ATM (asynchronous transfer mode)
- 1. ASSIGNMENT # 1 SUBMITTED TO: MISSSARASARWAR SUBMITTED BY: RABIAZAFAR 17581556-045 BSIT(5th semester) Section‘A’ Internetarchitecture&protocols Topic: Asynchronoustransfermode
- 2. Asynchronous transfer mode switching: Asynchronous transfer mode (ATM) is a switching technique used by telecommunication networks that uses asynchronous time-division multiplexing to encode data into small, fixed-sized cells. This is different from Ethernet or internet, which use variable packet sizes for data or frames. ATM as a backbone technology: ATM is the core protocol used over the synchronous optical network (SONET) backbone of the integrated digital services network (ISDN). ATM DEVICES:
- 3. An ATM network made up of an ATM switch and ATM end points. An ATM switchis responsible for cell transit through an ATM network. The job of an ATM switch is well defined. It 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 towards its destination. An ATM endpoint (or end system) contains an ATM network interface adapter. Examples of ATM endpoints are workstations, routers, digital service units (DSUs), LAN switches, and video coder-decoders. ATM cell header format (UNI, NNI FRAME FORMATE): Asynchronous transfer mode was designed with cells in mind. This is because voice data is converted to packets and is forced to share a network with burst data (large packet data) passing through the same medium. So, no matter how small the voice packets are, they always encounter full-sized data packets, and could experience maximum queuing delays. This is why all data packets should be of the same size. The fixed cell structure of ATM means it can be easily switched by hardware without the delays introduced by routed frames and software switching. This is why some people believe that ATM is the key to the internet bandwidth problem. ATM creates fixed routes between two points before data transfer begins, which differs from TCP/IP, where data is divided into packets, each of which takes a different route to get to its destination. This makes it easier to bill data usage. However, an ATM network is less adaptable to a sudden network traffic surge. ATM services (SVC, PVC):
- 4. A network must establish a connection before two parties can send cells to each other. In ATM this is called a virtual circuit (VC). It can be a permanent virtual circuit (PVC), which is created administratively on the end points, or a switched virtual circuit (SVC), which is created as needed by the communicating parties. SVC creation is managed by signaling, in which the requesting party indicates the address of the receiving party, the type of service requested, and whatever traffic parameters may be applicable to the selected service. "Call admission" is then performed by the network to confirm that the requested resources are available and that a route exists for the connection. The ATM provides data link layer services that run on the OSI's Layer 1 physical links. It functions much like small-packet switched and circuit-switched networks, which makes it ideal for real-rime, low-latency data such as VoIP and video, as well as for high- throughput data traffic like file transfers. A virtual circuit or connection must be established before the two end points can actually exchange data. ATM services generally have four different bit rate choices: Available Bit Rate: Provides a guaranteed minimum capacity but data can be busted to higher capacities when network traffic is minimal. Constant Bit Rate: Specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line. Unspecified Bit Rate: Doesn’t guarantee any throughput level and is used for applications such as file transfers that can tolerate delays. Variable Bit Rate (VBR): Provides a specified throughput, but data is not sent evenly. This makes it a popular choice for voice and videoconferencing. Reference model: ATM specifies the following three layers: ATM adaptation layer (AAL) ATM layer 2, roughly corresponding to the OSI data link layer physical layer, equivalent to the OSI physical layer ATM network interfaces: User to Network Interface (UNI): Public UNI, Private UNI Network to Node Interface (NNI): Private NNI (P-NNI)
- 5. Public NNI = Inter-Switching System Interface (ISSI) Most ATM networks supporting SPVPs, SPVCs, and SVCs use the Private Network Node Interface or the Private Network-to-Network Interface (PNNI) protocol. PNNI uses the same shortest-path-first algorithm used by OSPF and IS-IS to route IP packets to share topology information between switches and select a route through a network. PNNI also includes a very powerful summarization mechanism to allow construction of very large networks, as well as a call admission control (CAC) algorithm which determines the availability of sufficient bandwidth on a proposed route through a network in order to satisfy the service requirements of a VC or VP. ATM virtual circuit: ATM operates as a channel-based transport layer, using virtual circuits (VCs). This is encompassed in the concept of the virtual paths (VP) and virtual channels. Every ATM cell has an 8- or 12-bit virtual path identifier (VPI) and 16-bit virtual channel identifier (VCI) pair defined in its header. The VCI, together with the VPI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. The length of the VPI varies according to whether the cell is sent on the user-network interface (on the edge of the network), or if it is sent on the network- network interface (inside the network). As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values (label swapping). Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others). ATM switches use the VPI/VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination. The function of the VCI is similar to that of the data link connection identifier (DLCI) in frame relay and the logical channel number and logical channel group number in X.25. ATM classes of services: Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, frame relay, n* 64 channels, IP). The VPI is useful for reducing the switching table of some virtual circuits which have common paths. Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch on the circuit is informed of the traffic class of the connection.
- 6. ATM traffic contracts form part of the mechanism by which "quality of service" (QOS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection. 1. CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant. 2. VBR - Variable bit rate: an average or Sustainable Cell Rate (SCR) is specified, which can peak at a certain level, a PCR, for a maximum interval before being problematic. 3. ABR - Available bit rate: a minimum guaranteed rate is specified. 4. UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity. Types of virtual circuits and paths: ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the circuit is composed of a series of segments, one for each pair of interfaces through which it passes. PVPs and PVCs, though conceptually simple, require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service "contract") and the two end points. ATM networks create and remove switched virtual circuits (SVCs) on demand when requested by an end piece of equipment. One application for SVCs is to carry individual telephone calls when a network of telephone switches are inter-connected using ATM. SVCs were also used in attempts to replace local area networks with ATM.