Quality of Service

As part of the negotiated connection, ATM endpoints establish a service contract that guarantees a specific quality of service. These Quality of Service (QoS) guarantees are not offered by traditional LAN technologies.

With a traditional LAN, any notion of service guarantee is based on priority, where one transmission receives delivery preference over others. Because the sending station does not know the condition of the network or the data recipient prior to transmission (traditional LANs are connectionless), traffic is subject to delay at routers and elsewhere. These unforeseen delays make bandwidth availability and delivery times difficult to predict. While higher-priority traffic generally reaches its destination prior to lower-priority traffic, it is possible that the higher priority traffic arrives too late for isochronous traffic.

Note

The term "QoS" applies to several forms of quality guarantees, including ATM QoS, RSVP, and Generic QoS. Of the three, only ATM QoS is implemented at the hardware level.

For more information about other types of QoS, see "Quality of Service" in the TCP/IP Core Networking Guide .

ATM offers granular, explicit service guarantees that are not based on a relative structure (such as priority). With ATM, a data supplier can request a specific bandwidth, maximum delay, delay variation tolerance, and so forth. Each ATM switch then determines whether or not it can meet the request after taking current allocations into consideration. If it can accommodate the transmission, it guarantees the service level and allocates the necessary resources. With ATM, the service contract is enforced and the bandwidth is allocated at the hardware level; all of the switches between the sender and receiver know and agree to the service level before the contract is granted. The source station hardware, also having agreed to the contract, is responsible for shaping the traffic to fit the connection contract before it enters the network.

ATM offers the following five service categories:

• Constant Bit Rate (CBR)
  Specifies a fixed bit rate. Data is sent in a steady stream with low cell loss. This is an expensive service because the granted bandwidth must be allocated, regardless of whether or not it is actually used. CBR is typically used for circuit emulation. This category is supported in Windows 2000.

• Variable Bit Rate (VBR)
  Specifies a throughput capacity over time, but data is not sent at a constant rate. This also specifies low cell loss. It is available in two varieties, real-time VBR for isochronous applications and non-real-time VBR for all others.

• Available Bit Rate (ABR)
  Ensures a guaranteed minimum capacity but allows data to be sent at higher capacities when the network is free. ABR adjusts the rate of transmission based on feedback. This specifies low cell loss. ABR provides better throughput than VBR, but is less expensive than CBR. It is important to note that ABR has only recently been fully defined and not all hardware and software support this service category. It is part of the UNI 4.0 specification.

• Unspecified Bit Rate (UBR)
  Does not guarantee bandwidth or throughput; cells can be dropped. A UBR connection does not have a contract with the ATM network. This category is supported in Windows 2000.

• Weighted Unspecified Bit Rate (WUBR)
  The newest service category put forward by the ATM Forum, and it functions by assigning different processing priorities to different types of traffic, similar to a traditional connectionless LAN. Each such type of traffic is carried across a different connection; cells in connections with lower priority are dropped before those with higher priority.

Guaranteed QoS allows ATM to support time-sensitive (isochronous) applications, such as video and voice, as well as more conventional network traffic. While 100-megabit Ethernet and other high speed networks can provide comparable bandwidth, only ATM can provide the QoS guarantees required for real-time telephony, VCR-quality video streaming, CD-quality sound, smooth videoconferencing, and other delay-sensitive voice and video applications.

QoS is so vital to the industry that several initiatives are underway to provide QoS support for connectionless TCP/IP–based networks. While these solutions are useful, they require that all nodes on the network participate — which can be difficult to guarantee on heterogeneous networks. Because these solutions shape the traffic in software, latency and variations in delay are sometimes introduced. This is not the case with ATM.

Most importantly, the acceptance of ATM as a common standard for both LANs and WANs enables enterprise deployment of QoS applications and integrated services. The deployment of ATM/Asymmetric Data Subscriber Line (ADSL) to the home enables residential access to these services. ADSL uses existing copper twisted pair telephone lines to transmit broadband data to the home, without requiring recabling or a new telephone infrastructure. This extends the reach of ATM networks from the home desktop to the business desktop and everywhere in between.