ASYNCHRONOUS TRANSFER MODE

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ASYNCHRONOUS TRANSFER MODE

• BY
• Er. Amit Mahajan

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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".
• High-speed transfer technology for voice, video,
  and data over public networks.

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ATM VS X.25

• As the speed and number of local area networks
  (LANs) continue their relentless growth,
  increasing demand is place on wide area
  packet-switching networks to support the
  tremendous throughput generated by these LANs.
• X.25 was designed to support direct connection
  of terminals and computers over long distances.
• X.25 packets may be of varying length,whereas ATM
  packets are of fixed size.
• X.25, with its substantial overhead, is being
  recognized as an inadequate tool for wide area
  networking.

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ATM VS FRAME RELAY

• ATM is normally utilized for high bandwidths of
  34 Mbps and upwards.
• At speeds of 2 Mbps and below, Frame Relay is
  more bandwidth efficient than ATM.
• ATM transmits only fixed-size frames, called
  cells, not variable-sized frames as frame relay
  do.
• There are two main drivers that caused
  businesses migrate from frame relay service to
  ATM
• 1. The need for greater capacity than frame
  relay can handle.
• 2. The need to support mixed-media traffic,
  especially voice and video.

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• Both frame relay and ATM take advantage of the
  reliability and fidelity of modern digital
  facilities to provide faster packet-switching
  than X.25.
• ATM is even more streamlined than frame relay in
  its functionality, and can support data rates
  several orders of magnitude greater than frame
  relay.
• Frame relay was developed as part of the work of
  ISDN
• ATM was developed as part of the work on
  broadband ISDN
•  

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ATM VS SONET

• The industry offers two solutions for achieving a
  large integrated network SONET/SDH and ATM .
• The SONET is a globally accepted, non-proprietary
  standard for broadband transmission through
  fiber-optic cables.
• It handles transmissions from 51 Mbps to 10 Gbps.
• SONET/SDH is a physical transport medium that
  occupies the two bottom layers of OSI model.
• ATM is a high-speed packet switching technique
  suitable for LAN, wide-area network and broadband
  ISDN.
• SONET is a physical structure, while ATM is a
  transmission protocol.
• If the ATM protocol is used, a transport medium
  is still needed to carry traffic over the network

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ARCHITECTUREATM Devices 

• An ATM network is made up of an ATM switch and
  ATM endpoints.
• ATM switch accepts the incoming cell from an ATM
  endpoint or another ATM switch. It then reads and
  updates the cell header information and quickly
  switches the cell to an output interface toward
  its destination.
• An ATM endpoint contains an ATM network
  interface adapter.
• Examples of ATM endpoints are workstations,
  routers, LAN switches, and video coder-decoders
  (CODECs).

ATM Network Comprises ATM
Switches and Endpoints
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ATM virtual connections

• A Transmission path (TP) is the physical
  connection (wire,cable,satellite,and so on)
  between an end point and a switch or between two
  switches
• A Virtual Path (VP) transports ATM cells
  belonging to virtual channels which share a
  common identifier, called the Virtual Path
  Identifier VPI. Connects two switches.
• A Virtual Channel (VC) provides the transport of
  ATM cells which have the same unique identifier,
  called the Virtual Channel Identifier (VCI).

ATM virtual connections
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ATM protocol reference model

• The Physical Layer
• This layer describes the physical transmission
  of information through an ATM network.
• The original design of ATM was based on SONET
  because high data rate of SONETs carrier , the
  boundaries of cells can be clearly defined .
• SONET specifies the use of pointer to define the
  beginning of a payload.
• Types of physical media specified for ATM include
  shielded and unshielded twisted-pair, coaxial
  cable, and fiber-optic cable, which provide cell
  transport capabilities ranging from a T1 rate of
  1.544Mbps to a SONET range of 622Mbps.

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The ATM Layer

• The ATM layer represents the physical interface
  between the ATM Adaptation layer (AAL) and the
  Physical layer. Thus, the ATM layer is
  responsible for relaying cells from the AAL to
  the Physical layer for transmission, and in the
  opposite direction from the Physical layer to the
  AAL for use in an endpoint.
• When transporting cells to the Physical layer,
  the ATM layer is responsible for generating the
  five-byte cell header for each cell. When
  receiving cells from the Physical layer, the ATM
  layer performs a reverse operation, extracting
  the five-byte header from each cell.

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ATM cell header format

• ATM transfer information in fixed-size units
  called cells.
• An ATM cell header can be one of two formats UNI
  or NNI. The UNI header is used for communication
  between ATM endpoints and ATM switches in private
  ATM networks. The NNI header is used for
  communication between ATM switches.
• Each cell consists of 53 octets, or bytes. The
  first 5 bytes contain cell-header information,
  and the remaining 48 contain the payload (user
  information).

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ATM Cell Header Fields

• Generic Flow Control (GFC)The 4-bit GFC field
  provides flow control at the UNI level.The ITU-T
  has determined that this level of flow control is
  not necessary at the NNI.
• Virtual Path Identifier (VPI)The VPI is an 8-bit
  field in a UNI cell a 12-bit field in an NNI
  cell.In conjunction with the VCI, identifies the
  next destination of a cell as it passes through a
  series of ATM switches on the way to its
  destination.
• Virtual Channel Identifier (VCI)VCI is 16-bit
  field in both.
• Payload Type (PT)Indicates in the first bit
  whether the cell contains user data or control
  data. If the cell contains user data, the bit is
  set to 0. If it contains control data, it is set
  to 1. The second bit indicates congestion (0 no
  congestion, 1 congestion), and the third bit
  indicates whether the cell is the last in a
  series of cells that represent a single AAL5
  frame (1 last cell for the frame).
• Cell Loss Priority (CLP)Indicates whether the
  cell should be discarded if it encounters extreme
  congestion as it moves through the network. If
  the CLP bit equals 1, the cell should be
  discarded in preference to cells with the CLP bit
  equal to 0.
• Header Error Control (HEC)Calculates checksum
  only on the first 4 bytes of the header. HEC can
  correct a single bit error in these bytes,
  thereby preserving the cell rather than
  discarding it.
•  

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The ATM Adaptation Layer

• Was developed to enable two ATM concepts.
• This layer is responsible for providing an
  interface between higher-layer protocols and the
  ATM layer.
• AAL maps the data stream originated by the
  higher-layer protocol into the 48-byte payload of
  ATM cells, with the header placement being
  assigned by the ATM layer.
• In the reverse direction, the AAL receives the
  payload of ATM cells in 48-byte increments from
  the ATM layer and maps those increments into the
  format recognized by the higher-layer protocol

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• Class A services are data streams with a constant
  bit rate, running over established connections.
• Class B services are similar, but instead of
  being locked to a regular data rate they send
  'peaks' of data at some times, and little or none
  at others. Examples include compressed video.
• Class C services are those carrying data
  messages on established connections. These are
  inherently variable bit-rate, as Class B.
  Examples include X.25 and Frame Relay.
• Class D services are the so called
  connectionless datagrams, where a packet of data
  is sent into the network and contains its own
  destination address. Examples include many
  traditional local-area networks such as
  Ethernet, wide area networks and the new
  switched multimegabit

Class Timing Relationship Bit Rate Type of Connection
A Yes Constant Connection-oriented
B Yes Variable Connection-oriented
C No Variable Connection-oriented
D No Variable Connectionless

The ATM Application Classes
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• In AAL1 cell payload the Sequence Number
  Protection (SNP) field protects the Sequence
  Number (SN) field from the effect of bit errors
  occurring during transmission, in effect
  providing a forward error detection and
  correction capability.
• AAL1 is designated for transporting constant bit
  rate (CBR) data, such as real-time voice and
  video traffic.
• First byte in the normal 48-byte cell payload is
  used for cell sequencing and protection of the
  sequence number, limiting the actual payload to
  47 bytes per AAL1-generated cell.

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• The AAL2 was intended to support a VBR .
• But now used for low-bit-rate traffic short
  frame traffic such as audio,video or fax .ex
  mobile telephony.
• AAL3 is designed to transport delay-insensitive
  user data, such as Frame Relay, X.25, or IP
  traffic.
• AAL3/4 uses four additional bytes beyond the cell
  header. The use of those bytes makes 44 bytes in
  the cell available for transporting the actual
  payload.
• In comparison, AAL5 uses all 48 bytes beyond the
  cell header to transport the payload, providing a
  minimum 10 enhanced throughput in comparison to
  AAL3/4.

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THE ATM CONNECTION-ESTABLISHMENT PROCESS

• ATM signalling uses the one-pass method of
  connection setup that is used in all modern
  telecommunication networks, such as the telephone
  network.
• First, a source end system sends a setup message,
  which is forwarded to the first ATM switch
  (ingress switch) in the network. This switch
  sends a call proceeding message and invokes an
  ATM routing protocol. The signaling request is
  propagated across the network. The exit switch
  (called the egress switch) that is attached to
  the destination end system receives the setup
  message. The egress switch forwards the setup
  message to the end system across its UNI, and the
  ATM end system sends a connect message if the
  connection is accepted. The connect message
  traverses back through the network along the same
  path to the source end system, which sends a
  connect acknowledge message back to the
  destination to acknowledge the connection. Data
  transfer can then begin.

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ATM LAN EMULATION

• LAN Emulation (LANE) is a standard defined by
  the ATM Forum that gives to stations attached via
  ATM the same capabilities that they normally
  obtain from legacy LANs.
• The LANE protocol defines mechanisms for
  emulating either an IEEE 802.3 Ethernet or an
  802.5 Token Ring LAN.

• The LANE protocols make an ATM network look and
  behave like an Ethernet or Token Ring
• LANE requires no modifications to higher-layer
  protocols to enable their operation over an ATM
  network.

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LANE Components 

• LAN Emulation client (LEC)- It is an entity in an
  end system that performs data forwarding, address
  resolution, and registration of MAC addresses
  with the LAN Emulation Server (LES). An ATM end
  system that connects to multiple ELANs has one
  LEC per ELAN. 
• LESThe LES provides a central control point for
  LECs to forward registration and control
  information.When a station receives a frame to be
  sent to another station using a physical address
  , LEC sends a special frame to the LES.
• The server creates a virtual circuit between the
  source the destination station.The source
  station can now use this virtual circuit( the
  corresponding identifier) to send the frame or
  frames to the destination.

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• Broadcast and Unknown Server (BUS)Multicasting
  Broadcasting require the use of another server
  called the broadcast/unknown server(BUS).If a
  station needs to send a frame to a group of
  stations or to every station,the frame first goes
  to the busthis server has permanent virtual
  connnection to every station.The server creates
  copies of the received frame sends a copy to a
  group of stations or to all stations,simulating a
  multicasting or broadcasting process.The server
  can also deliver a unicast frame by sending the
  frame to every station.In this case the
  destination address is unknown.This is sometimes
  more efficient then getting the connection
  identifier from the LES
• LAN Emulation Configuration Server (LECS)This is
  used for the initial connection between the
  client LANE.
• This server is always waiting to receive the
  initial contact.It has well known ATM address
  that is known to every client in the system.
• The LECS maintains a database of LECs and the
  ELANs to which they belong.

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ATM QUALITY OF SERVICE 

• Quality-of-service (QoS) that guarantees
  traffic contract, traffic shaping, and traffic
  policing is based on the service class , user
  related attributes, network-related attributes.
• Traffic contract specifies an envelope that
  describes the intended data flow. When an ATM end
  system connects to an ATM network, it enters a
  contract with the network, based on QoS
  parameters.
• Traffic shaping is the use of queues to constrain
  data bursts, limit peak data rate, and smooth
  jitters so that traffic will fit within the
  promised envelope.
• ATM switches can use traffic policing to enforce
  the contract. The switch can measure the actual
  traffic flow and compare it against the
  agreed-upon traffic envelope. If the switch finds
  that traffic is outside of the agreed-upon
  parameters, it can set the cell-loss priority
  (CLP) bit of the offending cells. Setting the CLP
  bit makes the cell discard eligible, which means
  that any switch handling the cell is allowed to
  drop the cell during periods of congestion for
  the multimedia applications and provide overall
  optimization of network resources

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Service Class Quality of Service Parameter
constant bit rate (CBR) It is designed for customers who need real time audio or video services. The cell rate is constant with time. CBR applications are quite sensitive to cell-delay variation. Examples of applications that can use CBR are telephone traffic , videoconferencing, and television.
variable bit ratenon-real time (VBRNRT) This class allows users to send traffic at a rate that varies with time depending on the availability of user information.It is designed for those users who do not need real time services but use compression techniques to create a variable bit rate. Multimedia e-mail is an example of VBRNRT.
variable bit ratereal time (VBRRT) This class is similar to VBRNRT but is designed for applications that are sensitive to cell-delay variation. It is designed for those users who need real time services use compression techniques to create a variable bit rate. Examples for real-time VBR is interactive compressed video.
available bit rate (ABR) This class of ATM services provides rate-based flow control and is aimed at data traffic such as file transfer and e-mail. Although the standard does not require the cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable for switches to minimize delay and loss as much as possible. Depending upon the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate.If more network capacity is available, this minimum rate can be exceeded.ABR is particularly suitable for applications that are bursty.
unspecified bit rate (UBR) This class is a best effort delivery service that does not guarantee anything and is widely used today for TCP/IP.
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User related attributes

• SCR The Sustained cell rate is the average cell
  rate over a long time interval.The actual cell
  rate may be lower or higher than this value, but
  the average should be equal to or less than the
  SCR.
• PCR The peak cell rate defines the senders
  maximum cell rate. The users cell rate can
  sometimes reach this peak,as long as the SCR is
  maintained.
• MCR The minimum cell rate defines the minimum
  cell rate acceptable to the sender.For example,if
  the MCR is 50,000, the network must guarantee
  that the sender can send atleast 50,000 cells per
  second.
• CVDT The cell variation delay tolerance is a
  measure of the variation in cell transmission
  times.For example,if the CVDT is 5 ns ,this means
  that the difference between the minimum the
  maximum delays in delivering the cells should not
  exceed 5 ns.

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Network related attributes

• The network related attributes are those that
  define characteristics of the network.The
  following are some network related attributes
• CLR The cell loss ratio defines the fraction of
  cells lost(or delivered so late that they are
  considered lost) during transmission.For example,
  if the sender sends 100 cells one of them is
  lost,the CLR is
• CLR
  1/100 10-2
• CTD The cell transfer delay is the average time
  needed for a cell to travel from source to
  destination. The maximum CTD the minimum CTD
  also considered attributes.
• CDV The cell delay variation is the difference
  between CTD maximum the CTD minimum.
• CER The cell error ratio defines the fraction
  of cells delivered in error.

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IP over ATM

• When IP works with ATM , the IP packets are
  segmented into fixed length cells of ATM,
  transmitted through the ATM network, then
  reassembled into IP packets at the receiving end.
• Each entry/exit point is a router. An ATM
  backbone can span an entire continent and may
  have tens or even hundreds of ATM switches.
• Most ATM backbones have a permanent virtual
  channel (VC) between each pair of entry/exit
  points.
•  

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• For n entry points, n(n - 1) permanent VCs are
  needed to directly connect n entry/exit points.
  Each router interface that connects to the ATM
  network will have two addresses. The router
  interface will have an IP address, as usual, and
  the router will have an ATM address, which is
  essentially a LAN address.
• Consider now an IP datagram that is to be moved
  across the ATM backbone To four IP routers, the
  backbone appears as a single logical linkATM
  interconnects these four routers just as Ethernet
  can be used to connect four routers.
• Let us refer to the router at which the datagram
  enters the ATM network as the entry router and
  the router at which the datagram leaves the
  network as the exit router.
• The entry router does the following
• 1. Examines the destination address of the
  datagram.
• 2. Indexes its routing table and determines
  the IP address of the exit router
• 3. To move the datagram to the next router,
  the physical address of the next-hop router must
  be determined.
• 4. IP in the entry router then passes down to
  the link layer (that is, ATM) the datagram along
  with the ATM address of the exit router.

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• ATM must now move the datagram to the ATM
  destination address. This task has two subtasks
• Determine the VCI for the VC that leads to the
  ATM destination address.
• Segment the datagram into cells at the sending
  side of the VC (that is, at the entry router),
  and reassemble the cells into the original
  datagram at the receiving side of the VC (that
  is, at the exit router).
• ATM uses AAL5 to provide a more efficient way to
  segment and reassemble a datagram. Recall that IP
  in the entry router passes the datagram down to
  ATM along with the ATM address of the exit
  router. ATM in the entry router indexes an ATM
  table to determine the VCI for the VC that leads
  to the ATM destination address. AAL5 then creates
  ATM cells out of the IP datagram
• The datagram is encapsulated in a CPCS-PDU using
  the format in fig.
• The CPCS-PDU is chopped up into 48-byte chunks.
  Each chunk is placed in the payload field of an
  ATM cell.
• All of the cells except for the last cell have
  the third bit of the PT field set to 0.

The AAL_indicate bit is used to reassemble IP
datagrams from ATM cells.
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• The last cell has the bit set to 1. AAL5 then
  passes the cells to the ATM layer. ATM sets the
  VCI and CLP fields and passes each cell to the TC
  sublayer. For each cell, the TC sublayer
  calculates the HEC and inserts it in the HEC
  field. The TC sublayer then inserts the bits of
  the cells into the PMD sublayer.
• The ATM network then moves each cell across the
  network to the ATM destination address. At each
  ATM switch between the ATM source and the ATM
  destination, the ATM cell is processed by the ATM
  physical and ATM layers, but not by the AAL
  layer. At each switch the VCI is typically
  translated and the HEC is recalculated.
• When the cells arrive at the ATM destination
  address, they are directed to an AAL buffer that
  has been put aside for the particular VC. The
  CPCS-PDU is reconstructed using the AAL_indicate
  bit to determine which cell is the last cell of
  the CPCS-PDU. Finally, the IP datagram is
  extracted out of the CPCSPDU and is passed up the
  protocol stack to the IP layer

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ATM Advantages

• Provides scalable bandwidth from a few megabits
  per second (Mbps) to many gigabits per second
  (Gbps). 
• Fixed-length cells enables low-cost hardware to
  be developed to perform required cell switching
  based on the contents of the cell header, without
  requiring more complex and costly software
• Fixed-size cells allow ATM to support
  quantifiable QoS
• Because of its asynchronous nature, ATM is more
  efficient than synchronous technologies, such as
  TDM.
• Simplified Network Management.
• ATM is a cell-switching and multiplexing
  technology that combines the benefits of circuit
  switching (guaranteed capacity and constant
  transmission delay) with those of packet
  switching (flexibility and efficiency ).

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ATM disadvantages

• Overhead of cell header (5 bytes per cell)
• Complex mechanisms for achieveing QoS
• Congestion may cause cell losses
• ATM handles data traffic smoothly, but runs into
  delay problems with voice transmissions.

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CONCLUSION

• ATM is a high-speed packet switching technique
  suitable for LAN, wide-area network and broadband
  ISDN (integrated services digital network)
  transmissions.
• The decision of when to use ATM and when to use
  frame relay largely depends on the applications
  businesses want to run over their enterprise
  networks, the amount of bandwidth they need and
  their performance requirements.
• ATM is ideally suited for converged voice,data
  and video networks because it assures quality of
  service.
• It also provides the high amounts of bandwidth
  that businesses are increasingly demanding for
  data and other applications.
• Frame relay, on the other hand, continues to be a
  highly economical and reliable choice, especially
  for medium-speed, data only applications.
• It provides scalable bandwidth from a few
  megabits per second (Mbps) to many gigabits per
  second (Gbps). ATM provides no retransmissions on
  a link-by-link basis.
• Combining the ATM SONET offers scalability and
  flexibility. While Asynchronous Transfer Mode
  (ATM) and Synchronous Optical Network (SONET)
  technologies are still emerging technologies, the
  combination of the two will drastically alter
  future corporate LAN design

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•  

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