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Title: ASYNCHRONOUS TRANSFER MODE


1
ASYNCHRONOUS TRANSFER MODE
  • BY
  • Er. Amit Mahajan

2
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.

3
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.

4
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.

5
  • 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
  •  

6
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

7
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
8
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
9
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.

10
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.

11
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).

12
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.
  •  

13
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

14
  • 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
15
  • 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.

16
  • 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.

17
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.

18
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.

19
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.

20
  • 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.

21
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

22
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.
23
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.

24
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.

25
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.
  •  

26
  • 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.

27
  • 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.
28
  • 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

29
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 ).

30
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.

31
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

32
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  •  

33
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