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ATM Description
Knits local and wide-area networks and services into a seamless whole Billing possible on per-cell basis Scalable - Works at different speeds and on different media Open-ended growth path - Not locked to any physical medium or speed Example:
Cell size: 53 bytes Speed: 155.52 Mbits/s, 622.08 Mbits/s
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Advantages of ATM
Flexible bandwidth allocation Simple routing due to connection oriented technology High bandwidth utilisation due to statistical multiplexing i.e. ``Central Limit Theorem'' ensures peak deviates little from average (requires several active message streams). Deviation proportional to square root of number of streams Potential QOS (Quality Of Service) guarantees
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ATM Benefits - I
One Network-ATM will provide a single network for all traffic types-voice, data, video. ATM allows for the integration of networks improving efficiency and manageability. Enables new applications-Due to its high speed and the integration of traffic types, ATM will enable the creation and expansion of new applications such as multimedia to the desktop. Compatibility-Because ATM is not based on a specific type of physical transport, it is compatible with currently deployed physical networks. ATM can be transported over twisted pair, coax and fiber optics.
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ATM Benefits - II
Incremental Migration-Efforts within the standards organisations and the ATM Forum continue to assure that embedded networks will be able to gain the benefits of ATM incrementally-upgrading portions of the network based on new application requirements and business needs. Simplified Network Management-ATM is evolving into a standard technology for local, campus/backbone and public and private wide area services. This uniformity is intended to simplify network management by using the same technology for all levels of the network.
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ATM Benefits - III
Long Architectural Lifetime-The information systems and telecommunications industries are focusing and standardising on ATM. ATM has been designed from the onset to be scalable and flexible in:
Geographic distance Number of users Access and trunk bandwidths (As of today, the speeds range from Megabits to Gigabits)
This flexibility and scalability assures that ATM will be around for a long time
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Disadvantages of ATM
Overhead of cell header (5 bytes per cell) Complex mechanisms for achieving QoS Congestion may cause cell losses
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ATM Forum
The ATM Forum was started in October of 1991 by a consortium of four computer and telecommunication vendors. Since its inception, it has seen unprecedented growth, and today (as of June 1994), has over 500 members. Today's membership is made up of network equipment providers, semiconductor manufacturers, service providers, carriers and, most recently, end users. The Forum is not a Standards body. The ATM Forum is a consortium of companies that writes specifications to accelerate the definition of ATM technology. These specifications are then passed up to ITU-T (Formerly the CCITT) for approval. The ITU-T standard body fully recognizes the ATM Forum as a credible working group.
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ATM Networks
NNI = Network Network Interface UNI = User Network Interface
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UNI and NNI
ENDPOINT Higher Layers ENDPOINT Higher Layers
ATM Adaptation Layer (Data Link Layer) ATM Layer
UNI
NNI SWITCH
ATM Layer SWITCH ATM Layer
UNI
ATM Adaptation Layer (Data Link Layer) ATM Layer
ATM Physical Layer
ATM Physical Layer
ATM Physical Layer
ATM Physical Layer
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UNI and NNI Signaling
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ATM Switch Operation
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ATM Switching
Connections (routes) set up by software
Routing (path through multiple-switch network) and resource allocation is performed once per connection by switch control CPU
Cells are switched by hardware
Hardware (table lookup + switching fabric) switches each incoming cell to appropriate output port Once a connection is established, cells are not touched by software
ATM LANs grow by adding more switches
More aggregate bandwidth Negligible additional latency (10-50 microseconds per switch hop vs. 10000 microseconds per router hop)
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VP Switching
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Connection Setup through ATM Signaling (SVC)
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Applications of ATM
On the WAN for Public Carriers On the LAN
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