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Towards ATM

Ram Dantu

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Overview Network Evolution Connection-oriented packet-switched

network Used in both WAN and LAN settings Signaling (connection setup) Protocol:

Q.2931 Specified by ATM Forum Packets are called cells: 5-byte header + 48-

byte payload Commonly transmitted over SONET (but not

necessarily)

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Network Evolution

From Circuit Switching to Packet Switching higher network utilization finer traffic control

From Packet Switching to Frame Relay from store-and-forward to end-to-end circuits Frame Relay uses Permanent Virtual Circuits

(PVC) no intermediate flow control or error recovery

From Frame Relay to Cell Switching Solves problems associated with the mix of

various packet sizes.

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ATM Hardware

The basic component of an ATM network is a special-purpose electronic switch designed to transfer data at extremely high speed.

A typical small switch can connect between 16 and 32 computers.

Each connection between a computer and an ATM switch uses a pair of optical fibers in order to allow high speed data transfer.

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Large ATM Networks Although a single ATM switch has finite capacity,

multiple switches can be interconnected to form a large network.

To connect computers at two sites to the same network, a switch can be installed at each site, and the two switches can then be connected.

User to Network Interface (UNI) is the interface between equipment at a customer’s site and the switching equipment owned by the common carrier; and Network to Network Interface (NNI) is the interface between switches owned and operated by two different phone companies.

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Logical View of an ATM Network

To a computer attached to an ATM network, an entire fabric of ATM switches appears to be a homogeneous network.

Despite a physical architecture that permits a switching fabric to contain multiple switches, ATM hardware provides attached computers with the appearance of a single, physical network.

Any computer on the ATM network can communicate directly with any other; the computers remain unaware of the physical network structure.

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The Two ATM Connection Paradigms

ATM provides a connection-oriented interface to attached hosts.

To reach a remote destination over an ATM network, a host must establish a connection, an abstraction that resembles a telephone call.

ATM offers two forms of connections: Switched Virtual Circuit (SVC); Permanent Virtual Circuit (PVC).

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Switch Virtual Circuit (SVC)

A host communicates with its local ATM switch to request that the switch establish an SVC by specifying the complete address of a remote host computer and the quality of service (QOS) required.

The ATM signaling system takes over and establishes a path from the originating host across the ATM network (possibly through multiple switches), to the remote host computer.

During signaling, each ATM switch along the path examines the QOS requested. If it agrees to forward data, a switch records information about the circuit and sends the request to next switch along the path.

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Such an agreement requires a commitment of hardware and software resources at each switch.

When signaling completes, the local ATM switch reports success to both ends of the switched virtual circuit.

The ATM interface uses a 24-bit integer to identify each virtual circuit.

When a host creates or accepts a new virtual circuit, the local ATM switch assigns an identifier to the circuit. A packet transmitted across an ATM network contains no address; instead, a host labels each outgoing packet and the switch labels each incoming packet with a circuit identifier.

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Permanent Virtual Circuit (PVC) An administrator interacts with switches in an ATM

network to configure virtual circuits by hand.

The administrator specifies the source and destination of the circuit, the QOS the circuit will receive, and the 24-bit identifiers each host uses to access the circuit.

PVCs are important because: without a standard signaling mechanism, two

vendors must use PVCs to interoperate. PVCs can be used on leased lines. PVCs can be used for network maintenance and

debugging.

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Paths, Circuits, and Identifiers

ATM assigns a unique identifier to each circuit a host has open; the host uses the identifier when performing I/O operations or when closing the circuit.

A circuit identifier is meaningful only across a single hop; the circuit identifiers obtained by hosts at the two ends of a given circuit usually differ; each ATM switch translates the circuit identifier in a packet as packet flows from one host to the other.

Connection Identifier (24-bit) = Virtual Path Identifier (VPI) (8-bit) + Virtual Circuit Identifier (VCI) (16-bit)

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If a set of virtual circuits follow the same path, an administrator can arrange for all circuits in the set to use the same VPI.

ATM hardware can use the VPI to route traffic efficiently.

Commercial carriers can use the VPI for accounting - a carrier can charge a customer for a virtual path, and then follow the customer to decide how to multiplex multiple virtual circuits over the path.

VPI Field (8-bit) VCI Field (16-bit)

24-bit Connection Id

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ATM Cell Transport At the lowest level, an ATM network uses fixed-size

frames called cells to carry data.

ATM requires all cells to be the same size because this makes it easy to build fast switching hardware.

Each ATM cell is 53 bytes long, and consists of 5-byte header followed by 48-byte of data.

Flow Control VPI (First 4 bits)VPI (Last 4 bits) VCI (First 4 bits)

VCI (Middle 8 bits)VCI (First 4 bits) Payload Type PRIO

Cyclic Redundancy Check (CRC)

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Cells

Variable versus Fixed-Length no optimal fixed-length

• if small: high header-to-data overhead • if large: low utilization for small messages

fixed-length are easier to switch in hardware

• simpler• enables parallelism

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Small size improves queue behavior finer-grained pre-emption point for scheduling

link• maximum packet = 4KB• link speed = 100Mbps• transmission time = 4096 x 8 / 100 = 327.68s• high priority packet may sit in the queue

327.68s• in contrast, 53 x 8 / 100 = 4.24s for ATM

near cut-through behavior• two 4KB packets arrive at same time• link idle for 327.68s while both arrive• at end of 327.68s, still have 8KB to transmit• in contrast, can transmit first cell after 4.24s• at end of 327.68s, just over 4KB left in queue

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Carrying Voice in Cells voice digitally encoded at 64Kbps (8-bit samples at

8KHz) need full cell's worth of samples before sending cell example: 1000-byte cells implies 125ms per cell (too

long) smaller latency implies no need for echo cancellation

Settled on compromise of 48 bytes: (32+64)/2

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Unlike most network frames that places control information in a header, AAL5 places control information in an 8 bytes trailer at the end of the packet.

Each AAL5 must be divided into cells for transport across an ATM network, and then must be recombined to form a packet before being delivered to the receiving host.

If the packet is an exact multiple of 48 bytes, the division will produce completely full cells; else, ATM will use padding by zeros, and then followed by the 8 bytes trailer.

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ATM Protocol Reference Model

ATM Layer cell routing, multiplexing-demultiplexing, generic

flow control

AAL Layer split frames, reassembly, handling lost cells, timing

recovery and interleaving of multiple connections.

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Classes of Service provided by ATM Applications of each class

Class A AAL/1 Circuit emulation and Constant Bit Rate Video and Audio

Class B AAL/2 Variable Bit Rate Video and Audio

Class C AAL3/4 Connection Oriented Data Transfer

Class D AAL 5 Connectionless Data Transfer

Corresponding to the OSI model, 4 service access points are also defined to access these 4 classes of services.

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ATM Adaptation Layers (AAL) Although ATM switches small cells at the lowest

level, application programs that transfer data over an ATM do not read or write cells.

Instead, a computer interacts with ATM through an ATM Adaptation Layer, which performs several functions, including detection and correction of errors.

When a virtual circuit is created, both ends of the circuit must agree on which adaptation will be used.

Only two interesting ATM Adaptation Layer protocol have been defined: AAL1 (for video transmission) and AAL5 (for data transmission).

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ATM Adaptation Layer 5 (AAL5) Computers uses AAL5 to send conventional data

packets across an ATM network. Although ATM uses small fixed-size cells at the

lowest level, AAL5 presents an interface that accepts and delivers large, variable-length packets.

Between 1 and 65535 bytes of data 8 bytes trailer

(a) Basic packet format that AAL5 accepts and delivers

8 bitUU

8 bitCPI

16 bitLength

32 bitFrame Checksum

(b) The fields in 8 bytes trailer that follows the data

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AAL5 Covergence, Segmentation and Reassembly

The sending AAL5 uses the low order bit of the payload type field of the ATM cell header to mark the final cell in a packet as the end-of-packet bit.

Covergence:The receiving AAL5 collects incoming cells until it finds one with the end-of-packet bit set.

A computer uses AAL5 to transfer a large block of data over ATM virtual circuit. On the sending host, AAL5 generates a trailer, divides the block of data into cells and sends each cell over the virtual circuit. On the receiving hosts, AAL5 reassembles the cells to send the original block of data to receiving host.

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Cell Format User-Network Interface (UNI)

GFC: Generic Flow Control (still being defined) VCI: Virtual Circuit Identifier VPI: Virtual Path Identifier Type: management, congestion control, AAL5 (later) CLP: Cell Loss Priority HEC: Header Error Check (CRC-8)

Network-Network Interface (NNI) switch-to-switch format GFC becomes part of VPI field

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Segmentation and Reassembly

ATM Adaptation Layer (AAL) AAL 1 and 2 designed for applications that

need guaranteed rate (e.g., voice, video) AAL 3/4 designed for packet data AAL 5 is an alternative standard for packet

data

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AAL 3/4

Convergence Sublayer Protocol Data Unit (CS-PDU)

CPI: common part indicator (version field) Btag/Etag: beginning and ending tag BAsize: hint on amount of buffer space to

allocate Length: size of whole PDU

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Cell Format

Type• BOM: beginning of message• COM: continuation of message• EOM: end of message

SEQ: sequence number MID: message id Length: number of bytes of PDU in this cell

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AAL5

CS-PDU Format

pad so trailer always falls at end of ATM cell Length: size of PDU (data only) CRC-32 (detects missing or misordered cells)

Cell Format end-of-PDU bit in Type field of ATM header