METHOD FOR SYNCHRONIZATION IN A LOCAL AREA NETWORK INCLUDING A STORE-AND-FORWARD DEVICE

Abstract: The present invention relates to a method for identifying inaccurate time measurements in a local area network. The local area network comprises a sending device (2), such as a time client, a destination device (3), such as a timeserver, and a switching device and/or a store-and-forward device (6). A time request packet (4) is rejected if it arrives at the time client or timeserver within a given or calculated minimum distance from the preceding network packet. The time request packet (4) is also rejected if it falls outside a set of calculated error limits based on a set of estimates. Those estimates are kept in the time client memory and updated according to a given set of rules.

Description: Method for synchroni zati on i n a local area network i ncl udi ng a store-and-forward device Field of the invention A method for identifying inaccurate time measurements in a local area network incorporating a central switch and/or a store-and-forward device . Background of the invention Several schemes exist for synchronizing a time-of-day clock in one node, such as a time client, to a reference time-of- day clock in another node, such as a timeserver, over a local area network (LAN) . Most of the time-of-day clocks attain their stated accuracy only when a network transfer delay between the time client and the timeserver is constant. Introduction of switched Ethernet created a class of local area networks where network capacity was greatly enhanced at the expense of a predictable delay between the time one network node transmits a time request packet and the time the destination node receives it. A reason for this is that the link to the destination node may be busy with other packets when the aforesaid packet is ready to be placed on that link. In such cases the packet is stored in the switch, which therefore is also a store-and-forward device, until the link to the destination node is free. Summary of the invention The object of the invention is to provide a method for identifying inaccurate time measurements in a local area network incorporating a central switch and/or a store-and- forward device . This object is achieved by a method according to the independent claims 1 and 2. A network incorporating a store-and-forward device, like the switch in a switched Ethernet network, has two distinct sources of delay. The first source of delay is the connection from the sending device, such as the time client, to the store-and-forward device and the second source of delay is the connection from the store-and-forward device to the destination device, such as the time server 4. This invention will address both sources, using two distinct algorithms . Brief description of the drawings Fig. 1 is a schematic block diagram of a computer network, such as a LAN, comprising a time client, a timeserver and a central switch and/or a store- and forward device. Fig. 2 shows an example of normal packet arrivals and queued packet arrivals . Fig. 3 shows an example of accepting or rejecting a measurement based on estimates. Detailed description of the preferred embodiments Figure 1 shows a schematic block-diagram of a computer network 1, such as a local area network, comprising a time client 2 and a timeserver 3. A switching device and/or a store-and-forward device 6 are/is in the path between the time client and the timeserver. A time request packet 4 is transmitted from the time client 2 to the timeserver 3 and from the timeserver to the time client. It is presupposed that the time request packets 4 are time stamped at the end of the time request packets. 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Φ Hi Φ P- C- O 3 o rt rt 3 P- hi hi X Cu ≤ CΛ hj s: 3 φ ? O tr t- O Cu CΛ o Φ Φ φ Φ hi Φ rt υa P- 3 3 hi ιQ P- Cu hi rt Φ tr CΛ Ό CU X? P- NJ ti φ CΛ α α φ u rt P- < • tr CΛ Λ Φ CΛ tr Φ 3 Φ * C- rt P- CΛ CΛ O Φ ιQ t- m rt φ o t- rt rt Cu hi o tr Cu tr CΛ P- O 3 CU tr Φ u rt rt ti hi P- P-< rt tr Φ C- Φ 1 cu the time stamps may be used to synchronize the time client 2 and the time server 3. If there are delays on the link from the sending device to the store-and-forward device, an alternative procedure for validating the time stamp accuracy must be used. Figure 3 shows how the time stamp accuracy is validated when it has been a delay on the link 1 from the sending device, for example, the time client 2, to the switching device and/or store-and-forward device. This procedure is based on a certain degree of confidence in the local time- of-day clock. This confidence is expressed in four variables: estimated relative local frequency deviation dfR, estimated local frequency accuracy d2fR, estimated local time precision dtP and a confidence level. Start values for the estimated relative local frequency deviation dfR and the estimated local frequency accuracy d2fR are stored in the time client when the time client is manufactured, together with a comparison value for the confidence level variable. The confidence level variable always starts at 0 and as long as it is less than the comparison value, there is a low confidence in the estimates. When the confidence level variable is greater than or equal to the comparison value, there is a high confidence in the estimates. When a new time adjustment tA is calculated from the time stamps, it is decided whether to keep the adjustment or reject it according to, for example, the following algo- rithm: 1. First an upper acceptance level tH or a lower acceptance level tL for the new time adjustment tA is calculated: tB = tP + (tR - tP)*(l + dfR) dtB = dtP + (tR - tP)*d2fR tH = tB + dtB tL = tB - dtB where tP is the value of the local clock at a previously accepted time adjustment, tR is the value of the local clock at the current time request, dfR is the estimated relative local frequency deviation, dtp is the estimated local time precision at the previously accepted time adjustment and d2fR is the estimated local frequency accuracy at the same point in time. The range between the upper acceptance level tH and the lower acceptance level tL is called the error range. 2. If tA falls within the error range, tA is accepted and used to update local time, estimated local frequency and estimated frequency drift. In addition, the confidence level is increased up to a predefined limit. 3. If there is a high confidence in our estimates and tA does not fall within the error range, tA is rejected and the confidence level in our estimates is decreased to a value greater than 0. 4. If there is a low confidence in our estimates and tA does not fall within the error range, there are several options. A first option is to reject tA, decrease the confidence level and hope for a better tA in the future. A second option is to introduce a new set of estimates together with a time-of-day offset. When the new set of estimates has been introduced every new time adjustment tA must be checked against every set of estimates until one of the confidence levels becomes high. At that point in time, that set of estimates is kept and the others are deleted. The next time adjustment tA will fulfill the requirements in either the time client 2 or the timeserver 3.

Claims: Claims 1. Method for identifying inaccurate time measurements when running time synchronization on a local area network, the local area network comprising a sending device (2), a destination device (3) and a switching device and/or a store-and-forward device (6) arranged on a path (1) between the sending device (2) and the destination device (3), c h a r a c t e r i z e d b y rejecting time request packets (4) that arrive to the destination device (3) within a given time interval t from a preceding network packet. 2. Method for identifying inaccurate time measurements when running time synchronization on a local area network, the local area network comprising a sending device (2) , a destination device (3) and a switching device and/or a store-and-forward device (6) arranged on a path (1) between the sending device (2) and the destination device (3), c h a r a c t e r i z e d b y keeping one or more estimates of the accuracy and precision of the local time-of day clock in a memory in the sending device (2) and accepting or rejecting the new time measure- ment based on a set of calculations made from these estimates . 3. Method according to claim 2, c h a r a c t e r i z e d i n that the sending device (2) is a time client.

Inventors: APNESETH CHRISTOFFER, HOLMEIDE OEYVIND, JOHANNESSEN SVEIN, LOEKSTAD TROND, SKEIE TOR, APNESETH, CHRISTOFFER, HOLMEIDE, OEYVIND, JOHANNESSEN, SVEIN, LOEKSTAD, TROND, SKEIE, TOR
Applicants: ABB RESEARCH LTD, APNESETH CHRISTOFFER, HOLMEIDE OEYVIND, JOHANNESSEN SVEIN, LOEKSTAD TROND, SKEIE TOR, ABB RESEARCH LTD., APNESETH, CHRISTOFFER, HOLMEIDE, OEYVIND, JOHANNESSEN, SVEIN, LOEKSTAD, TROND, SKEIE, TOR
Application year: 2001
Main classification: H04L7
IPC codes: H04J3, H04L12