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Asynchronous Transfer Mode(ATM)
Mathews VergisTEL 660
Winter trimester 2005 - 2006
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Topics to be discussed:
Need for network convergence
Introduction to ATM
ATM Interfaces and Service categories Basic ATM Concepts and Operation
ATM Cell Structure and Addressing
ATM Layers
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Popular misconceptions even among
Engineers
The phasor was invented by Captain Kirk of
Star Trek
Armature reaction is a chemical reaction
ATM stands for Automated Teller Machines
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Need for Network
Convergence PSTN sometimes used as a data network
backbone but since it is circuit switched
(voice optimized) not very WAN efficient
Delay sensitive traffic such as voice not
possible on data networks since there is no
guarantee of QoS
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Types of Traffic and demand on a
communication channelVoice
Its generation is asynchronous (a speaker may speakanytime)
Its transmission must be synchronous (once themessage starts, it must flow continuously as it isspoken)
The bandwidth required for a voice conversation indigital communication is relatively small and constant
(64K) The signals may contain a high degree of error and
the information can still be retrieved correctly
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Types of Traffic and demand on a
communication channel
Video
The generation is synchronous (continuous)
Its transmission is synchronous. Thebandwidth required is variable and it couldrange from under 64 Kbps to several Mbps inthe same session.
Error control should be tight - otherwise thewrong information on the monitor may triggersevere wrongful actions
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Types of Traffic and demand on a
communication channel
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)
The information is extremely error-sensitive, so
extreme caution must be exercised in transmissionand error control must be very tight.
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How can we combine voice , data
and video on the same link?
Fixed and relatively short packets
Delays associated with each packet are going
to be short and fixed predictable
transmission
If Voice and Video can be given priority
handling then mixing is possible without any
diminishing in quality
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Introduction
ATM Asynchronous Transfer Mode
It is a high speed, connection oriented
switching and multiplexing technology
capable of transmitting voice, video and data
and interconnecting LANs
ATM is asynchronous because information
streams can be sent independently without the
need of a common clock
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History of ATM
Developed in the early 80s as a switching technologyfor Broadband Integrated Services Digital Network
Anchorage Accord in 1996 declares availability of
specs required to implement a multi-service ATMnetwork
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Market Segments of ATM
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Features and Benefits of ATM
Convergence of Voice , Video and Data onone network
High speed switching at hardware level
Bandwidth on demand
Predefined and guaranteed QoS and CoS
Superior Management features
Scalability in network size and speed
Ease of integration with other technologies
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ATM Applications
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ATM Fast Packet Standards and
Services
Handles traffic through fast
packet switching
technique
Must be able to handle both
circuit and packet switching
Must also be able to
accommodate the different
bit rates variable (packet
switching) andconstant
(circuit switching)
Uses Cell relay technology
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Important terms relevant to ATM
Quality of Service (QoS) :A broadly used term that refers tothe performance attributes of an end-to-end connection. A QoSdefinition for data would address attributes such as error rates,lost packet rates, throughput, and delay
Class of Service (CoS) :It is a way of managing traffic in anetwork by grouping similar types of traffic together andtreating each type as a class with its own level of service
priority
Fast Packet Switching :A packet switching technique thatincreases the throughput by eliminating overhead. Overhead
reduction is accomplished by allocating flow control and errorcorrection functions to either the user applications or thenetwork nodes that interface with the user. Cell relay is animplementation of this.
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Cells and Cell relay
A Cell is a formatted packet that uses a fixed lengthdata unit
Cell relay is the process of moving these cellsthrough switching elements
Fixed size cells can be switched at a very high speedand add predictability to data transmissions
Variable length frames produce unpredictablepatterns and performances as the buffer time cannot
be determined Cell tax overhead imposed by ATM cells which can
cut into amount of data that can be transferred
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ATM Interfaces
ATM is a connectionoriented technique designedto transport both connectionand connection-less services
Operations at the boundaryof a network are connectionoriented
Within the network theoperation is connectionless
User to Network Interface (UNI) :Connection existing between theuser equipment and ATMequipment.
Network to Network Interface(NNI) : Connection via which
traffic travels between ATMdevices in the same network.
Intercarrier Interface (ICI) : Usedto send traffic across intermediatenetworks
Data Exchange Interface (DXI) :
Used to transmit packets ratherthan cells to the ATM interfacewhen non ATM equipment isused
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ATM Service Categories
Allow for traffic to be buffered and queued for later
transmission
Can permit loose timing and asynchronous operations between
sender and receiver
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Fundamental ATM Operations
Concept
A virtual or logical connection is established
ATM forms a packet of fixed length 53 octets ( 5octet header and 48 octet information field )
Cells are placed in a queue, on reaching ATM switch Cells are then multiplexed asynchronously with other
cells for transmission
Switch adapts the incoming bit rate to match the
transmit channel bit rate Switch inserts dummy cells to meet the aggregate bit
stream rate of 155.52 Mbps
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ATM Virtual paths and
ChannelsATM Virtual Circuit Terminology
Virtual Channel (VC) provides afixed pathway or route between 2points. Setup across an ATMnetwork whenever data transferbegins.
Virtual Path (VP) groups of VCsused to tell a switch how to forwardan ATM cell through an ATMnetwork
Virtual Path Identifier (VPI) inthe ATM header used to identifyroute established in the ATM Switch
Virtual Channel Identifier (VCI) -in the ATM header used to identify achannel within a VP
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ATM Virtual paths (contd)
Transmission path physicalmedia transporting the cells
Virtual Channel Connection(VCC) connection from sourceend user VCI to destination userVCI
Virtual path Connection (VPC) connection from source end userVPI to the destination end userVPI
Operation : Cell is receivedacross a link on a known VPI or
VCI value - their values areremapped as necessary as allVCIs and VPIs are onlysignificant to the local link
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ATM VCI and VPI Swapping
Logical ID Swapping : This is the technique by which the Logical IDof one link is changed to another one as the cell passes through it. ATMswitch changes the cell header VPI/VCI fields to reflect a new VPI andVCI for the outgoing cell. This can be done in one of 2 ways : VP is
predefined in the switch or VP is set up dynamically when cell reachesthe switch.
Forwarding process depends on 2 lookup tables within the switch: VP table record of VPs on each link
VC table maintain the output VP and VC to send the cell Connection Admission Control (CAC) - procedure used to decide if a
request for an ATM connection can be accepted based on the attributesof both the requested connection and the existing connections
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VC and VP Swapping
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ATM Cell Structure
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ATM Cell Structure
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ATM Addressing
Uses addressing similar tonumerical addressing for telephonenumbers
Uses E.164 addresses for publicATM (B- ISDN) networks
Extended ATM addressing to
include private networks Overlaymodel ATM layer maps networklayer (IP) addresses to ATMaddresses. Address format uses OSInetwork service access point(NSAP) addresses
NSAP address providing thelogical point between the networkand transport layers of the OSImodel the location of this point isidentified by network serviceprovider
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ATM Address formats
Currently 4 formats used for ATM networks
The fields are divided into 2 sections: network and user
Network prefixes : fields as needed by the network side of the
UNI User prefixes : Fields as needed by the user side of the UNI
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ATM address formats (contd)
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ATM Routing Domains and Areas
ATM address 20 byte string that has the following fields:
Country code
Administrative authority
Routing domain Area identifier
End system identifier
NSAP
International code
ISDN telephone number
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ATM Routing Domains and Areas
Authority and Format Identifier (AFI): This identifier ispart of the network level address header. Value for 1 st AFI fieldcan be : DCC (hex 39),E.164 (hex 45), ICD (hex 47)
Routing domains : used for traffic management and
allocating bandwidth capacity. Defined in the lookup tables inthe switch.
End System Identifier (ESI) : identifies an end system( computer or LAN) within an area
Selector Field not used by the ATM network
Purpose of the ATM address format is identify ATM devicesin an ATM network
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ATM Routing Domains and Areas
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ATM Layers
ATM architecture uses a logical reference modelto describe its functions
ATM functions correspond to physical layer and
part of the Data Link layer of the OSI model On its own ATM has function at layers 1and 2 of
the OSI model, but today TCP/IP is routed overATM networks which means it can also function
at layers 3 and 4 of the OSI model.
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ATM reference model
The ATM reference model constructed as 3 planes
which span all the layers : Control generates and manages the signaling requests User manages the transfer of data
Management contains 2 parts: Layer management : manages layer specific functions
Plane management : manages and co ordinates functions related to
the whole system
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ATM and OSI model
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ATM Adaptation layer
Has 2 sublayers:
Convergence Sublayer ( CS )
Determines the Class of service (CoS) for the incoming traffic
Provides a specific AAL service at an AAL network serviceaccess point (NSAP)
Segmentation and Reassembly Sublayer (SAR)
Segments higher level user data into 48 byte cells plus
necessary overhead at the sending node and reassembles cells
at the receiving node
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AAL Types and Class of Service
(CoS)
Depending on data type, the AAL protocol provides 5 AAL
types to accommodate a particular service class
AAL 5 is the most popular AAL type
For IP, LAN frames , signaling messages, frame relay , video
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ATM Layer
Performs the framing , multiplexing /demultiplexing of cells and also does theswitching
Generates cell headers on transmitting node based on information from higher layers
Generic flow control
VCI /VPI translation Extracts cell headers on a receiving node and
passes cell payload to higher layers
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Physical Layer
Transports ATM cells on a communications channeland defines mechanical specifications ( connectorsetc.)
2 sublayers: Transmission Convergence sublayer
Maps cells into the physical layer frame format on transmit anddelineates ATM cells in the received bit stream
Generates HEC on transmit
Generates idle cells for cell rate decoupling or speed matching Physical medium sublayer
Medium dependent function like bit transfer, bit alignment
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References:
http://www.techfest.com/networking/atm/atm.htm
http://www.dit.upm.es/snh/arhelp/glossaries/atmf/gloss-a.html
http://www.rhyshaden.com/atm.htm
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm
Trivedi, Carol, Wide Area Networks; EMCParadigm 2004