Carrying voice, video and data traffic on a single network—a vision born of the surge in telephone wide area networks (WANs), cable video networks and corporate computer networks—is now a reality with asynchronous transfer mode (ATM). But what problems can it solve, and how does it solve them? Where does it fit in the protocol stack? And how can we test ATM?
ATM provides the switching, signaling and control fabric necessary to move the combined voice, video and data traffic at maximum speed on that single network, the Broadband Integrated Services Digital Network (B-ISDN). The Synchronous Optical NETwork (SONET), which describes optical standards for data multiplexing and transmission at up to 2.4 Gb/s, and twisted-pair copper specifications up to 115 Mb/s carry the traffic.
A Fixed-Size Cell
Unlike frame relay, which has a variable length frame, ATM is based on a fixed-size cell (Figure 1). This simplifies control and switching. The ATM cell contains 53 bytes, 5 for the header and 48 for the payload.
The 48-byte payload is a compromise. Thirty-two bytes or less is needed to reduce payload waste for voice and similar isochronous (constant data rate) traffic, while 64 or more bytes lowers the header overhead penalty on large data transmissions. Any audio, video, graphics and data can be digitized, cut into equal segments, then put into ATM cells for transmission. At the far end, the information is reassembled.
Data Connections and Transmission
ATM is a connection-based technology, as opposed to an address-based technology. An address-based data link requires each cell to hold the complete sending and destination addresses, which would extract a heavy overhead penalty on a 53-byte cell, and would require significant decoding for routing at each node. The ATM cell header simply requires the connection between the two nodes to be identified on each link.
Connection information is carried as a combination Virtual Path Identifier/ Virtual Channel Identifier (VPI/VCI) field. You could think of the path as a bundle of wires within a cable physically connecting two devices; the channel could be a pair of wires within the bundle.
The User to Network Interface (UNI), which defines generic flow control (GFC), payload type and cell loss priority for a connection, has 24 bits allocated for the VPI/VCI, allowing more than 16 million combinations.
Each link, from a user to successive nodes to the far end user, is actually a separate virtual connection. The number of VPI/VCI combinations therefore only has to be sufficient to support the number of possible connections between two adjacent nodes.
Payloads and the AAL Concept
It’s not simple to snip the different kinds of traffic into 48-byte segments, ship it across the network, then reassemble it with no problems. For data, if you get it all at the other end in the correct sequence, it’s OK. Video and audio, transmitted real-time, are not so tolerant. Cell delay and delay variation become significant, so you have different Quality of Service (QoS) requirements.
To ship multimedia, frame relay or LAN data over an ATM network, you need the ATM Adaptation Layer (AAL). As the name implies, this layer adapts the service you’re working with to carry it over ATM. It takes your data, segments and inserts it into the 48-byte payload, and extracts and reassembles it at the other end. The adaptation process itself may use up to 4 bytes of the payload, leaving 44 bytes for the data (Figure 2).
Any service other than a cell relay service (which needs no adaptation) can be adapted through the AAL for transmission over ATM data links. Audio and video specifications are now evolving, and the ATM Forum started working on standards for voice in October 1995.
The Protocol Stack–Where ATM Fits In
The Open Systems Interconnection (OSI) standard seven-layer reference model is a handy tool to use in discussing what’s happening in a network. These layers can be subdivided into stacks of protocols which perform the multiple smaller functions required to connect two computers’ applications (Figure 3).
How you select operations, such as retrieving a record from your remote data-base application, and how the communications function is initiated are application-layer concerns.
The presentation layer handles format conversion, data compression and security functions. An Application Programming Interface (API) function used to support communications, for example, could be considered a presentation-layer program.
A session is an active connection between application programs on two different computers from call setup until termination. The session layer provides name recognition and address conversion, and handles call setup between the two computers. It is during call setup that the connection parameters are determined. For ATM these are:
Peak Cell Rate.
Sustainable Cell Rate.
Cell Delay Variation.
Maximum Burst Size.
Adaptation Layer Type.
Quality of Service.
The transport layer is the level designated to ensure an error-free data connection from end to end. Transmission control protocol/internet protocol (TCP/IP) is an example of a transport level protocol; Ethernet, a data link protocol, is also used as a transport mechanism.
The network services layer, Layer 3 in the OSI model, handles the internetwork addressing, routing, signaling and call setup through the network. Interim Local Management Interface (ILMI) and Simple Network Management Protocol (SNMP) management protocols also belong in Layer 3.
The ATM data link stack at OSI Layer 2 includes the adaptation and the cell multiplexing layers.
The physical layer upper stack is the convergence layer, which maps the cells onto the point-to-point transmission system. The physical transmission system includes the physical layer protocols and the electro/optical specifications.
ATM is primarily a data link service with network-level features and several physical-layer specifications.
Types of Testing
Application-level testing across the network must be performed for all media types, but results may be subjective. Objectively, three types of tests are needed: conformance, performance and interoperability.
Conformance Testing
An ATM product must conform to interface specifications at the network, data link and physical levels. Protocol Implementation Conformance Statements (PICS) proformas, available from the ATM Forum, Bellcore and various Regional Bell Operating Companies (RBOCs), are helpful. They exist for physical layer specifications, and the ATM Forum is working on PICS for signaling (UNI V.3.1), AAL5 and the ATM layer.
When filled in, these PICS make it easy to determine what is supported by the device and thus what needs to be tested. By the time you read this, the ATM Forum should have released conformance abstract test suites for the ATM layer for both intermediate and end systems.
In the development environment, it is important to test the physical layer over the operating extremes by varying clock frequencies, voltages, ambient temperature and simulated line transmission lengths to assure that you have a robust interface.
All signaling protocols supported should be thoroughly tested, as these set up the connection parameters for each virtual channel (VC). The ATM Forum is working on a test suite for the Service Specific Connection Oriented Protocol (SSCOP) sublayer of the Signaling ATM Adaptation Layer.
The SSCOP provides a simple, reliable data transfer of the Layer 3 signaling messages, or protocol data units (PDUs), which includes keeping the connection active and retransmitting PDUs not received (error recovery). The messages contain all of the parameters required to set up the call, including destination, AAL type and QoS parameter values.
Performance Testing
First, know how well the equipment works under ideal, low-load conditions; then crank up the overload.
Most of the current capacity of ATM networks is for data traffic, so start with that. Increase data through your system under test until data is discarded. You need to know if the data is being discarded intelligently, as this affects overload performance significantly. Are cells discarded at random? Are cells discarded from only one higher-level packet at a time? From one VC?
The ATM layer is not responsible for errors after call setup, and higher-level protocols assure that missing data is retransmitted.
Random cells discarded from multiple higher-level frames across multiple connections will lead to significant retransmission efforts, which aggravates congestion. For data, you want a low cell retransmission ratio (the number of cells retransmitted divided by the total minus the number retransmitted).
For constant bit rate (CBR) traffic such as voice and video, critical parameters are delay and delay variance or jitter. For variable bit rate (VBR) traffic such as multimedia, compressed video and bursty LAN data, delay variation is critical. Increase the rate-dependent traffic input to the system-under-test to verify that the CBR and VBR traffic timing is maintained at the QoS requirements.
For audio and video traffic where retransmission is not an issue, random cell discard is appropriate. For voice transmissions, loss of a few random cells would be heard as a few short clicks (under 6 ms for 64 kb/s voice), and small delay variances would be tolerable.
Long delays, even with everything else perfect, lead to significant problems such as both people starting to talk at once. Minimum total delay is what voice and audio need.
Video, especially if it is a movie video or TV broadcast, can withstand a large delay. A single cell loss would appear as loss of part of a line for uncompressed video, but with high compression a whole line or even frame sync could be lost. Interoperability Testing
Interoperability testing assures that your equipment works with that of other manufacturers. The tests often are performed by independent organizations which guarantee the confidentiality of your product. Bellcore and a number of universities have interoperability labs.
Conclusion
ATM, primarily a data link service, enables voice, video and data traffic to travel on a single network. Testing is a challenge because results may be subjective. Conformance, performance and interoperability tests are three objective test types that will determine whether ATM is achieving its promise.
About the Author
John Ramsay is Executive Director of the Suncoast Hi-Tech Council in Sarasota, where he also has an engineering consulting practice. He holds a B.Sc. (EE) honors degree from the University of Leeds, England. Mr. Ramsay has more than 25 years of design and management experience in embedded systems, including management of the ATM switch development program at Loral Data Systems. Prior to Loral, he was with Reflectone and GEC-Marconi Avionics. Suncoast Hi-Tech Council, P.O. Box 49946, Sarasota, FL 34230, (941) 379-8324.
Figure 2
AAL1
–Constant Bit Rate (CBR) Services. Used for timing critical applications with synchronous bit streams such as PCM encoded voice traffic and CBR video.AAL2
–Variable Bit Rate (VBR) Services. Used where timing is still important, but the bit rate may vary, such as for end-to-end signaling, compressed audio and video.AAL3/4
–VBR Services with additional data integrity and error checking. The AAL3 (Connection-Oriented) and AAL4 (Connection-Less) specifications were merged into a single standard. AAL3/4 is used to segment variable length data frames and packets into cells with additional data integrity and error-checking mechanisms. Used for short bursty transfers such as from LANs.AAL5
–Simple Efficient Adaptation Layer (SEAL). Simplified version of AAL3/4 used for transmitting bursty data like LAN traffic, and frame relay data.
Figure 3
Layer 7.
Application–Provides interface between the user’s application and the network services.Layer 6.
Presentation–Performs format conversion including compression. Also handles security.Layer 5.
Session–Manages end-to-end connections for application programs.Layer 4.
Transport–Ensures error free end-to-end delivery. Initiates retransmission of erroneous/lost data.Layer 3.
Network–Handles internetwork addressing, signal routing and congestion control.Layer 2.
Data Link–Performs local addressing and error detection, ATM cell multiplexing, AAL functions and cell routing.Layer 1.
Physical–Physical interfaces, bit transmission and conversion to electrical/optical signals. Includes packaging physical layer bits into frames and blocks (convergence).
Copyright 1996 Nelson Publishing Inc.
March 1996
|
|