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Scheduler with reordering for ATM Adaptation Layer 2 switches

IP.com Disclosure Number: IPCOM000017126D
Original Publication Date: 1999-Oct-01
Included in the Prior Art Database: 2003-Jul-22
Document File: 4 page(s) / 24K

Publishing Venue

Siemens

Related People

Alan Forbes: AUTHOR [+2]

Abstract

The AAL2 Switching ASIC ACE contains several AAL2 output queues. To determine, from which queue the next output ATM cell shall be built and sent from, a scheduler is implemented. The scheduler makes this decision based on the two parameters timeout and size. A queue should be scheduled, if either the timeout of the 1 st packet in the queue has occurred, or if the size of the queue exceeds 47 bytes. The scheduler is shown in figure 3. It is implemented as a calendar function where outgoing queues are marked. Time represented by the now pointer constantly increases and if it points to a filled calendar position, then the control block orders the transmission of a cell for that queue marked in the calendar. The implementation of the calendar is achieved using a RAM, covering an address range of 0 to 4095. Each address represents a timeslot of 2.7 us (representing one ATM cell at STM1 rate), resulting in a maximum timeout of approximately 11 ms (2.7us*4096). If the actual time, i.e. the now-pointer, points to address 4095, the next address will be 0.

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Information / Kommunikation

Scheduler with reordering for ATM Adaptation Layer 2 switches

Idee: Alan Forbes, München; Christian Huber, Rosenheim

The AAL2 Switching ASIC ACE contains several AAL2 output queues. To determine, fromwhich queue the next output ATM cell shall be built and sent from, a scheduler is implemented.The scheduler makes this decision based on the two parameters timeout and size. A queue shouldbe scheduled, if either the timeout of the 1 st� packet in the queue has occurred, or if the size of thequeue exceeds 47 bytes. The scheduler is shown in figure 3.

It is implemented as a calendar function where outgoing queues are marked. Time represented bythe now pointer constantly increases and if it points to a filled calendar position, then the controlblock orders the transmission of a cell for that queue marked in the calendar. The implementationof the calendar is achieved using a RAM, covering an address range of 0 to 4095. Each addressrepresents a timeslot of 2.7 us (representing one ATM cell at STM1 rate), resulting in a maximumtimeout of approximately 11 ms (2.7us*4096). If the actual time, i.e. the now-pointer, points toaddress 4095, the next address will be 0.

The calendar stores the queue number, from which an ATM cell will be assembled. An entry intothe calendar is created, when a new packet comes to the 1 st� place of an aal2 output queue or ifthe queue-size exceeds a threshold. In the 1 st� case the search engine has to determine whichtimeslot (calendar address) is free, beginning from a timeout-pointer down to the now-pointer. Inthe 2 nd� case a free slot near to now is searched up to the timeout-pointer. The timeout-pointer isthe now-pointer plus a timeout value. At any time, only one entry of each queue is allowed in thecalendar.

In case of exceeding a queue-size threshold equal to an ATM cell payload, a new calendar entryclose to now is generated. Thus the old entry has to be removed from the calendar. This action isdenoted as 'rescheduling'. The information, at which timeslot the old entry was scheduled, isstored in a look-up table. The timer block generates a sync signal each 2.7us, which is used tosynchronize incoming ATM cells to the internal time scheme and to start a new output cycle. Italso generates a now-pointer representing the actual time.

Siemens Technik Report

Jahrgang 2� Nr. 5� Oktober 1999

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search hit

search address

queue nr.

current slot of queue nr.

11

3Kbit

look up table

calendar address

0

32Kbit

7

16

queuenr.

calendar

0

search engine

0

4095

255

0

reschedule

old calendar address

delete(sent)

start search

timestamp (13 bit)

sync

now

read output

(now pointer)

write reschedule

write schedule

delete pointer, due to timeout

qeue nr.

reschedule

sync (time incremented)

Timer

control block & sequencer

now pointer

ptr. to 1st packet in qeue queue nr. reschedule timeout value packet start search hit

search address

current slot of queue nr.

Figure 3

AAL2 Scheduler

The search engine provides t...