CA2094640C

CA2094640C - Arrangement for bounding jitter in a priority-based switching system

Info

Publication number: CA2094640C
Application number: CA002094640A
Authority: CA
Inventors: Ronald Anthony Spanke
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1992-06-11
Filing date: 1993-04-22
Publication date: 1997-01-07
Anticipated expiration: 2013-04-22
Also published as: EP0577269A3; EP0577269A2; EP0577269B1; DE69331454D1; US5287347A; JPH0662043A; CA2094640A1; ATE212164T1; DE69331454T2; JP2957062B2

Abstract

An arrangement where a switching system receives parameter(s) concerning the traffic expected on a call from a first user to a second user, the system determines a priority for the call based on the expected traffic parameter(s), and information is transmitted to the second user during the call based on the determined priority and with less than a maximum jitter. The priority is selected from a predefined priority table based on expected traffic parameters. The priority table is usable for both constant bit rate and statistical calls.

Description

- 20g4640 ARRANGEMENT FOR BOUNDING Jl l-l ~;K IN A
PRIORITY-BASED SWITCHING ~Y~
Technical Field This invention relates to co~ n;c~tions ~ .t~ s.
5 Bacl~ m.~d of the In~ tio..
The term brsnrlh~nd covers a host of new pluducls, technologies, services, and nelwc.lks. One way to define b~ nd r.~tw~lks is to cat~,~olize them as those nclw~lLs that support services l~uiring bit rates well above one megabits per second. Business and residçnti~l subscribers will be connc~led to bro~1b~nd 10 n~;lwolks via a COml~ll access, opel~tillg at 150 megabits per second or above, that can handle a range of dirr~ l broAdb~nd service t,vpes. ATM (asynchronous transfer mode) has been chosen as the cQ~ ic~tiQn principle on which bro~db~n-1 n(,~wolks will be based. A future broad~n~l ISDN (h.~gl~t~cl services digital n~,~w~lk) will offer the flexibility needed to handle diverse services ranging from 15 basic telephone service to high speed data transfer, videotele~hony, and high quality television distribution. The key to this flexibility is ATM which carries digital ihlrc.. ~ n in special cells. This allows the netwull~ to be used effiriently by appli-~ation~ and services with widely differing bandwidth re~lui~ s and call chara~tç~i~tics Priority-based ~ . have been dçsigne~l for ~wi~ ng ATM cells or pe~ rO. ~ .~;ng other packet ~.wi~hillg functior~ In such ~y~t.,ms, all cells (packets) for a given priority are tl;.n~ to their destin~tion before the first cell of the next lower priority. Within a given priority, cells are tr~ncmitte~ on a first come-&t serve basis. All equal priority cells arriv,ing at a given cell time are ~l~n.~m;~
25 before the same priority cells arriving at the next cell time are started. This becomes the root cause of jitter becanse one high bandwidth call could have a cell arriving every other cell time; all of a sudden, ten or twenty cells from other calls and having the sarne priority as the one high bandwidth call alTive in one of the open cell times.
All ten or twenty of these cells will be l,~-.s.--;l~ before the next cell of the high 30 bandwidth call c?using an a~osolu~e jitter of ten or twenty cell times. During this interval, cells for the high bandwidth call keep arriving every other cell time and are queued up. When the high bandwidth call does begin ~,i.n~ ;ng its cells again, they will be l-,.n~ ;u~ every cell time, back-to-back, until the queue is empty. Not only is such jitter unaccep~ble in many applic~tisn~ subst~nti~l re~ur~es are 35 r~uil~d for buffering and receiver buildout delay.

209~6~0 In ~d~liti~n to being "l,pol~lt in co~ nl bit rate applications, jitter is also an illlpul~ll pa-~,t~,l for stati~tical (bursty) traffic. Variable bit rate video il~iS very low delay, has very high bandwidth and relatively low burstiness.
Variable bit rate voice also r~uil~,s moderately low delay, has relatively low 5 bandwidth and low b~lrstiness~ Both of these applic~tionc must have relatively low jitter to gU~u~ll~ that cells do not arrive "too late" and the buffer at the receiving end does not become empty. Even though this is st~ticti- ~1 traffic, it is jitter sensitive.
Other applicationc, including file transfers and screen image dumps, do 10 not typically require low delay. Such appli~ationc can have either low or high average bandwidth; however, they are very bursty. Such applicationc could significantly il t~lUp~ the delay sensitive applic~tionc resultine in excessive queuing delays and very large buffer buildouts on the receiving end.
In view of the fc,.~,goillg, a recognized need in the art exists for an 15 arr~ng~ment usable in ATM or other packet ~wilcl~ g ~ h~S which will bound jitter to an acceptable amount and which is applicable to both cons~nt bit rate and statistical traffic.
Solution This need is met and a ~hnical advance is achieved in accor~-ce with 20 the invention in an e~empl~ry arrangement where a ~.. ilchh~g system receivesp~u~et~(s) conc. ..;ng the traffic e~cted on a call from a first user to a second user, the system adv~nt~gçously det~,..llilKs a priority for the call based on the expected traffic pal~t.,l(s), and iur~l~alion is tr~n~...;~t~d to the second user during the call based on thc dct~ ...;.-e~l priority and with less than a ...~ ,... jitter. 5 The priority is selecte~ from a predefined priority table based on the eYpecte~ traffic t~lS. Signifi~ntly~ the same priority table is usable for both col ~t~nt bit rate and st~tistical calls.
A method in accordance with the invention is usable in a swilcl~ g system serving a null~ber of users. The system receives one or more pd~ t 30 conc~ll,ing the traffic e~cled on a call from a first user to a second user. The system tr~n~mits hlr,....~;on to the second user during the call based on the ~pect~d traffic p~llet~s and with a n~ . jitter.
In an illustrative rnethod when the call is a cons~nt bit rate call, the expected traffic pa~ct~ls include a bandwidth p&a~ t~,~, BW, and the priority is35 deterrnine~ based on BW. The priority is selected from a priority table having a number, P, of priority bands and having a IllL~illlUIll bandwidth, BW High, and a 2094641) -...inin~.l... bandwidth, Bw~ow~ specified for each band. The ratio BWHigh/BWLow is a con~ nt The priority is select~ by determining the band that includes BW. The lllaXilllUl~ jitter is a worst case relative jitter. The constant bit rate call is made up of fixed length cells and the worst case relative jitter, in intercell arrival periods, is S given by the CQI-~t~--t ratio, BWHigh/BWLow~ e terms "relative jitter" and "intercell arrival period" are describe~l later herein.) If the priority table is used for constant bit rate calls only, it may include one ~ditiQnDl priority band below the P
priority bands and P and BwHighlBwLow satisfy a relationship [BWHighlBwLow] P
= BWIF/BWM~ where BWIp and BWMin are the m~iml-rn and l..;n;
10 bandwidths ~l~.~n each of the users and the ~witcl~ng system.
In a further illllc~rative m~tho~l, the priority table is used for both cM.~l~nt bit rate calls and st~tisti~l calls. When the call is a st~ti~t~ l call, the expected traffic pd~ t~ include an average bandwidth p~l~ t~,l, BWAV8, and a burstiness index, BI, and the priority is d~te~ ~l by det~,. ..~;ning the band in the 15 priority table that incllldes the rado BWAVg/BI. The st~ti~ti~l call is made up of fixed length cells and the worst case relative jitter, in intercell arrival p~io~1~, is given by the product of BI and the cons~ll ratio, BWHUh/BWLoW. P and BWHigh/BWLow satisfy a rel~ti~nchip tBwH~ BwLow]p = BW~BIM~JBWMu,.
where BWIp and BWM~, aTc the ...~ .. and ~ini~n~ bandwidths and BIM"" is 20 the .~ . b~ t sc bCI.~. C~"~ each of the users and the swilchillg system.In both illustrative l..t !i~o~ls~ cells received from the first user during thecall are dr~ped when the cclls cause the e~ct~;l traffic p~ te.~ to bc eYcee~ed D. a~ , D~s~;. ;plion FIG. 1 is a ~lia~m of an exemplary priority-based swilching system 25 which in~ ec an arr~ng~mt nt for bo~ g jitter, FIG. 2 is a ~lia~m of an input procescing unit includell in the system of FIG. l;
FIG. 3 is a diagram of a prioritizing output queue includcd in the system of FIG. l;
FIG. 4 is a r1ia~m of a call setup cell t~ c.~ t~l from users to the system of FIG. l; and FIG. 5 is a diagram of a sctup response cell .a~-cl~ d from the system of FIG. 1 to system users.

209q640 - Detailed Descl~iptio..
Switchin~ System 100 FIG. 1 is a diagram of an exemplary priority-based swilchillg system 100 which includes an arrangement for bounding jitter in accorlance with the 5 present invention. The ~wilching function is p~"Ç~ ..ed within system 100 by filters 104-1 through 104-8 and prioriti~ing output queues 105-1 through 105-8.
System 100 serves seven users Ul through U7. Input ~cessing units 101-1 through 101-8 process cells from users Ul through U7 as well as cells from a controller 109 which controls the operation of system 100. The ~oce~sit-g ~Ço~uled by 10 ~ cessing units 101-1 through 101-8 is describe~l further herein. Controlla 109 co.. ~n;~atçs with users U1 through U7 by ~.~nc.. ;ll;ng cells via path 108 to pnxeccing unit 101-8 and on through system 100 to the desired destin~tion usa.
One of the functions of controller 109 is to ~ete n~;ne a priority value for each virtual circuit set up bel~,el1 users based on palaul~,tels received from the users conce lfing 15 the e~;led traffic. Such priority values are co..-....,l-i.~te 3 from controller 109 to processing units 101-1 through 101-8 via a path 107 and are inserted in each cell of a virtual circuit for tr~ncmi~ n within system 100. Cells from all of the input pl~ces2~;ng units 101-1 through 101-8 are distlibuted to each of the eight filters 104-1 through 104-8 via paths 102-1 th~ugh 102-8 of an int~,lconncc~ion a~ngen~ent 103.
20 Each of the filters 104-1 through 104-8 only ll~,s-..its cells having a specific destin~tion For example, Iter 104-1 only tla~ s cells having the user Ul as a destin~fiQn. Since it is possible that as many as eight cells are received at the same cell time destined for Ul, there are eight paths 111-1 through 111-8 ~~ ,l. Iter104-1 and queue 105-1. Filter 104-8 only l-~n~ cells having controller 109 as a 25 destinati. n There are eight paths 118-1 through 118-8 bel-. cen filter 104-8 and queue 105-8. Queues 105-1 through 105-8 each ~ .---l cells in priority order as described further herein.
Cells are received from user Ul in format 290 shown in FIG. 2. Format 290(FIG. 2) includes an incollfillg virtual circuit idenfifier (VCr) or virtual path 30 i-len~ifi~ (VPI) followed by data. A VPI comprises several VCIs. In ATM, cells are 53 bytes in length. Within input pl~cesc;ng unit 101-l(FIG. 2), a cell is received by a unit 201, the VCI(VPI) is recovered and t~ ,d to validity checLl 211, leaky bucket enfo~r 212, cell counter 213, physical destin~tion table 214, priority table 215 and new VCI(VPI) table 216. Validit,,v checkel 211 deh ...;-~cs whether the 35 VCI(VPI) is valid based on infollnation received from controller 109 via path 107; if the VCI(VPI) is not valid, it causes a cell drop unit 202 to drop the cell. Leaky -bucket enfc l~;er 212 receives eA~t~ traffic pal~ll~t~ l j, for example, eApeuled bandwidth and eA~;t~,d l,ul~iness index, and when enfolc~r 212 dete~ es that a cell of a given virtual circuit results in the p ~ ,te. ~ being eYcee~erl, it causes cell drop unit 202 to drop the cell. A cell counter 213 simply counts cells in each virtual S circuit, and reports such counts to controller 109. A physical ~le;.~ ;Qn table 214 receives infclmation from controller 109 defining the ~estin~tion associated with each virtual circuit, and, based on the VCI(VPI), causes a destin~tion h~Çcllllation insertion unit 203 to insert the de~ A~;on infolll~alion in each cell. A priority table 215 receives a priority value from controller 109, and based on the VCI(VPI), causes a priority i~.. ~AI;on insertion unit 204 to insert thc priority inrc.. ~1;0n in each cell. A new VCI(VPr) tablc 216 receives a new VCI(VPI) value and causes a VCI(VPI) overwrite unit 205 to o-e~ the incorning VCI field of the user cell format 290 with an outgoing VCI in the system cell format 291. Format 291 also includes a desdnadon field as well as a priority field.
Cells ~ ~1 by input ~lvces~;ng unit 101-1 (FIG. 1) are distributed via path 102-1 of i~lt~,r~oml~lion alT~ngernent 103 to all of the filters 104-1 through 104-8. Cells ~estin~l for user Ul are tr~ h~l by filter 1~1 via paths 111- 1 through 111-8 to p~riti7ing output queue 105-1. Queue 105-1 (FIG. 3) includes a sort n~,lw~ 314 which can be constructed in the u~uler described, for example, by 20 Batcher in U. S. Patent 3,428,946 issued re~lu~ 18, 1969, or as described by Huang et al., in U. S. Patent 4,516,238 issued May 7, 1985. Sort network 314 sorts received cells in accol~ce with the priority field, and ~ s the highest prioritycell to user Ul. The ~ ing cells are L~ l~ via paths 313-1 through 313-R(R, being a large n-l-~hcl, e.g., greater than 1000) to delay unit 315, which provides 25 a one cell time delay, and l~ s the delayed cells back to sort n~.h. vll- 314 via paths 312-1 through 312-R. By operation of sort n~,lwvlk 314, older cells within a given pnority are ~ ;lh~i to user Ul before newer cells.
The mdl~ner of operation of queue 105-1 can be better understood by con~ide~ing the eY~nlrle of Table 1 with four active virtual circuits VCIl through 30 VCI4 having priorities 1 through 4"~ ely.

T~ne VCI l VCI 2 VCI 3 VC~ 4 Queue LRn8th C)utput 50% 33% 5% 2.5% P~ P~ P~ P~P~onty P~ P~ P~ P~
BI=2 BI=3 BI=l BIzl 1 2 3 4 1 2 3 4 ~ 1 ~2 ~3 P~4 0 . . . . O O O O Idle 1 1 2 3 4 1 1 1 1 1 *1 2 1 2 . . 1 2 3 . 2 . . 0 3 1 1 2 ~ 2 4 . . . . 0 2 1 1 2 ~2 s 1 . . . 1 1 1 1 1 ~1 7 . . . . 0 1 1 1 2 ~ ~2 8 . . . . 0 0 1 1 3 3 9 1 . . . 1 0 0 1 1 *1 1 2 . . 1 1 0 11 . 2 . . 0 2 0 1 2 ~ 2 12 . 2 . . 0 2 0 1 2 2 13 1 . . . 1 1 0 1 1 ~1 14 1 . . . 1 1 0 lS . . . . 0 1 0 1 2 ~ 2 16 . . . . 0 0 0 1 4 ~ 4 17 1 . . . 1 0 0 0 1 ~1 18 1 . . . 1 0 0 0 19 . 2 . . 0 1 0 0 2 ~ ~2 . 2 . . 0 1 0 0 2 2 21 1 2 3 . 1 1 1 0 1 ~1 22 1 . . . 1 1 1 0 23 . . . . 0 1 1 0 2 ~ 2 24 . . . . 0 0 1 o 3 3 1 . . . 1 0 0 0 1 ~1 27 . . . . O O O OIdle 28 . 2 . . 0 1 0 0 2 ~2 29 1 2 . . 1 1 0 0 1 *1 1 2 . . 1 2 0 0 31 . . . . 0 2 0 0 2 ~ ~2 32 . . . . 0 1 0 0 2 2 33 1 . . . 1 0 0 0 1 ~1 34 1 . . . 1 0 0 0 . . . . O O O OIdle 36 . . . . O O O OIdle 37 1 2 . . 1 1 0 0 1 ~1 38 1 2 . . 1 2 0 0 39 . 2 . . 0 3 0 0 2 ~ 2 Table 1 Bo~ded Ji~er F.y~mrle (Co~ ed) 2~9g640 Table 1 Bounded Jitter Example (Conhnued from previous page) Time VCI lVCI 2 VCI 3 VCI 4 Queue LengthOutput ~ V#=actual 50% 33% 5% 2.5% Pri Pri Pri PriPriorityPri Pri Pri Pri BI=2 BI=3 BI=l BI=l 1 2 3 4 1 2 3 4 Pri 1 Pri2 Pri3 Pri4 . . . . 0 2 0 0 2 ~2 41 1 . 3 4 1 1 1 1 1 ~1 ~ *

43 . . . . 0 1 1 1 2 ~ ~2 44 . . , . 0 0 1 1 3 3 1 . . . 1 0 0 1 1 ~1 46 1 2 . . 1 1 0 47 . 2 . . 0 2 0 1 2 ~ 2 48 . 2 . . 0 2 0 1 2 2 49 1 . . . 1 1 0 1 1 ~1 1 . . . 1 1 0 51 . . . . 0 1 0 1 2 ~ 2 52 . . . . 0 0 0 1 4 ~ 4 53 1 . . . 1 0 0 0 1 ~1 54 1 . . . 1 0 0 0 . 2 . . 0 1 0 0 2 ~ ~2 56 . 2 . . 0 1 0 0 2 2 57 1 2 . . 1 1 0 0 1 ~1 58 1 . . . 1 1 0 0 59 . . . . 0 1 0 0 2 ~ 2 . . . . O O O O Idle Table 1 Rounded Jitter FY~rnP1e 209464~

Note that virtual circuits VCl, VC2, VC3, and VC4 have occnr~ncies of 50%, 33%, 5%, and 2.5%, re~e~ u~ely, and the CA~ ;t~ cell arrivals (acsuming constant bit rate traffic) coll~s~ol1ding to those occup~-~es are shown by ,q,cteri~ on the right in Table 1. Virtual circuits VCl, VC2, VC3, and VC4 have burstiness indices of 2, 3, S 1, 1, respectively (burstiness index is ~i~cu~se~ later herein). Note that at cell time 1, cells are received in virtual circuits 1, 2, 3, and 4, p, ;~ ;es 1, 2, 3, and 4 have 1 cell each, the output priority is 1, and the cell ~ d is in virtual circuit VCl.
At cell time 2, cells are received in virtual circuits 1 and 2, ~iorities 1, 2, 3, and 4 have 1, 2, 1, and 1 cells, l~ ely, the output priority is again 1, and the cell 10 tr~n~mitte~l is again in virtual circuit VCl. At cell time 3, a cell is received in virtual circuit 2, pri~ritie~ 1, 2, 3, and 4 have 0, 3, 1, and 1 cells, les~li~ely, the output priority is 2, and the cell l~n~ ;t~d is in virtual circuit VC2. Note that all cells are n~ ed within (2 times BI) eA~t~i intercell arrival periods from the expected arrival times. The m~ximum jitter in this example is (2 times BI) eA~l intercell15 arrival periods.
An illustrative call setup cell received from a user, e.g., user Ul, is shown in FIG. 4. The five-byte header defines that VCI=l, which defines the cell as a sign~lling cell. VCIl is p~defi~-ed as a valid VCI in ch~L, 21 l(FIG. 2), as having controller 109 as the destinstion in table 214, and having a new VCI in table 20 216 for insertion in the system cell format. That new VCI also serves as a source i~el-~;fi~. At the be.~5;nni-~g and end of the 48-byte data po~ion of the call setup cell(FIG. 4) are fields en~losing the l~.nqi-.in~ data in an ATM adaption layer pl.Jt(Jcol. The data includes a mac~ge type defined as call setup, and three pa~ t~ -the eAp~t~l bandwidth (seven bytes), the e-~te~l bursdness (three 25 bytes), and the l~ucst~ I;on tel~holK numb~,~ (eight bytes). The call setup cell is co~ xl via in~..;onn~ ;Qn arpng~ment 103, filter 104-8, and queue 105-8 to controlla 109. Controller 109 uses the expected bandwidth and expected bu. ~lincss to assign a priority to the call by using a table of the type shown herein as Table 4. If the call is a c~ t bit rate (CBR) call (having BI=1), the priority is 30 det~nined by placing the ~A~t~d bandwidth in one of the ranges of the table. If the call is a st~ticti~ call (having BI>l), the 5~t~1 bandwidth is an average bandwidth, BWA~,8, and the priority is ~ict~, .n;n~d by placing BWA~,g/BI in one of the ranges of the table. Controller 109 also ~.rolms a translation on the requested ~1es~in~tion telephone number to obtain a physical ~lestin~tion~ e.g, user U5, and 35 assigns a VCI for use by user Ul and a VCI for use by user U5 for the call.
Controller 109 returns a setup r~sponse cell of the type shown in FIG. S via path 108, 2~94640 g proces~ing unit 101-8, inle~on~-P,c!;on alT~ngçn~nt 103, filter 104-1, and queue105-1 to user Ul. The setup response cell indicates whether the requested call has been approved, echoes the p~ et~ and defines the ~ssigne~ VCI to be used subsequently by user Ul for the call. Controller 109 returns a similar setup response S cell to user U5. Controller 109 transmits info~ ;Qn via path 107 and input proces~ing unit 101-1 to validity chec~ 211(FIG. 2) defining the ~signe-l VCI asvalid, to ellrol.;er 212 defining the bandwidth and bu~ ess pdlan~te~ for the ~igned VCI, to table 214 defining the physical destin~tion ~soci~ted with the ~igns~ VCI, to table 215 defining the priority a~soci~ed with the a~si n~d VCI, 10 and to table 216 defining the outgoing VCI to be used for inco.~ g cells having the a~igned VCI. If a return inle..o~-n~l;on from user U5 to user Ul is also set up,controller 109 uses the bandwidth and burstiness pa~ t~l~ for the return path toc~lc~ te the priority for the return path. Controller 109 tl~Sl~ similar inrollllalion via path 107 to proce~sing unit 101-5(FIG. 1) to set up the return path.
15 ~Q~ ;n~ Cell Jitter Usin~ P~-D~ Levels The following d~scli~J~ion provides bac~glo~ d iriÇollllalion on the number of delay priority levels l~uiled for an cxemrl~ry ATM fabric. It discusses thc pkc.-o...~ nol~ of cell jitter for CBR (cons~--t bit rate) t~ffic and shows how this ..... ~;.. ~.-- cell jitter can be bu~-d~l by ~llr"cqtine a priority level to a range of 20 frquencies. Based on these results, the nulll~, of priority levels luluil~d for CBR
traffic is det~ ;ned St~ti~tir~l traffic can also be ch~;~ le. ;7~1 by jitter. Even though st~tistic~l VCs (virtual circuits) can tolerate a higher amount of jitter, this jitter must still be absolutely bounded for many st~ti~tir~l applic~tion~ to function correctly. St~ti~tic~l traffic can also be divided into priority levels. A unified 25 priority ~nangemPnt is ~escribe~ which overlaps the CBR and st~ti~tir~l priorities such that both CBR and st~tistir~l VCs exist in the same priority S~CI1I1I1L The total nu~b~r of priority levels lC~luU~d for this co~..binful CBR and st~tistiral traffic is de~f ....~ l to ...~ re~n~le upper bounds on cell jitter.
Cell Jitter for CBR Traf~ c A CBR channel at a given rl~ue"lc~ expects a cell to arrive periodically with a given inter-cell sr~ n~- As an PY~mrle, on a 2.488 Gb/s int~ r~e a 100 Mb/s video call is set up. This call expects to receive a cell every 25 cell times.
This 25 cell period is defined as the e~ t~cl intercell arrival period. Due to conl.,.llion and queuing in the ATM (asynchronous transfer mode) fabric, the cell 35 may not be output exactly when it is expected. "Absolute Jitter" is defined as the amount that the actual cell arrival time differs from the expected cell arrival time.

A low bandwidth virtual chqnnel with an expected hlte,a"ival time of 2000-3000 cell times would not be signifil~ntly arr~t~ by an al solute jitter of300-500 cells, whereas a 300-S00 cell absolute jitter would be dev~t~ting to a high bandwidth virtual channel that eA~ect~,d a cell every 2-3 cell times. A more S ~pl~ sen~a~ive term is the "Relative Jitter". Relative Jitter is defined as follows:

J. Worst Case ~bsnlute Jitter FYpected Intercell Arrival Period This worst case relative jitter ~ se.ll~ the In~ ,, dirre.~nce in earliest arrival to latest arrival that can be eApe~;~d, co,~a..,d to the ~;Ap~;~d intercell arrival time. A
Relative Jitter of 1 to 2 eAp~t~,d intercell afrival times would be very good. A10 relative jitter of many intercell arrival periods would be very poor, and would require signifir~nt burr~ g at the receiver to insure that the playback data does not run out.
Many CBR services such as audio or video expect the next cell to arrive at the e~ ~l time so they can begin displaying the info.l.l~tion A "just-in-time"
cell arrival would ...;n;n-;,~ the overall dday through the switch. However, because lS of cell jitter, just-in-time cell arrivals cannot be ~ual~u~d, and the application is forced to queue up several cells on the receiving end to insure that the playback of infrJ. n~ does not run out of data while waiting for the next ATM cell to arrive.
To the extent that overall jitter in the switch can be reduced, this will also reduce the amount of b.... .........~ g and receiver buildout delay ~uil~,d for CBR services.
In a priority based fab~ic, all cells for a given priority will be output before the first cell of the next lower priority. Within a given priority, cells are output on a first come-first serve basis. That is, all equal priority cells arriving at a given cell time will be h~n~ before the same priority cells arnving at the next cell time are started. This ~c~..ncs the root cause of jitter ~ause one VC could25 havc a cell arriving every other cell time; all of a surlden~ ten or twenty cells having the same priority as the one VC but from dir~nt VCs arrive in one of the open cell times. All ten or twenty of these cells will be ~ before the next cell of the high bandwidth VC, c~u~ing an absolute jitter of ten or twenty cell times. During this interval, cells for the high bandwidth VC keep arriving every other cell time and 30 are queued up. When the high bandwidth VC does begin l, .n~.n;~ g its cells again, they will be l,i.l-~,~-;lle-l every cell time, back-to-back, until the queue is empty.

2~946~0 Calc.~ - of Possible Cell Jitter An upper bound for the worst case ~bsol~lt~, and relative jitter can be ~te.l~ined by a simple technique. Onc det~ ;neS the highest bandwidth channel and the lowest bandwidth channel that are allowed within a given priority band. For 5 this technique, the highest BW VC has an upper limit of 1/2 of the total interface rate. This value will gc~ne~al~ the worst case jitter. One c4~ ules the e~ ed intercell arrival period for this highest bandwidth chqnn~l One then CO~ ut~S N,the ...~ n -.. nulll~r of the low bandwidth ch~vnn~l~ that can be active on the ,. r~ce~ along with the one highest bandwidth ch~rlnel This is given by the 10 following formula:

N BW~J"erf", e - BWH~t A~wod BWLow~t ALlowod The worst case jitter would occur when a cell from all N low bandwidth sources arrived just before a cell from the 1 high bandwidth source. The cell from the high bandwidth VC would have waited its average intercell arrival period, and then just 15 before it was output, N cells from the low bandwidth VCs must be output before the next cell from the high bandwidth VC. The actual int~a~ al interval ~I~.~n cellsfrom the high bandwidth VC is the e~d inte.a.li~al period + N cells. The Absolute Jitter is this actual i,lte~ al interval minus the r~ t~l inlc~ al interval, which is always just equal to N cell times. Relative Jitter is then the 20 Absolute Jitter divided by the ~ct~ inte~ ;val intenral.
Concider the following example. A 150 Mb/s inte- r~e has only a single priority level and has no limit on how small a VC bandwidth can be. For this e, the highest bandwidth VC is li~ted to 1/2 of the inte~ e rate or 75 MB/s. ~rhis gives an e-l~ct~ intercell arrival time of 2 cells. (i.e. a cell should 2S arrive every other cell time.) Now since there is no limit on the ~m~llest VC bandwidth, assume that this bandwidth is--bits/sec. (i.e. very, very small). The number of these low bandwidth sources that can be ~u~pc.l~,d on the rem~ining 75Mb/s is therefore oo. The worst case jitter would occur when a cell from all ~VCs was received just before the cell from the high bandwidth VC reslllting in an 30 Absolute Jitter equal to N or oo celltimes The Relative Jitter is this ~ divided by 2 which is still OO. This unrealistic example is included merely to show that a single priority system with no lower bound on the VC bandwidth will generate up to an oo 2~9464û

amount of relative jitter. Clearly this is to be avoided.
Consider a more realistic e..;~le, still with a single priority system, but now a ..-in;-.~----- VC bandwidth of 64Kb/s is set. The In~Xil~ bandwidth VC
allowed is 75 Mb/s with an intercell arrival interval of 2 cells. The numbe. of low S bandwidthsourcesisN = (lSOMb-75Mb)/64Kb = 1171. The~bsolutejitteris N = 1171 cell times. The relative jitter is 1171/2 = 585 inte.~li~al times. Again, a jitter this large is to be clearly avoided.
('on~ e~ an example with mlllti. le pri~ies and having a very restricted band. The highest BW allowed in a given priority band is 10 Mb/s and the 10 lowest BW allowed in that same priority band is S Mb/s. The t;A~iled intercell alTival time is lS cell times for the highest BW ch~nnel. The llu~bcr of low BW
channels that can be ~,.p~lt~,d is N=(lSOMb lOMb)/SMb = 28. The worst case ~bsolute jitter is N = 28 cell times. The relative jitter is 28/lS or less than 2 ihlte,~li~/al intervals. This repl~is~n~ a very well bolln~e~l jitter specificadon.
All that is neces~ to c~lc~ te the relative jitter is the ratio bel~ecn the highest BW channel within the priority band and the lowest priority channel within the priority band. If the actual bandwidth ~ tri~ltion among the prioritylevels results in li~ ratios, then the largest ratio should be used. In the previous eY~mple, the highest BW tb lowest BW ratio is lOMB/SMB = 2.0, and for a 20 bandwidth ratio of 2 the .~ abso!ute jitter will be less than 2.0 int~,.~li~al times. The previous c~lc~ tinn resulted in a relative jitter of 28/lS, which is within this range. Table 2 lists the ..~ .. relative jitter for various ratios of highest bandwidth to lowest bandwidth.

Largest High Ratio Worst Case Relative Jitter BW L~w S 1.0 Sl.0 Inle.allival Interval 1.25 ~1.25 I~ al Intervals 1.5 ~l.S ~ .~liVal Intervals 1.75 ~1.75 Inte~ l Intervals 2 S2.0 Int~,~li~al Intervals 3 S3.0 Illtc.~li.~al Intervals 4 <4.0 Inte,d~ al Intervals S <5.0 Inlcla~ al Intervals lS 10 S10 Inte.~li~al Intervals <'~0 Illte~ ral Intervals <50 Illt~ al Intervals 100 <100 Int~ /al Intervals 200 <200 Int.,.~li~ral Intervals 500 5500 Illt~ al Intervals 1000 <1000 Inte.a.li~al Intervals 2000 S2000 I~ al Intervals 2~ S000 SS000 Inle,~li~al Intervals Table 2 Upper Bound of Relative Jitter for Given Ratio of BW~gh to BWLOW
30 Number of Priority Levels Required for CBR Tramc The nu~ of priority levels is Aet~ ;neA solely by the bandwidth of the int~ - ~CC, BW~, the minimllm bandwidth allowed on the illte.race, BWMin~ and the extent that jitter on the switch is to be bounded. The ratio BW gives the total bandwidth range that must be covered by the various priority bands. As shown 35 earlier, the rado BW gh gives the bandwidth range that is covered within one priority band. Since this is a ~COllleLIiC reladonship, the nulll~r of priorities, P, required can be dett.~ ed by the simple relationship:

20946~0 BWHi~ BW~F
BWL~W BWM1n Given the ~ x;~ .. and ~ bandwidths ~Uppolt~l on the link, and the number of priority bands for CBR traffic, the ratio of BW gh can be co~lpu~d for a given priority band. Given this ratio, the upper bound for the jitter on the switch is S also hlown. This is shown in Table 3.

BW~p BWM;T~ Numberof BW gh Ratio WorstCaseRelativeJitter CBR Priorities within 1 Band 2.488 Gb 9.6 Kb 1 260,000 <260,000 Inte~ /al Periods 2 509 SS09 Inlela.li~Jal Periods 3 63.75 <63.75 In~e~ al Periods 4 22.5 S22.5 Int~,l~lival Periods 6 8.0 ~8.0 Inlel~lival Periods 8 4.75 S4.75 Inter~lival Periods 12 2.82 S2.82 Int~.~li~al Periods 16 2.17 <2.17 Int~ al Periods 24 1.68 ~1.68 Inle.~lival Periods 32 1.47 <1.47 Int~l~liv~l Periods 48 1.30 Sl.30 Int~l~lival Periods 64 1.21 <1.21 Inle.~lival Periods Table 3 Number of P2iorities for CBR and Resulting Jitter In ~ldition to the number of priorities listed in Table 3, one nd~ition~l priority 20 should be ~ te~l for all CBR traffic below the ,n;~ u~pol~d bandwidth.
No reladve jitter ~ci r.~ O~ can be guaranteed for this lowest priority, as it could potentially approach infinity.
It should be a goal of the ATM switch to bound relative jitter to the ~m~llest possible ~rnollnt A practical limit would be to bound the ~Ax j~ n~
25 relative jitter to within 2 expected in~ ulival periods. In this fashion, all CBR
applic~tion~ would only have to use a build out delay at the receiving end of 2 ~ldition~l cells. Any additional delay is critical, especially in 64Kb/s or lower voice traffic. This may be a good re(luire~lelll for the mqxi.. ~ cell jitter allowed. Given this requh~ ll for an eYempl~ry 2.488 Gb/s switch, the lt;col~nu ~ o4 number of 30 priorities for CBR traffic would be ~t~n 20 and 24 priority levels. If 20 priority levels were used, this would include the ~dition~l priority level for all CBR traffic below the lowest BW, for which no jitter specifi~tion can be ~;u~anteed.

209~640 A practical scale for grouping bandwidths into priority bands to guarantee relative jitter within 2 expected inte~ val periods is to set the BW gh ratio for each priority band exactly equal to 2Ø In this &shion, it is very easy to co~ le the priority bands as shown in Table 4.

20946~0 .

Prioritv Band Bandwidth Range BWLOW BWHigh ~1244 Mb/s ~2488 Mb/s 2 ~622 Mb/s S 1244 Mb/s 3 >311 Mb/s 5 622 Mb/s 4 >155.52 Mb/s S 311.04 Mb/s S >77.76 Mb/s ~ 155.52 Mb/s 6 >38.88 Mb/s S 77.76 Mb/s 7 >19.44 Mb/s 5 38.88 Mb/s 8 >9.72 Mb/s S 19.44 Mb/s 9 >4.86 Mb/s ~ 9.72 Mb/s >2.43 Mb/s S 4.86 Mb/s 11 >1.215Mb/s <2.43Mb/s 12 >607.5 Kb/s S 1215 Kb/s 13 >303.8 Kb/s < 607.5 Kb/s 14 >lSl.9 Kb/s S 303.8 Kb/s lS >75.94 Kb/s S lSl.9 Kb/s 16 >37.97 Kb/s ~ 75.94 Kb/s 17 >18.98 Kb/s S 37.97 Kb/s 18 >9.492 Kb/s S 18.98 Kb/s 19 >4.746 Kb/s S 9.492 Kb/s >Ot S 4.746 Kb/s tNote: No Jitter Spec can be guaranteed for the lowest BW range.

Table 4 ~uposed Priority Bands for CBR Traffic with BW gh = 2.0 P~ .o. ;1~ Bands for Statisffcal Tramc Relative Jitter is also an un~ul~lt pala.ll~lel for st~ti~tir~l traffic. In 30 st~ tic~l traffic there are some applications that require very low delay such as variable bit rate video. This application has very high bandwidth and relatively low burstiness (BI = 2 to S). Variable bit rate voice also requires moderately low delay.

VBR voice has relaively low bandwidth (8-32Kb/s) and low burstiness (BI = 2 to 5).
These must hold a relatively tight relative jitter sperifiration, to ~:ualanlce that cells do not arrive "too late" and the delay buildout buffer on the receiving end does not b~o,l,e empty. Even though this is st~tistir~l traffic, it is jitter sensitive.
Other applir~tion~, including file transfers and screen image dumps, usually do not require low delay. These application~ can be either low or high average bandwidth, however they tend to be very, very bursty (BI=1000's). If multiple priorities did not exist within st~fi~tic~l traffic, these very bursty applications would ~ignifir~rltly inte,lul)t the delay sensitive applications resulting in 10 excessive queuing delays and very large buffer buildouts on the receiving end to co,.~ sate for the delay v~ri~tion~
The general priority philosophy within st~ishçql traffic is based on both bandwidth and b~ ess of the VC. Just as in CBR traffic, to minimi7~ jitter effects, higher priority is given to the higher BW VCs and lower priority is given to 15 the lower BW VCs. However, conc~- ..; ng burstiness, higher priority is given to the low burstiness VCs and lower p~iority is given to the high bul~Liness VCs.

~0946~0 Low Bul~ ess High Burstiness High Highest Priority ~ iUIll Priority BW
(VBR Video) Low Mediuln Priority Lowest Priority BW
(VBR Voice) (File Transfers) Table S
l 5 Priority Philosophy for St~tictic~l T~ffic High speed and low b~ es~ appli~l;o--c such as VBR Video receive the highest st~ti~tic~l priority. Medium priority goes to low bandwidth, low bU1~eSS VCs such as VBR voice. Medium priority also goes to high bandwidth, high bul~u~ess 20 VCs. necn~lse of their high average bandwidth, there cannot be many of the VCs S?mUll~neO~cly~ the.~,f~ their contrib~lti~m to the total queuing delays are lcss.
Finally lowest priority goes to low bandwidth high bul~iness VCs, such as cCc~;on~l megabyte file transfers, that are not delay sensitive. Not only do they have large burst lengthC~ their average bandwidth is low. Th~ref~ many of these 25 VCs could be s;m~ ewcly eq~fip~ on a given interf~ce- The res~llting collision of many of these high bursty VCs creates very, very large queuing delays.
The ill~ul t~t ratio to use when allocating a priority to a statistical VC
is the BI g ratio. Large BI 8 ratio VCs will receive the highest priority.
Very small BI g ratio VCs will receive the lowest priority. BWAV8 ~ Sen~S
30 the long term average bandwidth. Over a shorter interval, these st~tictic~l VCs may operate at a much higher peak bandwidth, but that is not iln~ t. The long term average bandwidth is the cridcal factor used in setdng up the call and dete.llfining jitter bounds.

Intermixin~ CBR and Statistical Tramc - Unified Priority S,,elrul..
Col-sider the following example where one variable bit rate (VBR) video chq-nn~l operates at an average BW of 30 Mb/s. However, it is slightly bursty;
assume that BI = 5. Its eA~ct~ intercell arrival period (if the average BW were 5 evenly spaced on a 2.488 Gb/s intf r~ce) is 83 cell times. If it were CBR traffic we would want to hold its Relative Jitter to less than 2 intercell amval periods.
However, since it is bursty traffic al~ywa~, the receiver is ~1esi~ned to have a buildout buffer that can acc~... ~e a larger relative jitter. It cannot, ho.. _.er, tolerate a very large relative jitter, otherwise, the delay buildout buffer would be very large, 10 and the overall delay would grow to I ~c~?lable levels for real-time voice/video applic~tions. Assume that this application can tolerate a relative jitter of up to 5-10 intf;l~,ival periods. This turns out to be a reasonable numbel since the source has BI=5. Because it is bursty, it is c~ct~d to tolerate SO"I~ .. l,at more jitter.
Since this is a stqti~tirql VC, if the highest priority were qlloc~q~teA just 15 below the CBR traffic, an i~ ing phe~o~r~o~ could result. The 4.7 Kb/s to 9.5Kb/s CBR band would have higher priority than this 30 MB/s, Bk5 traffic. One cancom~ulci the worst case jitter by vlloc~q~ting the rest of the 2.488 Gb/s i~le~ r,-ce (2.458 Gb/s rt...~i~.ing) to the lowest BW CBR call in that priorit,v level (4.7 Kb/s). This means that potentially 522,978 CBR VCs at 4.7 Kb/s can exist ~imull~neously with20 the 30 Mb/s VBR video chq-rtnel The worst case ~bsolute jitter would be 522,978 cell times. The worst case reladve jitter would be 522,978 / 83 = 6300 intercellarrival periods. Even though this is a stqti~ticql VC, there is no way that the VBR
video applicadon can handle a reladve jitter in this range.
Certain stq-tictir~q~l VCs must have higher priority than some CBR VCs.
25 In other words, the CBR and st~ti~tical pniorities must be il~t~ ed CBR traffic should not qlltomqtir~lly have higher priority than sPti~ticql traffic.
If we i,~t..... i~ CBR traffic and stq-ti~cql traffic on the same priorit,v scale, two illl~.~nt cons~ t~ must be observed:

1. The reladve jitter for any CBR VC must sdll be able to be kept to ~ 2 intercell arrival periods.

2. The reladve jitter for any stvti~icql VC must be kept to within a revqconq-b!e nu~l~ber of intercell arrival periods. This number should be bounded to a finite value, so that cu~luluer applicadon e~luip~ nt can be 2~94640 built accoldillgly.

Low burstiness VCs, such as VBR video and VBR voice must be bouncl.-A to within a very low number of intercell arrival periods. Higher burstiness VCs, such as file transfers, can be bounded to a much higher number of intercell arrival periods. A
5 relative jitter bound is still illl~l~lt for these VCs, because it defines the amount of l~Clll~ )l,y that is required on the receiving side to receive a file.
It is plopow~d here that a re~con~ble bound for relative jitter that appears to meet all st~ticSi~ ~1 applic~tionc is to set the m~Yim~lm bound on the relative jitter to be l,lopullional to the burstiness index of the traffic. The specific method outlined 10 below with BW gh = 2.0 in each priority band will limit the relative jitter to 2~BI
for a statict1cal VC with a given Bul~ ess Index, although tighter bounds can beobtained by increasing the number of priority levels. A CBR VC (BI = 1.0) would have its relative jitter bo~ d~d to within 2 intercell arrival times. A low bursty st~tictic~l VC (BI = 2 to 5) would have its relative jitter bounded to within 4 to 10 15 intercell arrival times lespecli~ely. This will t_en work for VBR video or VBR
voice traffic. A more bursty VC (VC= 50) would have its relative jitter bounded to within 100 intercell arrival times. Finally a highly bursty VC (BI = 1000) wouldhave its relative jitter bo~ln~ecl to within 2000 intercell arrival times.
In this fachinn, the average BW and the bul;~iness will be specified for 20 an applic~tiQn The switch will police the input to ~u~t~ that these values are not eYcee~l~ The applieation will ~ut~mati~ ~lly know the .. .~ . . . buildout buffer that it must allow on its receiver to gU~al~t~ that it will not run out of cells. CBR is now just a special case of statictic~l traffic, with BI = 1Ø
Tbe following scheme is usable for shanng the same priority levels 25 ~h.~ CBR and statictic~l traffic:

1. When a CBR call is set up, its BW is added to the total of BW of all VCs on the interface This total BW
of all VCs on the int~ re cannot be overalloc~te~.

2. When a st~tictic~l call is set up, its long term Average BW is added to the total of BW of all VCs on the inlelrace. This total BW of all VCs on the interface cannot be overallocated.

209~640 -3. When a CBR call is set up, it is given the priority level co~ ~n~ling to its bandwidth. Higher BW calls receive higher priority. Lower BW calls receive lower priority.

4. When a st~ti~ti~al call is set up, it is given the prioritylevelcull~;,~ndingtoits BI g factor. A
st~ti~ti~ VC will be ~lloc?~e~ the same priority as a CBR call of BI times its bandwidth.

5. If BW g = 2.0 in each priority band, a CBR call will have its relative jitter bounded to within 2 intercell arrival periods.

6. On this same priorit,v scale, a st~tisti~ ~l call will have its relative jitter bounded to within 2~BI intercell arrival pçrior 15 Unified ~ c ;l~ S~Je~l- .,... - Example~
Certain CBR traffic will have lower prioqity than some st~ ~tic~l calls.
With the above scheme, it is still possible to ~ual~tcc a relative jitter of less than 2 intercell arrival times for these CBR VCs, even though there is st~ti~ti~ traffic with higher priority above it.
An eY~mF'- is in order. Consi~ler one 64 Kb/s voice call. It is a CBR
call with an e~ e~l interceU arrival time on the 2.488 Gb/s in~v. ri~e of 38875 cells. A relative jitter of 2 intercell arrival times must be ~ r~1 Consi~ler a large number (248) of bursty LANs with an average BW of 10 MB/s. (Note that the peak BWs will be higher.) Assume each LAN has a ~A~ bu~ s of BI=100.
The 248 10 MB/s (average BW) and the single 64Kb/s CBR voice call will all fit on the same 2.488 Gb/s interface (99.7% OC~;~J~ Cy)- Each 10 Mb/s LAN VC is allocatedapriorityco~ ,on~lingto loMbls(average) or100Kb/s B d Table 4 this receives priority 15. The 64Kb/s CBR call receives priority 16. Note that all of the highly bursty LAN VCs receive a higher priority than the single 64 Kb/s voice call.
The CBR voice call still m~intAin~ its relative jitter to within 2 intercell arrival periods. The worst case example is if right before a cell for the 64 Kb/s VC
S was received, all 248 LANs received a burst of 100 cells before the next cell for the 64 Kb VC arrived. To make matters worse, since they are 10 Mb/s average channels, each channel should have received an average of 156 cells (worst case is 2 bursts of 100 cells) ~l~.e~,n consecutive cells for the 64 Kb/s call. They could not have received more than this because the input policing would have limited the VCs 10 to a BI=100, and worst case two bursts within this time period. Since all of the LAN
cells are a higher priority than the 64 KB/s VC, they will be output first, resulting in a worst case absolute jitter of 248$2* 100 = 49600 cell times. The relative jitter is only 49600/38875 = 1.276 illlwal.ival periods. The upper bound for relative jitter for CBR t~ffic has been ~ n~i~ l within 2 intercell arrival periods even though 15 there is sig~iricAIll st~Ati~tic-Al traffic with a higher priorit,v.
A second c~calnplc is in order to illustrate that the relative jitter for statistical calls can be ~ inl;.in~d to 2*BI intercell arrival times. Assume 1 VBR
video call at 3 Mb/s average bandwidth with a BI = 5. This has an expected intercell arrival period (A~ ming evenly spaced cells) of 829 cell times on a 2.488 Gb/s BWAVg = 3 Mb/s 20 interface. ThisisAllocAte~apriorityco~ ondingto BI=5 600 Kb/s. From Table 4, this results in a priority value of 13. 600 Kb/s is close to the highest BW allowed in this priority level. To COm~ the worst case jitter, assumethat the ",...Aini.~g bandwidth (2.488 Gb/s - 3 Mb/s = 2.485 Gb/s) is made up of calls with the lowest BW allowed in this same priority class (303.75 Kb/s). This results in 25 N = 8182 VCs at 303.75 Kb/s. If all 8182 VCs arrived just before the cells of the 3 Mb/s VBR call, the worst case A~bso!llte jitter would be 8182 cell times. The relative jitter is 8182 / 829 = 9.87 intercell arrival periods. This is within the upper bound of 10 intercell arrival periods that was e~,t~ for a BI=5 VC. In this example, the receiver for the VBR video call can implement a minim~l buildout buffer of 9 cells 30 and gualantee that jitter will always fall within this ammlnt The buildout buffer will never becollle empty, and it will impose the ...i~ .. excess delay on the video call.
Number of Unified P~ io~ Levels for CBR and Statistical Tramc The total null~bcl of priority levels le~luih~d for the fabric is determined solely by the following four criteria:

1. The bandwidth of the Tnte~ce, BWIF-2. The ~ ,n~ bandwidth allowed on the interf~e~
BWMin 3. The ,.u~i...,~..~ burstiness allowed for a VC, BIM,~

4. The extent that jitter is to be bo~n~le~l on the switch.

The factors are the same that were required for determining the CBR priority levels, with the addidon of BIM",~. The total Bandwidth-Bursdness range that must be covered by the various priority bands is given by the rado BW M . As shown earlier, the rado BW gh gives the bandwidth range that is covered within one 10 priority band. Since this is a gc~ ,1. ;c reladonship, the nu~ of priorides, P, required can be dele~ ned by the simple relationship:

BW High BW~ BIMa%
BW L~w, BW Min If the rado BWH gh is chosen to be exactly equal 2.0, then the 20 bandwidth ranges given in Table 4 for CBR traffic are used. However, Table 4 is mo~1ifipA by adding 15 q(l-iitionql priorities to handle bursty traffic. This in.il~ases the low end of the priority scale, as the burstiness h~cleases for low bandwidth VCs. Table 6 recreates Table 4, but extends it to handle various burstiness factors.

Bandwid~ Range Assigned Priori~ Level BWIow BWHigh BI=l BL2 BL4 B1~8 BL<16 BL<64 BL256 BL1024 BI<4096 (CBR) >1.244 Gb/s<2.488 Gb/s 1 2 3 4 5 7 9 11 13 5~622 Mb/sS 1.244 Gb/s 2 3 4 S 6 8 10 12 14 >311 Mb/s S6æMb/s 3 4 S 6 7 9 11 13 lS
>155.52 Mb/sS 311.04 Mb/s 4 S 6 7 8 10 12 14 16 >77.76 Mb/sS 155.52 Mb/s S 6 7 8 9 11 13 lS 17 >38.88 Mb/sS 77.76 Mb/s 6 7 8 9 10 12 14 16 18 1 0>19.44 M~/sS 38.88 Mb/s 7 8 9 10 11 13 lS 17 19 ~9.72 Mb/s< 19.44 Mb/s 8 9 10 11 12 14 16 18 20 ~4.86 Mb/s S 9.72 Mb/s 9 10 11 12 13 lS 17 19 21 ~2.43 Mb/s S 4.86 Mb/s 10 11 12 13 14 16 18 20 22 >1.215 Mb/sS 2.43 Mb/s 11 12 13 14 lS 17 19 21 23 15>607.5 Kb/sS 1215Kb/s 12 13 14 lS 16 18 20 22 24 >303.8 Kb/sS 607.5 Kb/s 13 14 lS 16 17 19 21 23 25 >lSl.9 Kb/s< 303.8 Kb/s 14 lS 16 17 18 20 22 24 26 >75.94 Kb/s5 lSl.9 Kb/s lS 16 17 18 19 21 23 25 27 >37.97 Kb/sS 75.94 Kb/s 16 17 18 19 20 22 24 26 28 20>18.98 Kb/sS 37.97 Kb/s 17 18 19 20 21 23 35 27 29 >9.492 Kb/sS 18.98 Kb/s 18 19 20 21 22 24 26 28 30 ~4.746 Kb/sS 9.492 Kb/s 19 20 21 22 23 25 27 29 31 S 4.746 Kb/s 20 21 22 23 24 26 28 30 32t Boul~dforRelativeJiuer 52 S4 S8 S16 S32 S128 SS12 S2048 58192 25antenurivalperiods) fNote: No Jitter Spec can be gu~a,l~eed for the lowest P~onty range.

Table 6 ProposedUnifiedPrioritySp~ Bandswith BW gh =2.0 Bul~iness levels BIS32, BI<128, BIS512, and BIS2048 were omitted for table 20g46~0 brevity; however they can be easily colllpuled. Table 6 is not used to determine the priority level, it is shown only to visualize how the bursdness affects the priority level assigned to st~ti~ti~ ~l calls. In practice, only the CBR column and the bandwidth ranges are required; however they are co.ll~ul~ from priority 1 to S priority 32. For statistical VCs, the rado BI g is used and the result is looked up in the CBR column. Table 6 has 32 priority levels that will allow the fabric to handle traffic from CBR at 2.488 Gb/s to 4.7 Kb/s with BI=4096. A bound for the reladve jitter can be absolutely guaranteed for any VC, CBR or st~ti~ti~l The bound for the reladve jitter on st~ti~ti~l VCs is d~ ~,....inPA only by its bursdness, 10 not by its bandwidth, and the same reladve jitter bound applies for all bandwidth ranges except for priority level = 32. By S~Ci~illg bul~iness, m~ximl~m relativejitter can be ~ u~u~llecd for an application and hal.l~ lesign~ ~plo~,liately.
Looser or dghter bounds for the reladve jitter can be realized by using dirre~ t numbers of priority levels. This is shown in Table 7. For co~a,;son, 15 BWIF, and BIM,~ have been held COrisl~nl at 2.488 Gb/s and 4096 respecdvely, such that 32 priority levels gives a BWHigh = 2.0, which .n~lcl ~s the priority bands in Table 6. In Table 7, the nulll~ of priority levels is varied and coll~,~n~ g bounds on the Reladve Jitter are given.

20946~0 Number of BW gl Ratio Worst Case Relative Jitter Priority within 1 Band (Intc.~li~al Periods) Levels CBRSt~tictie~l 4.3 E9 4.3 E94.3 E9*BI
2 65536 c65536~65536*BI
3 1625.5 S1625.5S1625.5*BI
4 256.0 S256.0S256*BI
6 40.3 <40.3S40.3*BI
8 16.0 <16.0S16*BI
12 6.35 S6.35S6.35*BI
16 4.00 ~4.00S4*BI
3.03 S3.03S3.03*BI
24 2.52 ~2.52S2.52*BI
28 2.21 52.2152.1.*BI
32 2.00 ~2.00S2*BI
48 1.58 Sl.58Sl.58*BI
64 1.41 Sl.41Sl.41*BI
96 1.26 Sl.26Sl.26*BI
128 1.19 <1.19<l.l9*BI
~ 1.00 Sl.00 SBI

Table 7 Number of Unified S~;l,.ull Priorities and Resulting Jitter

Claims

1. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from a first one of said users to a second one of said users, and transmitting information to said second user during said call based on said expected traffic parameters and with less than a maximum jitter, wherein said transmitting comprises determining a priority based on said expected traffic parameters for said call, and transmitting said information to said second user during said call based on said priority and with less than said maximum jitter, wherein said call is a statistical call, said expected traffic parameters include an average bandwidth parameter, BW AVg, and a burstiness index, BI, and said priority is determined based on BW AVg and BI.

2. A method in accordance with claim 1 wherein said priority is determined based on a ratio, BW Avg/BI.

3. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from a first one of said users to a second one of said users, and transmitting information to said second user during said call based on said expected traffic parameters and with less than a maximum jitter, wherein said transmitting comprises determining a priority based on said expected traffic parameters for said call, and transmitting said information to said second user during said call based on said priority and with less than said maximum jitter, wherein said determining comprises selecting said priority from a priority table where some statistical calls are given higher priority than some constant bit rate calls.

4. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from a first one of said users to a second one of said users, and transmitting information to said second user during said call based on said expected traffic parameters and with less than a maximum jitter, wherein said transmitting comprises determining a priority based on said expected traffic parameters for said call, and transmitting said information to said second user during said call based on said priority and with less than said maximum jitter, wherein said determining comprises selecting said priority from a priority table having a number, P, of priority bands and having a maximum bandwidth, BW High, and a minimum bandwidth, BW Low, specified for each of said bands, wherein said call is a statistical call, said expected traffic parameters include an average bandwidth parameter, BW AVg, and a burstiness index, BI, and said selecting comprises determining the one of said bands including BW AVg/BI.

5. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from a first one of said users to a second one of said users, and transmitting information to said second user during said call based on said expected traffic parameters and with less than a maximum jitter, wherein said transmitting comprises determining a priority based on said expected traffic parameters for said call, and transmitting said information to said second user during said call based on said priority and with less than said maximum jitter, wherein said determining comprises selecting said priority from a priority table having a number, P, of priority bands and having a maximum bandwidth, BW High, and a minimum bandwidth, BW Low, specified for each of said bands, wherein the ratio BW High/BW LoW is a constant for each of said bands.

6. A method in accordance with claim 5 wherein said maximum jitter is a worst ease relative jitter.

7. A method in accordance with claim 6 wherein said call is a constant bit rate call comprising fixed length cells and said worst case relative jitter, in intercell arrival periods, is given by said constant ratio, BW High/BWLow.

8. A method in accordance with claim 6 wherein said call is a statistical call comprising fixed length cells, said expected traffic parameters include a burstiness index, BI, and said worst case relative jitter, in intercell arrival periods, is given by the product of BI and said constant ratio, BW High/BW Low.

9. A method in accordance with claim 6 wherein said priority table is used for constant bit rate calls only and may include one additional priority band below said P priority bands, and with P and BW High/BW Low satisfying a relationship [BW High/BW Low] P = BW IF/BW Min, where BW IF and BW Min are the maximum and minimum bandwidths between each of said users and said switching system.

10. A method in accordance with claim 6 wherein said priority table is used for both constant bit rate calls and statistical calls and may include one additional priority band below said P priority bands, and with P and BW High/BW LoW satisfying a relationship [BW High/BW LoW] P =
BW IF*BI Ma/BW Min, where BW IF and BW Min are the maximum and minimum bandwidths and BI Max is the maximum burstiness between each of said users and said switching system.

11. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from said first user to a second one of said users, determining a priority based on said expected traffic parameters, and transmitting information to said second user during said call based on said priority, wherein said call is a statistical call, said expected traffic parameters include an average bandwidth parameter, BW AVg, and a burstiness index, BI, and said priority is determined based on BW AVg and BI.

12. A method in accordance with claim 11 wherein said priority is determined based on a ratio, BWAvg/BI

13. In a switching system serving a plurality of users, a method comprising receiving one or more parameters concerning the traffic expected on a call from said first user to a second one of said users, determining a priority based on said expected traffic parameters, and transmitting information to said second user during said call based on said priority, wherein said determining comprises selecting said priority from a priority table where some statistical calls are given higher priority than some constant bit rate calls.

14. A switching system serving a plurality of users comprising means for receiving one or more parameters concerning the traffic expected on a call from a first one of said users to a second one of said users, and means for transmitting information to said second user during said call based on said expected traffic parameters and with less than a maximum jitter, wherein said transmitting means comprises means for determining a priority based on said expected traffic parameters for said call, and means for transmitting said information to said second user during said call based on said priority and with less than said maximum jitter, wherein said determining means comprises means for selecting said priority from a priority table where some statistical calls are given higher priority than some constant bit rate calls.