Motivations Future applications require: higher bandwidth, generate a heterogeneous mix of network traffic, low latency. supporting all classes of traffics (voice, video, data and etc). Promises to provide: Greater integration of capabilities and services, Increased and more flexible access to the network, More efficient and economical service.
Asynchronous Transfer Mode (ATM) A network technology for both LANs and WANs. supports real time voice and video as well as data. topology uses switches that establish a logical circuit: guarantees quality of service (QoS). widely used as a backbone technology in carrier networks and large enterprises, never became popular as in LANs.
Basics fixed-size packets (cells) 5 bytes of header, 48-byte information field (payload). a connection-oriented technology similar to the telephone networks. each connection: virtual circuit. Allow network to guarantee the quality of service (QoS) by limiting the number of VCs. works at layer 2 of the OSI model. typically uses SONET for framing and error correction over the wire.
ATM Protocol Reference Model based on standards developed by the ITU. three layers: the ATM adaptation layer (AAL), the ATM layer, and the physical layer.
ATM Adaption Layer (AAL) interfaces the higher layer protocols to the ATM Layer. Four service classes and corresponding types: Class A - Constant Bit Rate (CBR) service: ALL1, Class B - Variable Bit Rate (VBR) service: AAL2, Class C - Connection-oriented data service: AAL3/4 or AAL5, Class D - Connectionless data service: AAL3/4 or AAL5.
ATM Layer provides an interface between the AAL and the physical layer. Inside a switch: Determine where the cells should be forwarded to, resets the connection identifiers, forwards the cells to the next link, buffers incoming and outgoing cells, handles various traffic management functions such as cell loss priority marking, congestion indication, and generic flow control access.
ATM Layer(2) Inside an end system: receives data stream of cells form physical layer, transmit either cells with new data or empty cell. fields in the ATM header define the functionality of the ATM layer. use at the user-to-network interface (UNI), use internal to the network, the network-to-node interface (NNI).
ATM Layer (3) Header: generic flow control (GFC): four bits. Routing field: VCI+VPI. payload type indicator (PTI). cell loss priority (CLP) bit. header error check (HEC).
Physical Layer defines the bit timing and other characteristics for encoding and decoding the data into suitable electrical/optical waveforms. The Synchronous Optical Network (SONET) is often used for framing and synchronization at the physical layer. ATM Forum has proposed a variety of physical layer standards, such as ATM over twisted-pair wire: accelerate the acceptance of ATM.
LAN Emulation approaches have been proposed to support existing LAN applications: consider ATM as a new link layer. provision of an ATM protocol to emulate existing LAN services: provide a huge cost benefit, prevents higher layer applications from accessing ATM's unique services, bandwidth management, major difference: LANs are connectionless.
LAN Emulation Architecture Three components: LAN emulation client (LEC), LAN emulation server (LES), Broadcast/Unknown Server (BUS).
Traffic Management deliver guaranteed quality of service on demand, utilization of available network resources, every aspect of ATM network operation contains some traffic management mechanisms.
Quality of Service Attributes Peak Cell Rate (PCR): The maximum instantaneous rate at which the user will transmit. Sustained Cell Rate (SCR): This is the average rate as measured over a long time interval. Cell Loss Ratio (CLR): The percentage of cells that are lost in the network. Cell Transfer Delay (CTD): The delay experienced by a cell between network entry and exit Points.
Quality of Service Attributes (1) Cell Delay Variation (CDV): a measure of variance of CTD. Burst Tolerance (BT): This determines the maximum burst size that can be sent at the peak rate. Minimum Cell Rate (MCR): This is the minimum rate desired by a user.
Traffic Contract a traffic contract specifies one of the following classes of traffic: Constant Bit Rate (CBR) Variable Bit Rate (VBR) real-time VBR (VBR-RT). nonreal-time VBR (VBR-NRT). Available Bit Rate (ABR) Unspecified Bit Rate (UBR) ABR or UBR: best effort traffic.
Congestion Control Techniques Connection Admission Control (CAC), Generic Cell Rate Algorithm (GCRA), Feedback Congestion Control.
References K. Y. Siu and R. Jain, `A brief overview of ATM: Protocol layers, LAN emulation, and traffic management,` Computer Communication Review 25, 2 (April 1995), pp 6-20. R. J. Vetter, `ATM concepts, architectures, and protocols,` Communications of the ACM, 38(2):30- 38,109, February 1995. http://www.infosyssec.net/infosyssec/security/secatm 1.htm