ATM: DESCRIPTIONS M. RAZIF AZMAL B. M. OTHMANWET SHAH RIZAD B. ISMAIL WET RASHA AFZAL B. SHAFII WET AHMAD AFIFI B. MOHD ZAKI WET MOHD FAIRUZ B. MOHD YUSOF WET GROUP 10

What is Asynchronous Transfer Mode (ATM)?  Asynchronous Transfer Mode (ATM) is a connection oriented data transmission protocol based on groups of fixed length cells. These cells provide high speed transmission of a wide range of computer traffic, including voice, data, and video.  In addition, ATM technology allows you to set up a number of virtual connections to carry cells from different applications over the same physical connection.

The basic characteristics of ATM are: 1.Cell-based technology 2.Service transparent 3.Connection-oriented 4.User-to-network Interface (UNI) 5.Occupies one layer in network architecture

1. ATM is cell-based  ATM uses fixed length packets called cells. ATM cells are 53 bytes long. The first 5 bytes = header (contains the information necessary to deliver the cell to the correct destination). The remaining 48 bytes of the cell contain data = payload.  Fixed-length cells provide predictable switching patterns, because cell switching is less complex than switching packets of variable length.  Having all data in the same size cell format provides increased transmission speeds, by eliminating protocol recognition tasks and decoding.

2. ATM is service transparent  The cell format means that more than one service can be multiplexed over the same physical line.  Network growth and scalability are easily accomplished by significantly reducing the number of changes needed to add new service traffic types.

3. ATM is connection-oriented  ATM is a connection-oriented transport service that uses communications channels to transport of ATM cells between ATM end-systems sequentially.  An end-station uses the Signalling protocol to request a communication channel between itself and other end-stations. The resulting connection is labeled with a unique number. Each cell header contains the connection label rather than the full address of the destination device. ATM Switches switch the cells between devices based on the connection label.

4. ATM provides a User-to- Network Interface (UNI)  The User-to-Network Interface (UNI) is used to connect an ATM edge device to an ATM switch that is managed as part of the same network.  The UNI is managed by the Interim Local Management Interface (ILMI) protocol.  ATM also provides a Network-to-Network Interface (NNI) that is typically used to interconnect two ATM switches managed as part of the same network.

5. Occupies one layer in network architecture  ATM is only one layer within the network architecture Layered architecture of an ATM network

Physical Layer

 The physical layer is responsible for transmitting and receiving ATM cells over a physical transport medium. It is also responsible for checking the integrity of the bits being transferred over a physical media, and making sure that they are error-free.  The ATM Module is compliant with both SONET STS-3c and SDH-STM-1 physical layer standards.

SONET STS-3c & SDH STM-1 1.SONET STS-3c  Synchronous Optical Network (SONET) is the physical layer most often associated with ATM. SONET provides the mechanism for the transport of ATM cells through a framing structure. Using this medium, data can be transferred at Mbps. 2.SDH STM-1  SDH STM-1 is a physical layer similar to the SONET layer, but with some differences in frame fields. SDH STM-1 is the physical layer commonly used in Europe.

ATM Layer

The ATM Layer  The primary responsibility of the ATM Layer is managing the sending and receiving of cells between the user and the network.  The ATM Layer also manages the header for the ATM cell.  This section describes the cell structure, and how the ATM Layer uses the information stored in the cell header to perform each of its tasks.

ATM Cell Structure  The ATM cell has 48 bytes of payload (information to be carried) and 5 bytes of header information, making the cell 53 bytes in length.  The cell header contains the information used by the network to forward each cell to its destination. ATM cell structure

The ATM cell header The ATM cell header consists of the following fields: Generic Flow Control (GFC)  Provides local functions, such as flow control. The value encoded in the GFC is not carried end-to-end and can be overwritten by the ATM Switch. Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI)  The VPI/VCI value allows the network to associate a cell with a given connection, so that the cell can be switched to its destination.

Payload Type Indicator (PTI)  The PTI is used to indicate whether the cell contains user information or connection associated layer management information (F5 flow). The PTI is also used for network resource management or to carry information about network congestion. This field is used to mark the end of the frame for the AAL5 Adaptation Layer. Cell Loss Priority (CLP)  The purpose of the Cell Loss Priority (CLP) bit in the ATM cell is to indicate that cells with this bit set should be discarded before cells which do not have the CLP bit set. Cells can be discarded based on CLP condition and according to the network load. When the network overloads, a discard mechanism, based on the value of the CLP bit in the cell header, may come into operation. Header Error Checksum (HEC)  The HEC field is used for detecting bit errors in the cell header. It is also used for cell delineation, defining where the cell begins in a SONET frame.

ATM Layer Functionality  The ATM Layer accepts the cell payload from a higher layer, adds the cell header, and passes the resulting 53 byte cell to the physical layer. In addition, it also receives cells from the physical layer, strips off the cell header and passes the remaining 48 bytes to the higher layer protocols.

ATM Switching  ATM switching is performed at the ATM layer by defining connection-oriented communications channels for sequential transport of ATM cells between ATM end-systems. These communications channels are called Virtual Channels (VC). Many virtual channels can exist on a physical link.  Cells are switched through the network based on the cell header, and switching is performed independently for every cell. Each cell takes a virtual channel.  There are two types of communication channels: Virtual Path Connection (VPC) Virtual Channel Connection (VCC).

 A Virtual Path Connection (VPC) contains Virtual Channel Connections (VCC).  A Virtual Path Connection (VPC) is identified by a Virtual Path Identifier (VPI). A Virtual Channel (VC) is identified by a Virtual Channel Identifier (VCI).  The ATM Module only manages Virtual Channel Connections (VCC). Communication Channels

 The ATM switching mechanism delivers cells from their source to their destination using a Virtual Channel Connection (VCC). A VCC is defined as spanning end- to-end, whereas a Virtual Channel is only a section of the VCC.  On each physical link, each Virtual Channel Connection (VCC) is uniquely identified using the Virtual Path Identifier (VPI) and the Virtual Channel Identifier (VCI).  The VPI/VCI is only meaningful in the context of the user to switch, or switch to switch, interface. The combination of the VPI/VCI values allows the network to associate each cell with a given virtual channel, so that it can be switched to the right destination. Identical VPI/VCI combinations may exist on different interfaces within the network.

 Every ATM cell header contains the VPI and VCI values associated with the connection. It is the user or network that specifies the connection VPI/VCI identifiers.  ATM cells are switched through the ATM network. The end-to-end connection or VCC consists of a number of Virtual Channels, and each Virtual Channel has different VPI and VCI values. The VPI and VCI values are only unique to the local interface between two devices.

Switched Virtual Circuit (SVCs)  ATM connections can be established dynamically by the signalling protocol (Switched Virtual Circuits).  SVCs use the signalling protocol to dynamically define connections as they are needed and to release them when they are no longer needed. SVCs use signalling for: Connections initiated by the user/application. Connections established and dropped dynamically. Connections established via standard signalling protocol. Varied connection time. Connections not automatically re-established after network failure.

UNI 3.1 Signalling (Q2931)  ATM uses UNI 3.1 signalling (Q.2931) for dynamically establishing, maintaining and clearing ATM connections between devices. The major features of this type of signalling include: On demand connections. Point-to-point and point-to-multipoint connections. Multiple address formats. Dynamic VPI/VCI assignment. Traffic parameters exchange. Characteristics per call per application.  Each ATM device requires a number of control connections to be setup before it can communicate directly with the destination device.

ATM Adaptation Layer (AAL)

 It is the AAL layer that makes it possible for the ATM Layer to support multiple services and traffic types.  ATM transmits data in fixed length cells. Each cell contains 48 bytes of user data. It is the AAL's Segmentation and Reassembly (SAR) sub-layer that converts the user data into fixed length cells for transmission across the ATM network, and then converts it back into the appropriate user data again at the far end of the connection.  As ATM can carry multiple traffic types, several adaptation protocols, each operating simultaneously, exist at the adaptation layer.  The ATM Module uses the AAL5 adaptation protocol, which is the data-oriented adaptation layer. The ATM Module will only work with other AAL5 devices.

LAN Emulation Layer  The LAN Emulation Layer enables existing applications to access an ATM network as if they were operating over traditional LAN technology. Upper Protocol Layer  This layer relates to the application that is requesting a service. For example, this could be a file transfer application. Interim Local Management Interface (ILMI)  The ATM Forum produced the Interim Local Management Interface (ILMI) to increase monitoring and diagnostic facilities, and to provide ATM address registration at the User-to-Network Interface (UNI). Other Important Layers in ATM Layered Architecture

Summary  Asynchronous Transfer Mode (ATM) is a connection oriented data transmission protocol based on groups of fixed length cells.  The basic characteristics of ATM are: 1.Cell-based technology 2.Service transparent 3.Connection-oriented 4.User-to-network Interface (UNI) 5.Occupies one layer in network architecture

 3 important layers in Layered Architecture of an ATM network are: 1.Physical Layer: responsible for transmitting and receiving ATM cells over a physical transport medium. 2.ATM Layer: managing the sending and receiving of cells between the user and the network. 3.ATM Adaptation Layer (AAL): makes it possible for the ATM Layer to support multiple services and traffic types.