Method and apparatus for performing service level analysis of communications network performance metrics

• US 6,058,102 A
• Filed: 11/06/1998
• Issued: 05/02/2000
• Est. Priority Date: 11/07/1997
• Status: Expired due to Term

• Current Assignee: NetScout Systems Incorporated
• Original Assignee: Visual Networks Technologies, Inc. (Fortive Corp.)
• Inventors: Nisbet, Thomas R., Troutman, Robert C., Hasselkus, John E., Ennis, James D. Jr., Wheeler, David E. Jr., Rehbehn, Kenneth J., Drysdale, R. Scott
• Primary Examiner(s): VU, HUY DUY
• Application Number:
  US09/187,611
• Time in Patent Office: 543 Days
• Field of Search: 370/241, 370/242, 370/243, 370/244, 370/245, 370/248, 370/249, 370/250, 370/251, 370/252, 370/253, 340/825.06, 340/825.17
• US Class Current: 370/252
• CPC Class Codes: H04L 1/24 Testing correct operation
• CPC Class Codes: H04L 2012/5628 Testing
• CPC Class Codes: H04L 2012/5647 Cell loss
• CPC Class Codes: H04L 2012/5649 Cell delay or jitter
• CPC Class Codes: H04L 41/00 Arrangements for maintenance, administration or management of data switching networks, e.g. of packet switching networks
• CPC Class Codes: H04L 41/5009 Determining service level performance parameters or violations of service level contracts, e.g. violations of agreed response time or mean time between failures [MTBF]
• CPC Class Codes: H04L 41/5019 Ensuring fulfilment of SLA
• CPC Class Codes: H04L 43/062 related to network traffic
• CPC Class Codes: H04L 43/0811 by checking connectivity
• CPC Class Codes: H04L 43/0829 Packet loss
• CPC Class Codes: H04L 43/0864 Round trip delays
• CPC Class Codes: H04L 43/0888 Throughput
• CPC Class Codes: H04L 43/106 using time related information in packets, e.g. by adding timestamps
• CPC Class Codes: H04L 43/12 Network monitoring probes
• CPC Class Codes: H04L 43/55 Testing of service level quality, e.g. simulating service usage
• CPC Class Codes: H04Q 11/0478 Provisions for broadband connections

Abstract

A data transmission system includes probes connected between end user sites and a data switching network. Each probe is connected to the switching network via an access channel wherein transmission circuits establish paths between the sites through the access channel and switching network. The probes capture and retransmit data traveling between the sites over respective transmission circuits, and can thereby insert service level analysis (SLA) messages into the data traffic in order to actively communicate network performance information to other probes. For each transmission circuit, the probes periodically collect measurements related to one or more network performance metrics, including: round-trip delay (RTD), data delivery ratio (DDR) and network availability. During each SLA measurement cycle, a sequence of SLA messages is exchanged over each transmission circuit, which messages contain data used to determine RTD and DDR. Optionally, the SLA message contents and protocol are designed to allow the SLA messages to be encapsulated in a single, standard data unit of any conventional data transmission protocol, such as an ATM cell. Consequently, the messaging system can be used with any data transmission protocol and in interworked networks without modification of the message data payload. The SLA measurements collected by the probes can be transmitted to a console in communication with the probes for processing, display, and archiving.

Claims

1. An apparatus for acquiring data used to perform service level analysis of network performance metrics in a data transmission system comprising at least a first site and a second site and at least a first data transmission circuit establishing a communication path through a switching network between the first and second sites, the apparatus comprising:

at least a first data acquisition device and a second data acquisition device respectively associated with said first and second sites;

said first data acquisition device inserting a first inter-device message into data traffic bound for the second site that is offered by the first site to the switching network over the first data transmission circuit, the first inter-device message delimiting a set of the data traffic bound for the second site from subsequent data traffic;

said second data acquisition device receiving the first inter-device message with data traffic originating from the first site and delivered by the switching network to the second site over the first data transmission circuit;

said first data acquisition device determining a count OFF_A indicating an amount of data traffic offered by the first site in the set delimited by the first inter-device message;

said second data acquisition device determining a count DEL_B indicating an amount of data traffic, in the set delimited by the first inter-device message, that is delivered by the switching network to the second site over the first data transmission circuit.

2. The apparatus of claim 1, wherein said first data acquisition device transmits the count OFF_A to said second data acquisition device, and said second data acquisition device stores data delivery performance data based on the count OFF_A and the count DEL_B.

3. The apparatus of claim 1, wherein:

said second data acquisition device inserts a second inter-device message into data traffic bound for the first site that is offered by the second site to the switching network over the first data transmission circuit, the second inter-device message delimiting a set of the data traffic bound for the first site from subsequent data traffic;

said first data acquisition device receiving the second inter-device message with data traffic originating from the second site and delivered by the switching network to the first site over the first data transmission circuit;

said second data acquisition device determining a count OFF_B indicating an amount of data traffic offered by the second site in the set delimited by the second inter-device message; and

said first data acquisition device determining a count DEL_A indicating an amount of data traffic, in the set delimited by the second inter-device message, that is delivered by the switching network to the first site over the first data transmission circuit.

4. The apparatus of claim 3, wherein said second data acquisition device transmits the count OFF_B to said first data acquisition device, and said first data acquisition device stores data delivery performance data based on the count OFF_B and the count DEL_A.

5. The apparatus of claim 3, wherein:

said first data acquisition device transmits a plurality of first inter-device messages, such that successive first inter-device messages define first data collection intervals corresponding to successive sets of data traffic bound for the second site, said first data acquisition device determining the count OFF_A for each of the first data collection intervals, and said second data acquisition device determining the count DEL_B for each of the first data collection intervals;

said second data acquisition device transmits a plurality of second inter-device messages, such that successive second inter-device messages define second data collection intervals corresponding to successive sets of data traffic bound for the first site, said second data acquisition device determining the count OFF_B for each of the second data collection intervals, and the first data acquisition device determining the count DEL_A for each of the second data collection intervals.

6. The apparatus of claim 5, wherein:

for each of the first data collection intervals, at least one of said first and second data acquisition devices determines a count Δ

DEL_A indicating a number of protocol data units (PDUs) originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit during the first data collection interval, and a count Δ

OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit during the first data collection interval; and

for each of the second data collection intervals, at least one of said first and second data acquisition devices determines a count Δ

DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit during the second data collection interval, and a count Δ

OFF_A indicating a number of PDUs bound for the second site that are offered by the first site to the switching network over the first data transmission circuit during the second data collection interval.

7. The apparatus of claim 6, further comprising a console in communication with said first and second data acquisition devices, wherein at least one of said console, said first data acquisition device and said second data acquisition device determines data delivery performance of the first data transmission circuit as a function of the counts Δ

DEL_A, Δ

OFF_B, Δ

DEL_B and Δ

OFF_A.

8. The apparatus of claim 7, wherein at least one of said console, said first data acquisition device and second data acquisition device calculates a data delivery ratio of a number of PDUs delivered on the first data transmission circuit by the switching network to a number of PDUs offered to the switching network on the first transmission circuit as (Δ

DEL_A +Δ

DEL_B)/(Δ

OFF_A +Δ

OFF_B).

9. The apparatus of claim 7, wherein at least one of said console, said first data acquisition device and said second data acquisition device determines a round trip delay of data transmitted over the first data transmission circuit based on the transmit and receive times of the first and second inter-device messages.

10. The apparatus of claim 7, wherein at least one of said console, said first data acquisition device and said second data acquisition device determines an amount of time that the first data transmission circuit was available during a predetermined time period.

11. The apparatus of claim 6, wherein:

said first data acquisition device computes a one-way data delivery performance metric for the first data transmission circuit as a function of the counts Δ

DEL_A and Δ

OFF_B ; and

said second data acquisition device computes a one-way data delivery performance metric for the first data transmission circuit as a function of the counts Δ

DEL_B and Δ

OFF_A.

12. The apparatus of claim 11, wherein:

said first data acquisition device calculates a one-way data delivery ratio of a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit to a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit as Δ

DEL_A /Δ

OFF_B ; and

said second data acquisition device calculates a one-way data delivery ratio of a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit to a number of PDUs bound for the second site that are offered by the first site to the switching network over the first data transmission circuit as Δ

DEL_B /Δ

OFF_A.

13. The apparatus of claim 3, wherein said first and second data acquisition devices exchange a sequence of inter-device messages over the first transmission circuit, the sequence of inter-device messages including said first and second inter-device messages, wherein the count OFF_A is transmitted from said first data acquisition device to said second data acquisition device in a message in the sequence of inter-device messages, and the count OFF_B is transmitted from said second data acquisition device to said first data acquisition device in a message in the sequence of inter-device messages.

14. The apparatus of claim 13, wherein said first and second data acquisition devices form each message of the sequence of inter-device messages in accordance with an inter-device message structure, wherein each inter-device message includes:

a message type field identifying the inter-device message as a particular message within the sequence; and

a count field for transmitting a count indicating an amount of data traffic offered to the switching network for transmission over the first data transmission circuit.

15. The apparatus of claim 14, wherein said first and second data acquisition devices form each message of the sequence of inter-device messages in accordance with the inter-device message structure, wherein each inter-device message further includes:

at least one timestamp field for transmitting timestamp data relating to timing of a message in the sequence.

16. The apparatus of claim 15, wherein said at least one timestamp field includes:

a first timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was received; and

a second timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was transmitted.

17. The apparatus of claim 15, said first and second data acquisition devices form each message of the sequence of inter-device messages such that each message is capable of being encapsulated in a single ATM cell and in a single frame relay frame.

18. The apparatus of claim 3, wherein:

said first data acquisition device maintains the count OFF_A indicating a number of protocol data units (PDUs) bound for the second site that are offered by the first site to the switching network over the first data transmission circuit;

said first data acquisition device maintains the count DEL_A indicating a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit;

said second data acquisition device maintains the count OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit; and

said second data acquisition device maintains the count DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit.

19. The apparatus of claim 18, wherein:

the counts OFF_A, OFF_B, DEL_A and DEL_B maintained by said first and second data acquisition devices are running totals counted from a time of counter initialization or a time of counter rollover; and

said first and second data acquisition devices subtract counts from previous data collection intervals from the counts OFF_A, OFF_B, DEL_A and DEL_B to determine numbers of PDUs offered and delivered by said first and second data acquisition devices over the first data transmission circuit during a latest data collection interval.

20. The apparatus of claim 18, wherein the counts OFF_A, OFF_B, DEL_A and DEL_B maintained by said first and second data acquisition devices indicate numbers of PDUs offered and delivered by said first and second data acquisition devices during individual data collection intervals.

21. The apparatus of claim 3, wherein, said first and second data acquisition devices exchange a sequence of inter-device messages, such that:

said first data acquisition device transmits the first inter-device message to said second data acquisition device at a transmit time T₁, the first inter-device message being received at said second data acquisition device at time T₂ ;

said second data acquisition device determines the count DEL_B as of time T₂ ;

said second data acquisition device transmits the second inter-device message to said first data acquisition device at a transmit time T₃, the second inter-device message being received at said first data acquisition device at time T₄ ;

said first data acquisition device determines the count DEL_A as of time T₄ ;

said first data acquisition device transmits a third inter-device message to said second data acquisition device containing the count OFF_A as of time T₁ ; and

said second data acquisition device transmits a fourth inter-device message to said first data acquisition device containing the count OFF_B as of time T₃.

22. The apparatus of claim 21, wherein said first and second data acquisition devices exchange the sequence of inter-device messages during a periodic measurement cycle.

23. The apparatus of claim 22, wherein said first and second data acquisition devices exchange the sequence of inter-device messages containing data collected during a data collection interval terminating during the periodic measurement cycle.

24. The apparatus of claim 21, wherein:

the third inter-device message contains timestamp information indicating values of the transmit time T₁ of the first message and the receive time T₄ of the second message; and

said second data acquisition device calculates the round trip delay over the first transmission circuit using the values of times T₁, T₂, T₃ and T₄.

25. The apparatus of claim 24, wherein:

the second inter-device message contains timestamp information indicating a value of the receive time T₂ of the first message;

the fourth inter-device message contains timestamp information indicating a value of the transmit time T₃ of the second message; and

said first data acquisition device calculates the round trip delay over the first transmission circuit using the values of times T₁, T₂, T₃ and T₄.

26. The apparatus of claim 3, wherein:

the switching network is an interworked network employing plural, different data transmission protocols;

said first data acquisition device constructs the first inter-device message such that the first inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols; and

said second data acquisition device constructs the second inter-device message such that the second inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols.

27. The apparatus of claim 26, wherein:

the interworked network includes an asynchronous transfer mode (ATM) network employing an ATM data transmission protocol and a frame relay network employing a frame relay data transmission protocol;

the first site is a site on one of the ATM network and the frame relay network, and the second site is a site on the other of the ATM network and the frame relay network;

said first data acquisition device constructs the first inter-device message such that the first inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame; and

said second data acquisition device constructs the second inter-device message such that the second inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame.

28. An apparatus for measuring round-trip delay in a data transmission system comprising at least a first site and a second site and at least a first data transmission circuit establishing a communication path through a switching network between the first and second sites, the apparatus comprising:

at least a first data acquisition device and a second data acquisition device respectively associated with said first and second sites and being configured to exchange a sequence of inter-device messages, such that;

said first data acquisition device transmits a first message to said second data acquisition device at time T₁, the first message being received at said second data acquisition device at time T₂ ;

said second data acquisition device transmits at time T₃ a second message to said first data acquisition, the second message being received at said first data acquisition device at time T₄ ;

said first data acquisition device transmits to said second data acquisition device a third message containing timestamp information indicating values of the transmit time T₁ of the first message and receive time T₄ of the second message; and

said second data acquisition device calculates the round trip delay over the first transmission circuit using the values of times T₁, T₂, T₃ and T₄.

29. The apparatus of claim 28, wherein said second data acquisition device computes the round trip delay as:

(T₄ -T₃)+(T₂ -T₁).

30. The apparatus of claim 29, wherein:

said second data acquisition device transmits in the second message timestamp information indicating a value of the receive time T₂ of the first message;

said second data acquisition device transmits to said first data acquisition device a fourth message containing timestamp information indicating a value of the transmit time T₃ of the second message; and

said first data acquisition device calculates the round trip delay over the first transmission circuit using the values of times T₁, T₂, T₃ and T₄.

31. The apparatus of claim 30, wherein said first data acquisition device computes the round trip delay as:

(T₄ -T₃)+(T₂ -T₁).

32. A data transmission system capable of performing service level analysis of network performance metrics, comprising:

a switching network;

at least a first site and a second site;

at least a first data transmission circuit establishing a communication path through the switching network between the first and second sites;

at least a first data acquisition device and a second data acquisition device respectively associated with said first and second sites; and

a console in communication with at least one of said first and second data acquisition devices;

said first data acquisition device being configured to;

collect first performance data related to performance of said first transmission circuit;

insert a first inter-device message into data traffic bound for the second site that is offered by the first site to the switching network over the first data transmission circuit; and

use the first inter-device message to delimit a set of the data traffic for which the first performance data is collected;

said second data acquisition device being configured to;

receive the first inter-device message with data traffic originating from the first site and delivered by the switching network to the second site over the first data transmission circuit; and

collect second performance data related to performance of said first transmission circuit for the set of data traffic delimited by the first inter-device message;

at least one of said first data acquisition device, said second data acquisition device and said console generating service level analysis data of at least one network performance metric from the first and second performance data; and

said console providing as an output the service level analysis data.

33. The system of claim 32, wherein said first data acquisition device transmits the first performance data to said second data acquisition device, and said second data acquisition device stores historical performance data based on the first and second performance data.

34. The system of claim 32, wherein:

said second data acquisition device is configured to;

collect third performance data related to performance of said first transmission circuit;

insert a second inter-device message into data traffic bound for the first site that is offered by the second site to the switching network over the first data transmission circuit; and

use the second inter-device message to delimit a set of the data traffic on which the third performance data is collected;

said first data acquisition device is configured to;

receive the second inter-device message with data traffic originating from the second site and delivered by the switching network to the first site over the first data transmission circuit; and

collect fourth performance data related to performance of said first transmission circuit for the set of data traffic delimited by the second inter-device message;

at least one of said first data acquisition device, said second data acquisition device and said console generates service level analysis data of at least one network performance metric from the first, second, third and fourth performance data; and

said console provides as an output the service level analysis data.

35. The system of claim 34, wherein said second data acquisition device transmits the third performance data to said first data acquisition device, and said first data acquisition device stores historical performance data based on the third and fourth performance data.

36. The system of claim 34, wherein:

said first performance data collected by said first data acquisition device indicates an amount of data traffic offered by the first site in the set delimited by the first inter-device message;

said second performance data collected by said second data acquisition device indicates an amount of data traffic, in the set delimited by the first inter-device message, that is delivered by the switching network to the second site over the first data transmission circuit;

said third performance data collected by said second data acquisition device indicates an amount of data traffic offered by the second site in the set delimited by the second inter-device message; and

said fourth performance data collected by said first data acquisition device indicates an amount of data traffic, in the set delimited by the second inter-device message, that is delivered by the switching network to the first site over the first data transmission circuit.

37. The system of claim 34, wherein at least one of said first data acquisition device, said second data acquisition device and said console determines data delivery performance of the first data transmission circuit from the first, second, third and fourth performance data.

38. The system of claim 34, wherein:

said first data acquisition device determines as the first performance data a count OFF_A indicating a number of protocol data units (PDUs) bound for the second site that are offered by the first site to the switching network over the first data transmission circuit;

said first data acquisition device maintains as the fourth performance data a count DEL_A indicating a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit;

said second data acquisition device maintains as the third performance data a count OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit; and

said second data acquisition device maintains as the second performance data a count DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit.

39. The system of claim 38, wherein:

said first data acquisition device transmits a plurality of first inter-device messages, such that successive first inter-device messages define first data collection intervals corresponding to successive sets of data traffic bound for the second site, said first data acquisition device determining the count OFF_A for each of the first data collection intervals, and said second data acquisition device determining the count DEL_B for each of the first data collection intervals;

said second data acquisition device transmits a plurality of second inter-device messages, such that successive second inter-device messages define second data collection intervals corresponding to successive sets of data traffic bound for the first site, said second data acquisition device determining the count OFF_B for each of the second data collection intervals, and the first data acquisition device determining the count DEL_A for each of the second data collection intervals.

40. The system of claim 39, wherein:

for each of the first data collection intervals, at least one of said first data acquisition device, said second data acquisition device and said console determines a count Δ

DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit during the first data collection interval, and a count Δ

OFF_A indicating a number of PDUs bound for the second site that are offered by the first site to the switching network over the first data transmission circuit during the second data collection interval; and

for each of the second data collection intervals, at least one of said first data acquisition device, said second data acquisition device and said console determines a count Δ

DEL_A indicating a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit during the second data collection interval, and a count Δ

OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit during the second data collection interval.

41. The system of claim 40, wherein at least one of said console, said first data acquisition device and said second data acquisition device determines data delivery performance of the first data transmission circuit as a function of the counts Δ

DEL_A, Δ

OFF_B, Δ

DEL_B and Δ

OFF_A.

42. The system of claim 41, wherein at least one of said console, said first data acquisition device and second data acquisition device calculates a data delivery ratio of a number of PDUs delivered on the first data transmission circuit by the switching network to a number of PDUs offered to the switching network on the first transmission circuit as (Δ

DEL_A +Δ

DEL_B)/(Δ

OFF_A +Δ

OFF_B).

43. The system of claim 40, wherein:

the counts OFF_A, OFF_B, DEL_A and DEL_B maintained by said first and second data acquisition devices are running totals counted from a time of counter initialization or a time of counter rollover, and the counts Δ

OFF_A, Δ

OFF_B, Δ

DEL_A and Δ

DEL_B are determined by subtracting counts from previous data collection intervals from the counts OFF_A, OFF_B, DEL_A and DEL_B.

44. The system of claim 39, wherein said first and second data acquisition devices exchange a sequence of inter-device messages over the first transmission circuit, the sequence of inter-device messages including said first and second inter-device messages, wherein at least one of the first and third performance data is transmitted between said first and second data acquisition devices in a message in the sequence of inter-device messages.

45. The system of claim 44, wherein at least one of said console, said first data acquisition device and said second data acquisition device determines a round trip delay of data transmitted over the first data transmission circuit using timestamp data transmitted in the sequence of inter-device messages.

46. The system of claim 44, wherein at least one of said first data acquisition device, said second data acquisition device and said console generates performance data indicating an amount of time that said first data transmission circuit was available during a predetermined time period.

47. The system of claim 44, wherein, said first and second data acquisition devices exchange the sequence of inter-device messages, such that:

said first data acquisition device transmits the first inter-device message to said second data acquisition device at a transmit time T₁, the first inter-device message being received at said second data acquisition device at time T₂ ;

said second data acquisition device determines the count DEL_B as of time T₂ ;

said second data acquisition device transmits the second inter-device message to said first data acquisition device at a transmit time T₃, the second inter-device message being received at said first data acquisition device at time T₄ ;

said first data acquisition device determines the count DEL_A as of time T₄ ;

said first data acquisition device transmits a third inter-device message to said second data acquisition device containing the count OFF_A as of time T₁ ; and

said second data acquisition device transmits a fourth inter-device message to said first data acquisition device containing the count OFF_B as of time T₃.

48. The system of claim 47, wherein:

the second inter-device message contains timestamp information indicating a value of the receive time T₂ ;

the third inter-device message contains timestamp information indicating values of the transmit time T₁ of the first inter-device message and the receive time T₄ of the second inter-device message;

the fourth inter-device message contains timestamp information indicating a value of the transmit time T₃ of the second message; and

at least one of said first data acquisition device, said second data acquisition device and said console calculates the round trip delay as;

(T₄ -T₃)+(T₂ -T₁).

49. The system of claim 34, wherein:

said switching network is an interworked network employing plural, different data transmission protocols;

said first data acquisition device constructs the first inter-device message such that the first inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols; and

said second data acquisition device constructs the second inter-device message such that the second inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols.

50. The system of claim 49, wherein:

the interworked network includes an asynchronous transfer mode (ATM) network employing an ATM data transmission protocol and a frame relay network employing a frame relay data transmission protocol;

the first site is a site on one of the ATM network and the frame relay network, and the second site is a site on the other of the ATM network and the frame relay network;

said first data acquisition device constructs the first inter-device message such that the first inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame; and

said second data acquisition device constructs the second inter-device message such that the second inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame.

51. A method of performing service level analysis of network performance metrics in a data transmission system that includes at least first and second sites;

at least first and second data acquisition devices respectively associated with the first and second sites; and

at least a first data transmission circuit establishing a communication path through a switching network between the first and second sites, the method comprising the steps of;

(a) collecting, at the first data acquisition device, first performance data related to performance of the first transmission circuit;

(b) collecting, at the second data acquisition device, second performance data related to performance of the first transmission circuit;

(c) inserting a first inter-device message into data traffic bound for the second site that is offered by the first site to the switching network over the first data transmission circuit;

(d) using the first inter-device message to delimit a set of the data traffic for which the first and second performance data are collected; and

(e) generating service level analysis data of at least one network performance metric from the first and second performance data.

52. The method of claim 51, further comprising the steps of:

(f) transmitting to the second data acquisition device a value of the first performance data; and

(g) storing at the second data acquisition device historical performance data based on the values of the first and second performance data.

53. The method of claim 51, further comprising the steps of:

(f) collecting, at the second data acquisition device, third performance data related to performance of the first transmission circuit;

(g) collecting, at the first data acquisition device, fourth performance data related to performance of the first transmission circuit;

(h) inserting a second inter-device message into data traffic bound for the first site that is offered by the second site to the switching network over the first data transmission circuit; and

(i) using the second inter-device message to delimit a set of the data traffic for which the third and fourth performance data are collected;

wherein step (e) includes generating service level analysis data of at least one network performance metric from the first, second, third and fourth performance data.

54. The method of claim 53, further comprising the steps of:

(k) transmitting to the first data acquisition device a value of the third performance data; and

(l) storing at the first data acquisition device historical performance data based on the values of the third and fourth performance data.

55. The method of claim 53, wherein:

step (a) includes maintaining as the first performance data an indication of an amount of data traffic offered by the first site in the set delimited by the first inter-device message;

step (b) includes maintaining as the second performance data an indication of an amount of data traffic, in the set delimited by the first inter-device message, that is delivered by the switching network to the second site over the first data transmission circuit;

step (f) includes maintaining as the third performance data an indication of an amount of data traffic offered by the second site in the set delimited by the second inter-device message; and

step (g) includes maintaining as the fourth performance data an indication of an amount of data traffic, in the set delimited by the second inter-device message, that is delivered by the switching network to the first site over the first data transmission circuit.

56. The method of claim 53, wherein step (e) includes determining data delivery performance of the first data transmission circuit from the first, second, third and fourth performance data.

57. The method of claim 53, wherein:

step (a) includes maintaining as the first performance data a count OFF_A indicating a number of protocol data units (PDUs) bound for the second site that are offered by the first site to the switching network over the first data transmission circuit;

step (b) includes maintaining as the second performance data a count DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit;

step (f) includes maintaining as the third performance data a count OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit; and

step (g) includes maintaining as the fourth performance data a count DEL_A indicating a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit.

58. The method of claim 57, wherein:

step (c) includes inserting a plurality of first inter-device messages into data traffic originating from the first site and bound for the second site, such that successive first inter-device messages define first data collection intervals corresponding to successive sets of data traffic bound for the second site;

step (a) includes determining the count OFF_A for each of the first data collection intervals;

step (b) includes determining the count DEL_B for each of the first data collection intervals;

step (h) includes inserting a plurality of second inter-device messages into data traffic originating from the second site and bound for the first site, such that successive second inter-device messages define second data collection intervals corresponding to successive sets of data traffic bound for the first site;

step (f) includes determining the count OFF_B for each of the first data collection intervals; and

step (g) includes determining the count DEL_A for each of the first data collection intervals.

59. The method of claim 58, further comprising the steps of:

(j) determining, for each of the first data collection intervals, a count Δ

DEL_B indicating a number of PDUs originating from the first site that are delivered by the switching network to the second site over the first data transmission circuit during the first data collection interval, and a count Δ

OFF_A indicating a number of PDUs bound for the second site that are offered by the first site to the switching network over the first data transmission circuit during the second data collection interval; and

(k) determining, for each of the second data collection intervals, a count Δ

DEL_A indicating a number of PDUs originating from the second site that are delivered by the switching network to the first site over the first data transmission circuit during the second data collection interval, and a count Δ

OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first data transmission circuit during the second data collection interval.

60. The method of claim 59, wherein step (e) includes determining data delivery performance of the first data transmission circuit as a function of the counts Δ

DEL_A, Δ

OFF_B, Δ

DEL_B and Δ

OFF_A.

61. The method of claim 60, wherein step (e) includes calculating a data delivery ratio of a number of PDUs delivered on the first data transmission circuit by the switching network to a number of PDUs offered to the switching network on the first transmission circuit as (Δ

DEL_A +Δ

DEL_B)/(Δ

OFF_A +Δ

OFF_B).

62. The method of claim 59, wherein:

steps (a), (f), (g) and (b) respectively include determining the counts OFF_A, OFF_B, DEL_A and DEL_B as running totals counted from a time of counter initialization or a time of counter rollover; and

steps (j) and (k) include determining the counts Δ

OFF_A, Δ

OFF_B, Δ

DEL_A and Δ

DEL_B by subtracting counts from previous data collection intervals from the counts OFF_A, OFF_B, DEL_A and DEL_B.

63. The method of claim 58, wherein said first and second data acquisition devices exchange a sequence of inter-device messages over the first transmission circuit, the sequence of inter-device messages including said first and second inter-device messages, the method further comprising the step of:

(j) transmitting at least one of the first and third performance data between said first and second data acquisition devices in a message in the sequence of inter-device messages.

64. The method of claim 63, further comprising the step of:

(k) determining a round trip delay of data transmitted over the first data transmission circuit using timestamp data transmitted in the sequence of inter-device messages.

65. The method of claim 63, further comprising the step of:

(k) generating performance data indicating an amount of time that said first data transmission circuit was available during a predetermined time period.

66. The method of claim 63, wherein:

step (c) includes transmitting the first inter-device message to said second data acquisition device at a transmit time T₁, the first inter-device message being received at said second data acquisition device at time T₂ ;

step (a) includes determining the count OFF_A as of time T₁ ;

step (b) includes determining the count DEL_B as of time T₂ ;

step (h) includes transmitting the second inter-device message to said first data acquisition device at a transmit time T₃, the second inter-device message being received at said first data acquisition device at time T₄ ;

step (f) includes determining the count OFF_B as of time T₃ ;

step (g) includes determining the count DEL_A as of time T₄ ;

the method further comprising the steps of;

(k) transmitting a third inter-device message to said second data acquisition device containing the count OFF_A ; and

(l) transmitting a fourth inter-device message to said first data acquisition device containing the count OFF_B.

67. The method of claim 66, wherein:

step (h) includes transmitting timestamp information indicating a value of the receive time T₂ within the second inter-device message;

step (k) includes transmitting timestamp information indicating values of the transmit time T₁ of the first inter-device message and the receive time T₄ of the second inter-device message in the third inter-device message;

step (l) includes transmitting timestamp information indicating a value of the transmit time T₃ of the second message in the fourth inter-device message;

the method further comprising the step of;

(m) calculating the round trip delay as;

(T₄ -T₃)+(T₂ -T₁).

68. The method of claim 53, wherein the switching network is an interworked network employing plural, different data transmission protocols;

step (c) includes constructing the first inter-device message such that the first inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols; and

step (h) includes constructing the second inter-device message such that the second inter-device message is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols.

69. The method of claim 68, wherein:

the interworked network includes an asynchronous transfer mode (ATM) network employing an ATM data transmission protocol and a frame relay network employing a frame relay data transmission protocol, the first site is a site on one of the ATM network and the frame relay network, and the second site is a site on the other of the ATM network and the frame relay network, wherein

step (c) includes constructing the first inter-device message such that the first inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame; and

step (h) includes constructing the second inter-device message such that the second inter-device message is capable of being encapsulated in a single ATM cell and a single frame relay frame.

70. A method of measuring data delivery performance of a data transmission circuit forming a transmission path between a first site and a second site through a switching network over which the first and second sites communicate, the method comprising the steps of:

(a) maintaining at a first site end of the data transmission circuit a count OFF_A of a number of protocol data units (PDUs) bound for the second site that are offered by the first site to the switching network over the data transmission circuit;

(b) maintaining at the first site end a count DEL_A of a number of PDUs originating from the second site that are delivered by the switching network to the first site over the data transmission circuit;

(c) maintaining at a second site end of the data transmission circuit a count OFF_B of a number of PDUs bound for the first site that are offered by the second site to the switching network over the data transmission circuit;

(d) maintaining at the second site end a count DEL_B of a number of PDUs originating from the first site that are delivered by the switching network to the second site over the data transmission circuit; and

(e) computing a measure of PDU delivery performance of the data transmission circuit from the counts OFF_A, OFF_B, DEL_A and DEL_B.

71. The method of claim 70, wherein step (e) includes calculating a data delivery ratio as the ratio of the number of PDUs delivered by the switching network to the first and second sites over the data transmission circuit to the number of PDUs offered to the switching network by the first and second sites for transmission over the data transmission circuit.

72. The method of claim 70, further comprising the steps of:

(f) periodically transmitting the count OFF_A from the first site end to the second site end over the data transmission circuit;

(g) periodically transmitting the count OFF_B from the second site end to the first site end over the data transmission circuit;

(h) periodically storing at the first site end of the data transmission circuit a count Δ

OFF_B of a number of PDUs bound for the first site that were offered by the second site to the switching network over the data transmission circuit during a data collection interval, and a count Δ

DEL_A of a number of PDUs originating from the second site that were delivered by the switching network to the first site over the data transmission circuit during the data collection interval; and

(i) periodically storing at the second site end of the data transmission circuit a count Δ

OFF_A of a number of PDUs bound for the second site that were offered by the first site to the switching network over the data transmission circuit during a data collection interval and a count Δ

DEL_B of a number of PDUs originating from the first site that were delivered by the switching network to the second site over the data transmission circuit during the data collection interval;

wherein step (e) includes computing the measure of PDU delivery performance from the counts periodically stored in steps (h) and (i).

73. The method of claim 72, wherein step (e) includes calculating a data delivery ratio as the ratio of the number of PDUs delivered by the switching network to the first and second sites over the data transmission circuit to the number of PDUs offered to the switching network by the first and second sites for transmission over the data transmission circuit as (Δ

DEL_A +Δ

DEL_B)/(Δ

OFF_A +Δ

OFF_B).

74. A method of calculating a round trip delay of data transmitted over a data transmission circuit forming a path through a packetized switching network between first and second sites, comprising the steps of:

(a) transmitting a first message from a first data acquisition device associated with the first site to a second data acquisition device associated with the second site at time T₁, the first message being received at the second data acquisition device at time T₂ ;

(b) transmitting a second message from the second data acquisition device to the first data acquisition device at time T₃, the second message being received at the first data acquisition device at time T₄ ;

(c) transmitting a third message from the first data acquisition device to the second data acquisition device containing timestamp information indicating values of the transmit time T₁ of the first message and the receive time T₄ of the second message; and

(d) computing the round trip delay using the values of times T₁, T₂, T₃ and T₄.

75. The method of claim 74, wherein step (d) includes computing the round trip delay as:

(T₄ -T₃)+(T₂ -T₁).

76. The method of claim 74, wherein step (b) includes transmitting in the second message timestamp information containing a value of the receive time T₂ of the first message, the method further comprising the step of:

(e) transmitting from the second data acquisition device to the first data acquisition device a fourth message containing a value of the transmit time T₃ of the second message;

wherein step (d) is performed by both the first data acquisition device and the second data acquisition device.

77. The method of claim 76, wherein step (d) includes computing the round trip delay as:

(T₄ -T₃)+(T₂ -T₁).

78. A data transmission structure for transmitting a sequence of inter-device messages over a data transmission circuit forming a path through a switching network between first and second sites, the switching network employing at least one data transmission protocol, the structure comprising:

a message type field identifying the inter-device message as a particular message within the sequence; and

a count field for transmitting an indicator of a number of protocol data units (PDUs) offered for transmission to the switching network over the data transmission circuit, wherein, when said message type field contains a first value, said count field contains a count OFF_A indicating a number of PDUs bound for the second site that are offered by the first site to the switching network over the first transmission circuit, and when said message type field contains a second value, said count field contains a count OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first transmission circuit;

wherein each inter-device message conforming to the data transmission structure is capable of being encapsulated in a single protocol data unit of said at least one data transmission protocol.

79. The structure of claim 78, wherein the switching network is an interworked switching network employing plural, different data transmission protocols, wherein each inter-device message conforming to the data transmission structure is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols.

80. The structure of claim 79, wherein said plural data transmission protocols includes at least one of:

Asynchronous Transfer Mode (ATM), Frame Relay, High Level Data Link Control (HDLC), X.25, and tunneled protocols.

81. The structure of claim 79, wherein:

the interworked network includes an asynchronous transfer mode (ATM) network employing an ATM data transmission protocol and a frame relay network employing a frame relay data transmission protocol;

the first site is a site on one of the ATM network and the frame relay network, and the second site is a site on the other of the ATM network and the frame relay network; and

each inter-device message conforming to the data transmission structure is capable of being encapsulated in a single ATM cell and in a single frame relay frame.

82. The structure of claim 78, wherein:

the sequence of data measurement messages includes first and third messages transmitted over the data transmission circuit from the first site to the second site in accordance with the data transmission structure, and second and fourth messages transmitted over the data transmission circuit from the second site to the first site in accordance with the data transmission structure;

when said message type field identifies the inter-device message as the third message in the sequence, said count field contains the count OFF_A ; and

when said message type field identifies the inter-device message as the fourth message in the sequence, the count field contains the count OFF_B.

83. The structure of claim 78, further comprising:

a first timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was received; and

a second timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was transmitted.

84. The structure of claim 83, wherein:

the sequence of data measurement messages includes first and third messages transmitted over the data transmission circuit from the first site to the second site in accordance with the data transmission structure, and second and fourth messages transmitted over the data transmission circuit from the second site to the first site in accordance with the data transmission structure;

when said message type field identifies the inter-device message as the second message in the sequence, said first timestamp field indicates a receive time of the first message;

when said message type field identifies the inter-device message as the third message in the sequence, said first timestamp field indicates a receive time of the second message, said second timestamp field indicates a transmit time of the first message, and said count field contains the count OFF_A ; and

when said message type field identifies the inter-device message as the fourth message in the sequence, said second timestamp field indicates a transmit time of the second message, and the count field contains the count OFF_B.

85. A method of transmitting a sequence of inter-device messages over a data transmission circuit forming a path through a switching network between first and second sites, the switching network employing at least one data transmission protocol, the inter-device messages containing data relating to transmission performance of the data transmission circuit, the method comprising the steps of:

(a) forming each of the inter-device messages in the sequence in accordance with an inter-device message transmission structure, such that each of the inter-device messages is capable of being encapsulated in a single protocol data unit of said at least one data transmission protocol of the switching network, wherein each inter-device message includes;

a message type field identifying the inter-device message as a particular message within the sequence; and

a count field for transmitting an indicator of a number of protocol data units (PDUs) offered for transmission to the switching network over the data transmission circuit, wherein, when the message type field contains a first value, the count field contains a count OFF_A indicating a number of PDUs bound for the second site that are offered by the first site to the switching network over the first transmission circuit, and when the message type field contains a second value, the count field contains a count OFF_B indicating a number of PDUs bound for the first site that are offered by the second site to the switching network over the first transmission circuit;

(b) inserting each of the inter-device messages into data traffic to be transmitted on the data transmission circuit between the first and second sites; and

(c) extracting the inter-device messages from the data traffic transmitted on the data transmission circuit.

86. The method of claim 85, wherein the switching network is an interworked switching network employing plural, different data transmission protocols, and wherein step (a) includes forming each of the inter-device messages, such that each of the inter-device messages is capable of being encapsulated in a single protocol data unit of each of the plural data transmission protocols of the interworked network.

87. The method of claim 86, wherein said plural data transmission protocols includes at least one of:

Asynchronous Transfer Mode (ATM), Frame Relay, High Level Data Link Control (HDLC), X.25, and tunneled protocols.

88. The method of claim 86, wherein:

the interworked network includes an asynchronous transfer mode (ATM) network employing an ATM data transmission protocol and a frame relay network employing a frame relay data transmission protocol;

the first site is a site on one of the ATM network and the frame relay network, and the second site is a site on the other of the ATM network and the frame relay network; and

step (a) includes forming each inter-device message to be capable of being encapsulated in a single ATM cell and in a single frame relay frame.

89. The method of claim 85, wherein:

the sequence of data measurement messages includes first and third messages transmitted over the data transmission circuit from the first site to the second site in accordance with the inter-device message transmission structure, and second and fourth messages transmitted over the data transmission circuit from the second site to the first site in accordance with the inter-device message transmission structure; and

step (a) includes forming the inter-device messages such that;

when said message type field identifies the inter-device message as the third message in the sequence, said count field contains the count OFF_A ; and

when said message type field identifies the inter-device message as the fourth message in the sequence, the count field contains the count OFF_B.

90. The method of claim 85, wherein step (a) includes forming each of the inter-device messages to include:

a first timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was received; and

a second timestamp field for transmitting timestamp data indicating a time at which a previous message in the sequence was transmitted.

91. The method of claim 90, wherein:

the sequence of data measurement messages includes first and third messages transmitted over the data transmission circuit from the first site to the second site in accordance with the inter-device message transmission structure, and second and fourth messages transmitted over the data transmission circuit from the second site to the first site in accordance with the inter-device message transmission structure; and

step (a) includes forming the inter-device messages such that;

when said message type field identifies the inter-device message as the second message in the sequence, said first timestamp field indicates a receive time of the first message;

when said message type field identifies the inter-device message as the third message in the sequence, said first timestamp field indicates a receive time of the second message, said second timestamp field indicates a transmit time of the first message, and said count field contains the count OFF_A ; and

when said message type field identifies the inter-device message as the fourth message in the sequence, said second timestamp field indicates a transmit time of the second message, and the count field contains the count OFF_B.