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ITU-T QoS | ATM Forum QoS | Service Class Category | Cell Delay | Cell Loss |
A | 1 | ABR | Very High (~150msec) | Very Low (10^-12) |
B | 2 | CBR | Very Low (~5msec) | Very Low (10^-12) |
C,D | 3,4 | VBR-nrt | High (~50msec) | Low (10^-6) |
B | 2 | VBR-rt | Low (~10msec) | Low (10^-6) |
C | 3 | UBR | Very High (~150msec) | Very High (10^-3) |
Table 6-5. Latency by Class of Service
Latency is extremely important to real-time applications such as video and audio. Too much latency, and the picture or audio begins to become distorted until the picture or sound is unrecognizable.
Port speeds are governed by the equipment the service provider has installed in the ATM backbone and the physical transmission medium speed accessible from the customer’s premises. If the service provider has done an adequate job of engineering its ATM network, the switch speed is much greater than the user’s access speed.
ATM UNI | Port Speed |
DS-0 | 0.064 Mbps |
DS-1 | 1.544 Mbps |
2xDS-1 | 3.0 Mbps |
3xDS-1 | 4.50 Mbps |
4xDS-1 | 6.0 Mbps |
5xDS-1 | 7.50 Mbps |
6xDS-1 | 9.0 Mbps |
7xDS-1 | 10.50 Mbps |
8xDS-1 | 12.0 Mbps |
DS-2 | 6.312 Mbps |
DS-3 | 44.736 Mbps |
OC-1 | 38 Mbps |
OC-3 | 155 Mbps |
OC-12 | 622 Mbps |
OC-48 | 2.4 Gbps |
OC-192 | 10 Gbps |
Table 6-6. Typical ATM port speeds
ATM traffic management and control is responsible for the orderly flow of data from source to destination. Traffic management and control issues include the type of connection established between the source and destination and traffic policing.
Communication between parties is established through a direct physical path that is expected to exist for the duration of the communication. Data is sent sequentially from source and arrives sequentially at destination. The PSTN, CATV, and Frame Relay networks are connection oriented.
Communication between parties does not have a direct physical path from source to destination. While data is sent sequentially from the source, the data can arrive in any order at the destination due to differing transmission paths the data elements may take. The Internet and LANs are considered connectionless oriented.
PVCs use switch connections to route data from source to destination. The cells contain the routing information in the header VCI and VPI fields necessary for the switch to direct the cells to the correct destination path. PVCs may exist for any time duration desired such as weeks, months, or years. The telecom industry refers to a PVC connection as a "nailed up" connection. A PVC does not require any call establishment procedures as everything necessary to get the data from source to destination exists in the 5-byte header.
SVCs use ATM signaling and ATM addressing to direct cells from source to destination. SVCs exist for the duration of the data transfer only, which may be milliseconds or seconds. SVCs require call establishment procedures including address screening and source address authentication. The source transmits a signaling request that is routed through the network, establishing the link from source to destination. The destination will accept or reject the source’s signaling request. If the destination accepts the source’s request to establish a path, each switch along the transmit path will update its VC translation table. After all the switches complete the VC translation, data is then allowed to transit the network.
Figure 6-13. Multicasting
Usage parameter control (UPC) is a high-tech way of saying "traffic cop." Usage parameter control is high-tech traffic policing of the ATM "superhighway." Usage parameter control detects and reports user contract violations and can prevent users from exceeding their contract specified traffic limitations. The intent is to ensure the network QoS is maintained. The ATM switch Message Information Base (MIB) stores individual user traffic usage parameters which are used by network control software to compare to contract parameters. The service provider has the options to ignore contract violations, notify the user of violations, and/or prevent user traffic from exceeding contract imposed limitations, thereby preventing violations from even occurring. The specific action the UPC performs after detecting a violation is to discard an illegal cell or tag as Discard Enable, meaning it will be discarded if the network becomes congested while the cell is in transit.
Figure 6-14. NNI-to-UNI/UNI-to-NNI interface
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