AU6339990A - An erase station and a method of erasing slots - Google Patents
An erase station and a method of erasing slotsInfo
- Publication number
- AU6339990A AU6339990A AU63399/90A AU6339990A AU6339990A AU 6339990 A AU6339990 A AU 6339990A AU 63399/90 A AU63399/90 A AU 63399/90A AU 6339990 A AU6339990 A AU 6339990A AU 6339990 A AU6339990 A AU 6339990A
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- erased
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- slot
- station
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Description
AN ERASE STATION AND A METHOD OF ERASING SLOTS
The present invention relates to an erase station for a network using a Distributed Queue Dual Bus (DQDB) communications protocol and a method of erasing slots in such a network.
The DQDB protocol and a QPSX network which employs the protocol are described in the specification of Australian Patent Application No 45968/89 and more fully in the report prepared by the IEEE 802/6 Working Group entitled "Proposed IEEE Standard 802.6 - Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)" Draft D10, May 1989, and the specification of International Application PCT/AU85/00304 (International Publication WO 86/03639), which are herein incorporated by reference. A discussion of the DQDB protocol is also provided in the International Journal of Digital and Analog Cabled Systems, Vol 2 179-186 (1989) which is also incorporated herein by reference.
In communications networks which employ the DQDB communications protocol, slots which are used to transmit data from one station of the network to another cannot be reused by subsequent stations, along the bus on which the data is transmitted, once the slot has been read by the destination station. Empty slots which can be used for data transmission between stations are only provided by Head of Bus (HOB) units of the network. This is also the case for networks which employ an improved DQDB protocol which enables guaranteed capacity to be allocated to stations in the network. The improved protocol is described in A. Kennignton, Guaranteed Bandwidth in Distributed Queuing, Contributions 802.6- 90/50-51 to the IEEE 802.6 Working Group, July 1990. Stations which operate on the basis of the basic DQDB protocol are hereinafter referred to as DQDB stations, and stations which operate according to the improved DQDB protocol and are able to provide guaranteed capacity for a priority level are hereinafter referred to as GBW stations.
To increase the capacity of a network which employs a DQDB protocol it is
_ _
have been received by all destinations, specified by their address field, so they can be used again before proceeding to an End of Bus (EOB) unit. Erasing a slot entails resetting the busy bit in the slot's header and thus, if necessary, resetting the information carrying bits of the slot so it may be used by another station to transmit data to a succeeding station. A distributed queue established between stations of the network however should be maintained if slots are erased and made available for use again before reaching an EOB unit.
In accordance with the present invention there is provided an erase station for a network using a DQDB communications protocol, comprising: means for erasing slots received on a transmit bus which have been used to transmit data to a station which precedes the erase station on said transmit bus, said erased slots thereafter being available for use by stations for data transmission on said transmit bus; counting means for maintaining an erased count of erased slots which may be used by stations to satisfy a request for a slot; and means for erasing a request received on a request bus when said erased count corresponding to said request is greater than a predetermined value.
Preferably said counting means includes outstanding counting means for maintaining an outstanding count of outstanding requests for slots, and erased counting means for maintaining said erased count.
Preferably said erased count is incremented when a slot is erased.
Preferably said outstanding count is decremented when an erased slot is released by said erase station.
Preferably said outstanding count is incremented when said request is received and said erased count is decremented when said request is erased.
Preferably said erased count is decremented if said erased count is greater
than a predetermined value and said erase station requires use of slot for transmission on said transmit bus.
Preferably said station maintains an outstanding count and an erased count for each request priority level. Preferably when a slot is erased said erased count is incremented at the highest priority level for which a request is outstanding or about to be sent by said station.
Preferably said station includes means for storing a slot until a previous slot received bit of the header of the succeeding slot is received by the station.
Preferably said predetermined value is zero.
In accordance with the present invention there is also provided a method of erasing slots in a network using a DQDB communications protocol, comprising: erasing slots received on a transmit bus which have been used to transmit data to a preceding station on said transmit bus, erased slots thereafter being available for use by stations for data transmission on said transmit bus; maintaining an erased count of erased slots which may be used by stations to satisfy a request for a slot; and erasing a request received on a request bus when said erased count corresponding to said request is greater than a predetermined value.
Preferably said method includes maintaining an outstanding count of outstanding requests for slots.
Preferably said erased count is incremented when a slot is erased.
Preferably said outstanding count is decremented when an erased slot is released.
Preferably said outstanding count is incremented when said request is
received and said erased count is decremented when said request is erased.
Preferably said erased count is decremented if said erased count is greater than a predetermined value and an erasing station requires use of a slot for transmission on said transmit bus.
Preferably said method involves maintaining an outstanding count and an erased count for each request priority level. Preferably said erased count is incremented at the highest priority level for which a request is outstanding, or about to be sent by said erasing station, when a slot is erased.
Preferably said method includes storing slots until a previous slot received bit of the header of the succeeding slot is received.
A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
Figure 1 is a block diagram of a network which uses a DQDB communications protocol and includes an erase station; and Figure 2 is a block diagram of an erase station.
A network 2, as shown in Figure 1, which employs a DQDB communications protocol includes a bus A 4 for transmitting slots in one direction and a parallel bus B 6 for transmitting slots in the opposite direction, and a plurality of stations 8 and 10 which are connected in parallel between the buses 4 and 6. Respective HOB units 12 and EOB units 14 are used to terminate the ends of the buses 4 and 6, slots being transmitted on a bus 4 or 6 from the respective HOB unit 12 to the respective EOB unit 14. The buses 4 and 6 are used to transmit both data and requests for slots and are preferably serial data lines.
The network 2 illustrated in Figure 1 includes four stations 8, which may be
DQDB or GBW stations, and an erase station 10 connected in parallel between the second and third stations 8. The erase station 10, as shown in Figure 2, includes
an erase unit 20 which erases slots received on bus A 4 or B 6 which have been used, DQDB count units 22 for both buses 4 and 6 and erase count units 24 for both buses 4 and 6. A DQDB count unit 22 includes request counters for each of the four request priority levels and an erase count unit 24 includes an erased slot counter for each priority level. The request counters operate in the same manner as request counters of the other stations 8 to maintain a record of outstanding slot requests. The erase station 10 is able to erase slots used in transmitting data between the first and second stations 8 so the erased slot can then be used to transmit data between the third and fourth stations 8 and similarly it can be used to erase slots used to transmit data between the fourth and third stations 8 so that the slots can then be used to transmit data between the second and first stations.
The erase station 10 may be a dedicated station for erasing slots only or it may be a station which is also able to transmit and receive data on slots in the same manner as the DQDB or GBW stations. A transmitting erase station would include all of the features of a dedicated non-transmitting erase station. The DQDB count unit 22 of a transmitting erase station, therefore, would further include countdown counters and request queue counters for each priority level and each bus 4 and 6, and the station would also include circuitry enabling it to access slots for transmitting and receiving data on both buses 54 and 6. The stations 8 and 10 of the network 2 are configured by standard microprocessor based and controlled high speed data processing components with appropriate software, which are chosen to meet the functional requirements described herein. The circuitry is similar to that described in the references listed on page 1.
To simplify the description we will hereinafter describe the operation of the erase station 10 with respect to data only being transmitted on bus A 4 and requests being transmitted on bus B 6. Furthermore, we will also consider the station 10 to be a transmitting station. It is to be understood the description also applied to data transmitted on bus B 6 and requests transmitted on bus A 4 and, in most respects, to non-transmitting erase stations.
The slots received by the erase station 10 on bus 4 are stored in the erase unit 20 until a previous slot received (PSR) bit of the header of the following slot is received. If the PSR bit is high this indicates the stored slot contains information which has been transmitted and received by a preceding station on bus A. Thus if the PSR bit is set the stored slot is erased by the station 10 and returned to bus A with its PSR bit reset to 0, otherwise the stored slot is merely returned to bus A unchanged. To be erased the stored slot must also have a high busy bit in its respective header, as empty slots are returned to bus A unchanged.
The erase station 10 maintains request and countdown counters, the values of which are Cr(i) and Cd(i), respectively, for each priority level i = 0, 1, 2, 3, where 0 is the lowest priority level and three is the highest level. The sum of the request and countdown counters for a given priority i represents the number of requests outstanding for that priority at the erase station 10. The request and countdown counters are decremented or incremented according to the DQDB protocol. Therefore, as for a DQDB station, Cr(i) or Cd(i) for all i is decremented by 1 (if not already 0), whenever an empty slot is released by the erase station 10 onto bus A, except this includes both empty slots which are received and created by the station 10. Cr(i) or Cd(i) is also incremented for i ≤ j if a request on priority j is received on bus B. For a non-transmitting erase station Cd(i) = 0 for ali i.
The erase station 10 also maintains a request queue counter for each priority level indicating the number of requests which the station has queued to send for each priority level i, where the value of the request queue count for priority level i is Cw(i). According to the basic DQDB protocol a station 8 or 10 can only transmit and have outstanding one request (i.e. one which has not been granted) at any given time. When a station 8,10 queues a segment for transmission a request queue counter at the appropriate priority level is incremented to place the segment in the distributed queue.
A request issued by a succeeding station on bus A, which has not been
received by a preceding station on bus B, but has already been granted, is hereinafter referred to as a stale request. If the erase station 100 erases slots and does not erase requests, the number of stale requests recorded by the station 10 will increase thereby causing unfair usage of capacity by particular stations and wastage of the transmitting capacity of the network 2. The erase station 10 therefore should cancel requests accordingly to ensure the distributed queue for the network 2 is not upset by stale requests. Ideally, following erasure of a slot which is subsequently used to satisfy a given priority k request, the erase station immediately, or shortly afterwards, should cancel a priority k request received on bus B. This, however, requires the erase station 10 to be able to firstly accurately guess for which priority level an erased slot is to be used and secondly to then locate in a short period of time a request of that priority to cancel. To cancel a request, the erase station may, after erasing a slot, either cancel a request queued in its request queue counter or wait for a request to arrive on bus B which can be cancelled. To try and achieve the ideal situation discussed above, the erase station 10 operates as follows.
The priority level for which an erased slot is to be used can be estimated by comparing values p and t, where p is the highest priority level for which the sum of the corresponding request and countdown counters is greater than 0 (Cr(i) + Cd(i) > 0), p being equal to -1 if the request and countdown counters are 0 for all values of i, and t is the highest priority level at which the erase station 10 has a segment queued and waiting to be transmitted, t being equal to -1 if there are no segments queued for transmission at any priority level. If p > t, the erased slot will probably be used to satisfy a priority p request issued by a succeeding station on bus A, whereas if t > p the erased slot will probably be used by the erase station 10 to transmit a segment queued at priority t. If t = p the erased slot may be used either by the erase station or by one of the succeeding stations at priority t. Therefore, an erased slot is most likely to be used to satisfy a request at priority m, m being the highest value of p or t, m = max(p,t).
The erased slot counters, provided for each priority level, record the number
of slots erased for the purpose of future request cancellation, and the value of the counters are Ce(i) for i = 0, 1, 2, 3. After the erase station erases a slot, the priority m erased slot counter of the station is incremented if m ≥ 0. When a request arrives on bus B, the corresponding erased slot counter determines whether the request is to be cancelled or not. In particular, if a priority k request arrives on bus B, and Ce(k) > 0, the request is cancelled, or erased and the priority k erased slot counter is decremented by 1.
If after erasing a slot m = -1, no erased slot counter should be incremented because the erase station 10 does not plan to cancel any future requests because it appears that none of the succeeding stations will use the erased slot.
When the erase station 10, after the erased slot counters have been updated, releases an empty slot received from a preceding station, or a slot which has just been erased, on bus A, the station 10 decrements its positive request and countdown counters for all priorities, according to the DQDB protocol.
When the station 10 receives a slot with an empty priority k request field on bus B, the erase station examines its priority k request queue counter and its priority k erased slot counter. If Cw(k) and C.(k) are both greater than 0 then they are both decremented by 1 and the priority k request field is not set to 1. The station 10 also decrements its request and countdown counters, accordingly, at priorities less than k to cancel the effect of the self requests generated when the station 10 queue the priority k request. If, however, the priority k erased slot counter is 0, the empty priority k request field is set to 1 by the station 10 and the priority k local request queue counter is decremented by 1, according to the DQDB protocol.
When a request at priority level k arrives on bus B at the erase station 10, the station 10 updates its request and countdown counters according to the DQDB protocol. Therefore, Cr(i) or Cd(i) is incremented for all priority levels i such that i ≤ k. If, the erased slot counter for priority level k is positive, then the request is
erased and the priority k erased slot counter is decremented by 1.
From the above, it will be apparent the erase station 10 decrements all of its positive request and countdown counters when an erased or empty slot is released on bus A in anticipation of the slot being used to satisfy the highest priority level request queued or recorded at the station 10. The request and countdown counters of the station 10 are then incremented according to the DQDB protocol when requests are received on bus B from succeeding stations on bus A. The erased slot counts are maintained and used to determine whether a request received on bus B by the station 10 is to be erased or allowed to proceed to the stations which precede the erase station 10 on bus A. The erased slot counters are used to prevent stale requests being sent to the preceding stations and upsetting the distributed queue of the network 2. Furthermore, if the erase station 10 is a transmitting station and the erased slot is to be used to satisfy a queued request of the station 10, then an empty request field can be left unchanged when received, and a request queue counter of the station 10 decremented accordingly.
A number of variations can be made to the method described above to achieve the same effect. For example, when an erased slot counter is incremented for a given priority, the station 10 can examine its local request queue counter for that priority and if found to be positive, the station 10 could decrement both counters by 1. Furthermore, whenever the local request queue counter is incremented, the erase station could examine its erased slot counter for that priority and if found to be positive, the station 10 would decrement both counters by 1.
The network 2 may be monitored and controlled by a central controller, which would adjust the traffic capacity of sections of the network 2, as desired. An erase station 10 may be used by the controller to assist in adjusting capacity allocations. For instance, the controller may adjust the number of requests erased by the station 10 by adjusting the level of at least one of the erased slot counters, i.e. the erased counts may be incremented and decremented accordingly to achieve a desired capacity distribution.
Claims (33)
1. An erase station (10) for a network (2) using a DQDB communications protocol, comprising: means (20) for erasing slots received on a transmit bus (4,6) which have been used to transmit data to a station (8) which precedes the erase station (10) on said transmit bus (4,6), said erased slots thereafter being available for use by stations (8,10) for data transmission on said transmit bus (4,6); counting means (22,24) for maintaining an erased count of erased slots which may be used by stations (8,10) to satisfy a request for a slot; and means (20) for erasing a request received on a request bus(6,4) when said erased count corresponding to said request is greater than a predetermined value.
2. An erase station (10) as claimed in claim 1, wherein said counting means (22,24) includes outstanding counting means (22) for maintaining an outstanding count of outstanding requests for slots and erased counting means (24) for maintaining said erased count.
3. An erase station (10) as claimed in claim 2, where said erased count is incremented when a slot is erased.
4. An erase station (10) as claimed in claim 3, wherein said outstanding count is decremented when an erased slot is released by said erase station (10).
5. An erase station (10) as claimed in claim 4, wherein said outstanding count is incremented when said request is received and said erased count is decremented when said request is erased.
6. An erase station (10) as claimed in claim 5, wherein said erased count is decremented if said erased count is greater than a predetermined value and said erase station (10) requires use of a slot for transmission on said transmit bus (4,6).
7. An erase station (10) as claimed in any one of claims 2 to 6, wherein said erase station (10) maintains an outstanding count and an erased count for each request priority level.
8. An erase station as claimed in claim 7, wherein said erase station (10) predicts the priority level for which an erased slot is to be used on the basis of said outstanding count and increments the erased count at said priority level, accordingly.
9. An erase station as claimed in claim 8, wherein said priority level is predicted on the basis of said outstanding count and on the basis of data or segments the erase station (10) has queued for transmission.
10. An erase station (10) as claimed in claim 8 or 9, wherein when a slot is erased said erased count is incremented at the highest priority level for which a request is outstanding or about to be sent by said erase station (10).
11. An erase station as claimed in claim 7 or 10, wherein when a slot is erased said erased count is incremented at priority level m, m being the maximum of p and t, where p is the highest priority level for which said outstanding count is greater than a predetermined value and t is the highest priority level at which the erase station has a segment queued for transmission.
12. An erase station (10) as claimed in claim 10 or 11, wherein said outstanding count is decremented at each priority level when an erased slot is released.
13. An erase station (10) as claimed in claim 12, wherein if said erased count is greater than a predetermined value and said erase station (10) has queued a priority k request for sending and a slot with a coπesponding empty request bit is received on a request bus (4,6), said request bit is not set to send the request, the priority k erased count is decremented and the outstanding counts are updated as if said priority k request had been sent.
14. An erase station (10) as claimed in any one of the preceding claims, wherein said station includes means (20) for storing a slot until a previous slot received bit of the header of the succeeding slot is received by the station.
15. An erase station (10) as claimed in any one of the preceding claims, wherein said predetermined value is zero.
16. A method of erasing slots in a network (2) using a DQDB communications protocol, comprising: erasing slots received on a transmit bus (4,6) which have been used to transmit data to a preceding station (8) on said transmit bus (4,6), erased slots thereafter being available for use by stations (8,10) for data transmission on said transmit bus (4,6); maintaining an erased count of erased slots which may be used by stations (8,10) to satisfy a request for a slot; and erasing a request received on a request bus (4,6) when said erased count corresponding to said request is greater than a predetermined value.
17. A method as claimed in claim 16, wherein said method includes maintaining an outstanding count of outstanding requests for slots.
18. A method as claimed in claim 17, wherein said erased count is incremented when a slot is erased.
19. A method as claimed in claim 18, wherein said outstanding count is decremented when an erased slot is released by said erase station.
20. A method as claimed in claim 19, wherein said outstanding count is incremented when said request is received, and said erased count is decremented when said request is erased.
21. A method as claimed in claim 20, wherein said erased count is decremented if said erased count is greater than a predetermined value and an erasing station (10) requires use of slot for transmission on said transmit bus.
22. A method as claimed in any one of claims 16 to 21, wherein said method involves maintaining at said erase station (10) an outstanding count and an erased count for each request priority level.
23. A method as claimed in claim 22, including predicting the priority level for which an erased slot is to be used on the basis of said outstanding count and incrementing the erased count at said priority level, accordingly.
24. A method as claimed in claim 23, wherein said priority level is predicted on the basis of said outstanding count and on the basis of data or segments the erase station (10) has queued for transmission.
25. A method as claimed in claim 23 or 24, wherein when a slot is erased said erased count is incremented at the highest priority level for which a request is outstanding or about to be sent by said erasing station (10).
26. A method as claimed in claim 22 or 25, wherein when a slot is erased said erased count is incremented at priority level m, m being the maximum of p and t, where p is the highest priority level for which said outstanding count is greater than a predetermined value and t is the highest priority level at which the erase station has a segment queued for transmission.
27. A method as claimed in claim 25 or 26, wherein said outstanding count is decremented at each priority level when an erased slot is released.
28. A method as claimed in claim 27, wherein if said erased count is greater than a predetermined value and said erase station (10) has queued a priority k request for sending and a slot with a coπesponding empty request bit is received on a request bus (4,6), said request bit is not set to send the request, the priority k erased count is decremented and the outstanding counts are updated as if said priority k request had been sent.
29. A method as claimed in any one of claims 16 to 28 wherein said method includes storing a slot until a previous slot received bit of the header of the succeeding slot is received.
30. A method as claimed in any one of claims 16 to 29, wherein said predetermined value is zero.
31. A method as claimed in any one of claims 16 to 30, wherein said erased count is adjusted by a network controller to adjust a capacity allocation.
32. A network (2) which employs a DQDB communications protocol having at least one erase station (10) as claimed in any one of claims 1 to 15.
33. A network (2) as claimed in claim 32, including a network controller which is adapted to adjust said erased count.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU63399/90A AU641688B2 (en) | 1989-09-13 | 1990-09-12 | An erase station and a method of erasing slots |
Applications Claiming Priority (7)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPJ632889 | 1989-09-13 | ||
| AUPJ6328 | 1989-09-13 | ||
| AUPJ9281 | 1990-03-23 | ||
| AUPJ928490 | 1990-03-23 | ||
| AUPJ928190 | 1990-03-23 | ||
| AUPJ9284 | 1990-03-23 | ||
| AU63399/90A AU641688B2 (en) | 1989-09-13 | 1990-09-12 | An erase station and a method of erasing slots |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| AU33893/93A Division AU661751B2 (en) | 1989-09-13 | 1993-03-01 | A method of erasing slots |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| AU6339990A true AU6339990A (en) | 1991-04-18 |
| AU641688B2 AU641688B2 (en) | 1993-09-30 |
Family
ID=27423632
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| AU63399/90A Ceased AU641688B2 (en) | 1989-09-13 | 1990-09-12 | An erase station and a method of erasing slots |
Country Status (1)
| Country | Link |
|---|---|
| AU (1) | AU641688B2 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU648799B2 (en) * | 1990-05-21 | 1994-05-05 | Koninklijke Kpn N.V. | Method for improving the transmission of information in the form of data packets, in accordance with a distributed queuing protocol |
| AU662578B2 (en) * | 1991-12-20 | 1995-09-07 | Telstra Corporation Limited | A method of transmitting data from a terminal to a digital network transmission line |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3755786A (en) * | 1972-04-27 | 1973-08-28 | Ibm | Serial loop data transmission system |
| JPH0666806B2 (en) * | 1988-02-08 | 1994-08-24 | 住友電気工業株式会社 | Token ring transmission method |
-
1990
- 1990-09-12 AU AU63399/90A patent/AU641688B2/en not_active Ceased
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU648799B2 (en) * | 1990-05-21 | 1994-05-05 | Koninklijke Kpn N.V. | Method for improving the transmission of information in the form of data packets, in accordance with a distributed queuing protocol |
| AU662578B2 (en) * | 1991-12-20 | 1995-09-07 | Telstra Corporation Limited | A method of transmitting data from a terminal to a digital network transmission line |
Also Published As
| Publication number | Publication date |
|---|---|
| AU641688B2 (en) | 1993-09-30 |
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