ATM payload structures command new Sonet network test requirements

Sept. 1, 1996

ATM payload structures command new Sonet network test requirements

william stumpf

hewlett-packard cerjac telecom operation

To check out the delivery of ATM payload services via an STS-12c format structure, novel test methods and analyses must be derived to verify the full-bandwidth performance of the Sonet OC-12 link

The growing presence of 599-Mbit/sec STS-12c payload structures has created a new set of Synchronous Optical Network (Sonet) testing requirements for telecommunications network and service providers. The STS-12c format is used exclusively for the delivery of

Asynchronous Transfer Mode (ATM) payload services and complements existing STS-3c and STS-1 structures at 149.76 and 51.84 Mbits/sec, respectively. By mapping ATM traffic directly to the STS-12c format, network and service providers can eliminate the extra step required to multiplex four STS-3c channels or 12 STS-1 channels for OC-12 transport at 622 Mbits/sec.

To efficiently test various OC-12 network equipment and service applications, however, network and service providers require test solutions that include methods for analyzing clear-channel and ATM formats, and must use several ATM cell structures and measurement techniques.

Two types of formats can be used to test an STS-12c network: clear-channel and structured. The clear-channel format fills the entire synchronous payload envelope with a test pattern to verify the full-bandwidth performance of the OC-12 link. This format is used primarily to perform facility-verification tests at the physical layer. For example, Sonet section-layer transmission elements, such as optical repeaters and the fiber-optic cabling itself, can be verified with the clear-channel format.

The structured STS-12c payload format is made up of back-to-back ATM cells, each 53 bytes long (see Fig.1). This format is used to perform ATM layer testing through network multiplexer and switching elements by initially specifying the ATM virtual channel address and bandwidth, and then making system performance and cell measurements.

ATM cell test formats

The most basic ATM cell format used for testing STS-12c networks is known as the ATM Adaptation Layer 0 (AAL0; see Fig. 2). This format fills the entire 48-byte information field of the ATM cell with a test pattern used to verify the transmission convergence sublayer of the Broadband Integrated Services Digital Network (Bisdn) protocol stack, which is responsible for mapping ATM cells into the transport stream. It extends across all the cells that contain the specified virtual path (VP) and virtual circuit (VC) channel assignments and facilitates the process of performing bit-error-rate (BER) testing on the ATM cell payload data.

The ATM channel under test is configured to a specific bandwidth allocation, up to the maximum STS-12c synchronous payload envelope capacity of 599 Mbits/sec; unused bandwidth is filled with idle cells. On the receive side, the incoming channel bandwidth is measured based upon the number of cells detected over the test interval; then, bit-error ratios and errored seconds are computed.

Another type of cell format used for testing ATM networks is called AAL1, which uses 47 bytes of the cell information field for a test pattern. The remaining, or 48th, byte is used, in part, for a 3-bit sequence number field that facilitates the detection of dropped cells within the STS-12c network. The sequence number field is incremented sequentially by the originating node. The receiver checks for missing or out-of-sequence cells and derives cell-loss measurements from this information. The sequence number is also accompanied by a sequence-number-protection field that contains a 3-bit cyclic-redundancy code and a 1-bit parity code to protect the number from error occurrences that would create false cell-loss interpretations.

When performing tests using the AAL1 format, advanced test instruments, such as the E4480A Cerjac 156MTS Sonet maintenance test set, report information regarding the sequence number, the cyclic-redundancy code and parity errors, and cell-loss measurements. The latter are important for testing service applications, such as local area network (LAN) file transfers, which require retransmission of the entire packet or file when a data loss occurs.

Another ATM format that can be used for testing is called the test cell, which provides a 4-byte time-stamp field and fills the remaining 44 information field bytes with a test pattern. The time-stamp information is used to perform cell-delay measurements through the network; these measurements check critical parameters for time-sensitive applications, such as voice and video services over ATM.

Using the test cell format requires that a single test instrument be used for both the cell generator and the cell receiver because the respective time-stamp circuits must be synchronized. Two types of delay measurements are performed: round-trip delay, which is important for voice-service applications, and cell inter-arrival time delay, which is critical for video-service applications.

Test applications for an OC-12 network carrying STS-12c services cover several methods of analysis, depending on the type of network equipment involved, available test access points and specific test objectives.

Some types of equipment do not require an ATM-structured STS-12c format to pass through the signals successfully; they require only that a valid concatenation indicator be present within the payload pointer bytes.

In these test applications, the clear-channel format can be used to simplify the test setup and perform full-bandwidth BER testing. This format would be used, for example, to test a pure Sonet digital crossconnect system that operates at the synchronous payload envelope level only and does not perform ATM cell processing.

Other types of equipment, such as a hybrid Sonet/ATM digital crossconnect or a broadband switching system, require the structured STS-12c/ATM format for testing because they perform switching functions based on the ATM virtual path and VC channel assignments. In these applications, any one of the three ATM cell formats can be used, depending on the type of service being tested.

In some applications, testing with all three types of cell formats is required, but on different VP/VC channels. Each channel might be carrying a different type of service, thereby dictating different test objectives, such as quantifying cell-loss performance instead of cell delay. In all three formats, however, bit-error measurements are performed on the cell-payload data.

Other factors in determining test methodology include network access points and the service-affecting nature of the test setup. For out-of-service testing, a Sonet test instrument can be used in terminal mode to source the entire STS-12c signal, thereby allowing full control over the transmitter setup.

Alternatively, the test instrument can be used in an add/drop mode, for example, when the OC-12 transport is being used to carry four STS-3c signals. In this situation, three of the four STS-3c signals are passed through the test set unchanged, while the fourth STS-3c signal is controlled as the channel under test.

As with the STS-12c payload application, the STS-3c signals can be tested in either the clear-channel or the ATM-structured format. For in-service testing, a nonobtrusive test access point must be available for monitoring the STS-12c traffic. To perform this test, a special monitor mode is used, and long-term measurements can be logged for later analysis.

In addition, a cell-scan function can be performed to identify the active VP/VC channels on the link and measure the bandwidth loading on each channel. In monitor mode, the transmitter is typically placed in full pass-through mode for out-of-service applications where STS-12c measurements are required, but the signal must be sent through the network unchanged (see Fig. 3).

Test setups, measurements and alarms abound for STS-12c and ATM applications (see table, page 60). At the STS-12c level, the test setup must specify whether the signal is clear-channel or ATM-structured. Specific STS-12c measurements are those that pertain to the concatenation aspects of the synchronous payload envelope, such as verification of the pointer values and the path-coding violation measurements.

At the ATM level, important setup parameters include the transmit bandwidth, distribution, ATM adaptation-layer type and header values. Also, a received VP/VC channel address must be specified, and operations and maintenance alarms may be generated and detected. u

William Stumpf is hardware engineer and scientist for Hewlett-Packard Cerjac Telecom Operation in Westford, MA.

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