ATM provides quality of service guarantees by establishing an end-to-end virtual connection in the network prior to transferring information. A contract must be negotiated between users and the ATM network layer to define the service between two or more nodes and specify connection parameters. The first part of the contract is the traffic descriptor, which characterizes the load to be offered, and the second specifies the QoS desired by the user and accepted by the network operator or carrier. To provide traffic contracts, ATM employs a number of QoS parameters, with a worst case performance specified for each, which the network operator must meet or exceed.
ATM allows applications to bypass the TCP/IP layer and talk directly to the hardware. Once a connection is made, connection-oriented media allow the status of the destination to be determined, and a virtual circuit to be established directly from one application to another. ATM takes large chunks of data, segments them into uniform cells for transmission, and reassembles them on the receiving end. Because ATM is connection-oriented, everything will arrive in sequence.
When a target application receives a MakeCall request, it examines the QoS to determine if it can meet the requested specifications—two megabits per second, for example. If the requested bandwidth is available through the network, the request will be granted, establishing a virtual circuit with a QoS guarantee which all of the intervening ATM hardware will honor. This guarantee eliminates the need for ATM to provide acknowledgment. In effect, ATM cards offer essentially the same set of services as a conventional transport driver does above Ethernet. Applications can talk directly to ATM, or to a protocol driver sending packets over ATM.
This virtual circuit capability can function, regardless of distance, over private ATM networks or public carrier ATM switches, providing ATM services with a global reach.
ATM gains speed and efficiency through the use of uniform cells. Each cell has a 5-byte header and a 48-byte payload. This fixed length eliminates the need to waste CPU cycles looking for the end of a packet. The header is just five bytes, so it doesn't have to provide size, delimiter, or similar information. This switching is so efficient that within a homogenous ATM environment there's no need for routers.
In contrast, in a routed environment, a large data packet might arrive just before a small voice segment. The large data packet may not be time-critical, but the system will process the large packet before it can get to that time-sensitive voice packet. This problem doesn't exist on an ATM network.
When a virtual connection is made, each packet is provided with a VC identifier (VCI), which is carried in the header and provided to a switch mapping table. The table handles the simple crossover process of mapping incoming VC cells from the incoming port to the assigned outgoing port, and then moving to the next incoming cell.
During transmission, the switch looks at the header information and at its mapping table, and it switches the VC to the appropriate port. The cells arrive at the destination ATM card, which also knows the VC number, reassembles the cells into the protocol data unit, and sends it out to whatever application or driver that VC number was linked to as the SAP during initial connection.
Because the VC path is used all the way through, there is nothing new about transferring data over a WAN which also has ATM switches. This is very different from using a router, as is done with conventional WANs.
Before ATM, the datacom and telecom worlds were separate and had different protocols. Whenever they met, such as at the router, significant and time-consuming translations were required. ATM brings the two worlds together. LAN ATM cards and switches connect to WAN ATM switches. This better enables voice to be done from a workstation, either through a separate physical line to a switch or using a phone as a peripheral to the PC, through an add-in card, to the switch and onto the network. With ATM switches from one end to the other, QoS can be absolutely guaranteed all the way through.
ATM provides great flexibility for applications to set quality-of-service requirements. Video conferencing might require a high QoS, while data may allow a lower setting. A very granular set of parameters exists by which to specify what QoS is required. An application can specify the amount of bandwidth, the maximum delay, and the maximum cell-loss rate. There are about twenty different parameters, all having to do with time guarantees. Some of the most important QoS parameters are listed below.
The maximum rate (in cells per second) at which the sender wants to send the cells.
The expected or required cell rate averaged over a long interval.
The minimum number of cells per second that the user considers acceptable. If the carrier cannot guarantee this much bandwidth, it must reject the connection.
How uniformly the ATM cells are delivered. ATM layer functions may alter the traffic characteristics by introducing cell delay variation. When cells from two or more ATM connections are multiplexed (MUX), cells of a given connection may be delayed when cells of another connection are being inserted at the output of the multiplexer. (Multiplexing at the transport layer refers to placing several transport connections onto one virtual circuit).
The amount of variation present in cell transmission times. CDVT is specified independently for peak cell rate and sustained cell rate. For a perfect source operating at PCR, every cell will appear exactly 1/PCR after the previous cell. However, for a real source operating at PCR, some variation in cell transmission time will occur. CDVT controls the amount of variation acceptable using a leaky bucket algorithm, described later in this section.
The fraction of the transmitted cells that are not delivered or are delivered so late as to be useless (for example, for real-time traffic).
The average transit time from source to destination.
The fraction of cells that are delivered with one or more bits wrong.
The fraction of N-cell blocks of which M or more cells contain an error.
The number of cells per second that are delivered to the wrong destination because of an undetected error in the cell header.
The QoS of an ATM connection refers to the cell loss, the delay, and the delay variation incurred by the cells belonging to that connection. QoS of an ATM connection is closely linked to the bandwidth it uses. For example, using more bandwidth increases the cell loss, the delay, and the delay variation incurred, therefore decreasing the QoS for cells of all connections that share those resources.