COST257 TD(97) 2 tJ Espoo, May 1997
Simple Integrated
Media Access (SIMA)
Kalevi Kilkki Nokia ResearchCenter E-mall:
[email protected]
Abstract The basic objectives of future Internet are to increasethe network capacity, to offer a practical real-time service, and to develop a feasible charging scheme.Theseobjectives introduce very strict requirementsfor the trafilc control system. This paper presents a new simple approach for traffic management: Simple IntegratedMedia Access (SIMA). According to the SIMA concept each customer shall define only two issuesbefore a connection establishment:a nominal bit rate (NBR) and the selection betweenreal-time and non-real-time service classes.NBR forms the basis of charging, and it defines how the network capacity is divided among different connectionsduring overload situations. Simplicity of SIMA meansthat, on the one hand, the network operator does not guaranteethe continuous availability of nominal bit rate, and on the other hand, the user is allowed to send data with any bit rate independently of the NBR. The strength of SIMA lies in its wide areaof applications. There is no needto build complex systemswith several service classeseachappropriateto only certain applications.This paperis basedmainly on an unfinished IETF draft (for this reasonsome figures and tables are in text format). Information about SIMA is also available from http://www-mc.nokia.com/sima/.
1. introduction The Internet is at a phase of great changes.There are several stringent new requirementsfor the network becauseof two reasons:the invasion of new users, and the rapid developmentof new applications. These requirements mean that network capacity must rapidly be increased, real-time service has to be fundamentally improved, and a feasible charging schememust be introduced. The current Internet approach for meeting these requirements consists of several service specifications, ResourceReservationProtocol, QoS routing, etc. We can make an interpretation on the basis of the service specifications that the basic philosophy of Internet development is to define different services for different basic communication needs.There seems to be demand for three elementary services: first one for very reliable and high quality connections,second for connections with less stringent quality requirements, and third one for data connections which can smoothly adapt their bit rate. As the requirements of these elementary classesdiffer significantly, an obvious approachis to have different service specifications like the servicesspecified by InternetEngineeringTask Force (IETF) or ATM Forum. The supposed advantageof this approach is that by dividing the service specification task into several smaller parts the specification processis easier than if the all the service types were included in a single specification. However, this advantageis somewhat questionablebecausethe whole service concept (with all the different service types) is what the network operatorshould manageand sell to customers and what the customer should buy and use. In particular, most Internet customers will be reluctant to learn several 1
complicated services which may even have very different structures,traffic parameters,charghtg schemes, etc. For real marketing purposes,the Internet service packagemust be simple, much simpler than what the current service models directly make possible.There are two main approachesto meet this requirement:to hide the complexity of network service from the end-useror to design an entirely new integrated service which is able to satisfy all the primary customerneeds.The approachusedin this documentbelongsto the later category.
2. Service specification The primary idea of the SIMA service is to maximize the exploitation of network resourceswith a simple control scheme while keeping the ratios of QoS levels offered to different flows unchanged under changeablet&tic conditions. The maximization is basedon threekey features:all flows with different QoS requirementssharethe total capacity of every link, the network attemptsto avoid any unnecessarypacket discarding, and flow (or call) level blocking can be totally avoided. The approximate constancy of QoS ratios and simplicity are achieved by using 8 priority levels which make possible a fair packet discarding scheme Inside the network without keeping track on the traffic of every flow (the technical details are presentedin chapter4). The SIMA specification coversthe whole Internetserviceincluding charging,QoS and performanceaspects, and traffic control functions in the network As opposedto most service specifications, charging is the starting point of the SIMA concept.The prevalent charging schemeapplied by Internet operatorsis a flat rate one with a constantmonthly fee. Although this schemeis most reasonablewhen the network service is basedon the best-effort principle, many network operatorsmay still be willing to apply this scheme even with more complicated servicemodels.TheSIMA servicemodel attemptsto meet this demand. 2.1 Nominal Bit Rate When the network operatoroffers the SIMA service,a customerfirst pays for some Nominal Bit Rate (NBR, kbit/s) and then he/she can trade the speedfor QoS. Let us assumethat a user pays X Ecu/month. This chargeis translatedto a NBR using an arbitrary function. The function NBR = F(X) could be linear, but thereis no reasonto specify the relationship betweenNBR andcharging.If NBR is permanent,it is probably related to an interface (if we assumethe same organizationthat owns the right to use a network interface buys the NBR). The next level of NBR is the NBR assignedto a user (or IF’-address).The bottom level is the NBR of a flow (determined,for instance,by a pair of IP addressandport number). Depending on the available information and the network capabilities, there are three basic approachesto manageNBRs. The simplest approachis to assign the NBR only to an interface, which means that the network measuresthe whole traftlc going through the interface and handles this traffic as an indivisible entity. The usersand flows that sharethe NBR obtain approximatelythe same QoS. In the secondapproach eachuser (identified by an IF’ address)has his/her own NBR. Now the network measuresthe total traffic generatedby a user, anddifferent flows competewith eachother on a best-effortbasis. Both these approacheshave the drawbackthat they do not separatedifferent applications properly: a highspeedfile transfer may disturb other flows, e.g.,real time video connections,althoughthe user may consider the file transfer a background process which uses only the capacity left by other more demanding applications. Therefore, as regardsthe performanceand QoS of the SIMA service the most useful approach is the one whereevery flow has its own NBR. Later in this documentwe supposethat the network is capable to identify andmeasureevery flow, andthat every flow has its own NBR. The question how theseNBRs are determined and managed can be left for network operators, and is, therefore, out of the scope of this document. 2.2 Real-time vs. Non-real-time The other part of the SIMA service concept is the possibility to request a real-time service. The user Is entitled to him/herself determine whether the flow is a real-time (it) or non-real-time (nrt) one. In practice, this decision can be made usually at the application level: a real-time service is requested only for 2
interactive audio or video applications. If a real-time service is requested,the SIMA network attempts to offer as short delay and small delay variation aspossible by using small buffers reservedonly for real-time Connections. The expenseof this choice is that, if there aretraffic variations of time scale horn 0.1 ms to 5 ms, small real-time buffers cannot filter these variations (see illustration in chapter 4.1). Therefore, the measurementfor the priority determination shall be more sensibleas regardsthe traffic variations in case of real-time service than with non-real-time service. If the user changes a VBR connection from mt-service to it-service without changing NBR or tmftlc process,the delay will decrease,but the cell loss ratio may increasebecausereal-time measurementgives worse priorities during peak rates. If the user wants to obtain the samequality, this impairment of loss ratio should be compensatedby increasing NBR. Real-time-servicecould, in this respect,be more expensivethan non-real-time service although there is no difference in the actual tariffs. In consequence,if the application is a real-time one, it is advantageousfor the user to selectthe real-time class, becauseit is the only way to attain small delay and delay variation. Furthermore,if the traffic variations are small enough,the user may always select a real-time service,becausethere is no difference in cell loss ratio betweenrt and mt-services. In contrast, if there are significant traffic variations as with typical data applications, non-real-time service gives better quality, that is, smaller packetloss ratio. 2.3 Quality of Service expectations The total SIMA service requestedby a user consistsof a nominal bit rate and of a possible real-time service request.This half of the service is as clear and reasonableas possible. The other half of the service is the expectedQoS of the flow, or actually, the expectedQoS of the application that the customer uses over the SIMA network An essentialissue for the successof the SIMA service is how reasonableand acceptablethis part of the service conceptwill be. Most customers have experience of circuit switched networks (like telephone networks) and packet networks with best-effort service (like the current Internet). In a circuit switched network a busy period meansthat the call blocking probability increases.In packet networks the packet loss ratio increasesduring busy periods, and effectively, the available capacity for a flow decreasesif a TCP/lP type of protocol is used. In a SIMA environment, when a user buys a NBR for a flow and then sendstraftlc into a SIMA network, there is usually no flow level blocking (although it is possible to protect the SIMA network from excessiveoverloadsby restricting the total sum of NBRs). The quality of the flow dependson two issues: the NBR to actual bit rate ratio, and total load in the network. Therefore, a potential difficulty is that the customer cannot precisely know what the QoS of a flow will be becauserapid trafhc variations may bring about unexpected changesof QoS (however, even in the case of services using resource reservation the actual quality of flows using certain quality class may vary signitlcantly, becausethe quality can only be determinedby using statistical parameters). Becausethe quality of existing flows is not in the same way predictableas with servicesusing complicated resourcereservationmechanism,the SIMA network shall be implementedin a way that the users can rely on the fairnessof the service. The fairnessof the SIMA serviceis basedon the fact that all flows with the same actual bit rate to NBR ratio perceivessimilar QoS. Thus, a home user with 10 kbit/s NBR receivesthe same QoS as a large company with NBR of 100 Mbit/s provided that both are transmitting at their own NBR. The SIMA service can offer this fairness feature during a short interval. In contrast, during a long period, like a month, fairness is not as clear, becausethe amount of transferredinformation dependsessentially on total length of active periods, whereasthe charging does not dependon the customer’s activity. This fairness problem common to any service with flat-rate charging can be solved, if needed,by using a time dependent charging. Another aspectof fairness is the possibility to obtain more quality with higher price or lower price with less quality by changingthe actual bit rate or NBR. This meansthat each customeris entitled to changethe NBR to actualbit rate ratio and by that meansto optimize his&r quality to chargeratio. If the ratio increases,the quality of the flow is enhanced.If the user sendstraffic by using a constantbit rate, the SIMA service offers 7 different quality levels (for variable bit rate traffic the levels are less distinct but basically the same).
3
Although the absolute quality of each priority level dependson the network dimensioning and on actual traffic process,the quality levels can be describedapproximately asfollows: I = reservedfor non-SIMA serviceswith resourcereservation 6 = excellentquality: negligible packetloss ratio 5 = high quality: packet lossesonly during exceptionaltraftic peaks 4 = good quality: small packetloss ratio evenduring busy hour 3 = moderatequality: usually small packetloss ratio exceptduring busy hours 2 = satisfactoryquality: from time to time very high packetloss ratio 1 = suitablefor best-efforttrafhc during busy hour 0 = unusableduring busy hour, but suitablefor best-efforttraffic during non-busyhours The charge of priority level j will be X*2A(j-4), if the charge of level 4 is X, and if the charging is proportional to NBR. However, quality level 0 can be in practice obtained free of charge,and the operator may use different chargingschemewith level 7. The network operatormay try to dimensionthe network in a way that the traffic of the threelowest levels is able to fill the network during busy hour. Now, as the charge of level 6 service is 16 times higher than that of level 2, we can assumethat there will be much more traftic offered to the lowest levels. For instance,there could be 10 times more traffic on the lowest levels and still the incomes from level 6 traffic is higher than thoseof levels 0, 1, and 2 together.Therefore, the traffic load of level 6 could be increasedby several hundredspercentsbefore there is any packet loss in that level (provided that the network is properly dimensioned).Note that even very high load of the low quality levels has no significant effect on the packetloss ratio of the highestlevels. It is reasonableto assumethat the most intensetraffic variations occur at the lowest quality levels, whereasthe charging may dampenthe variations at the highest quality levels. Thus, for most of the time the highest priority levels can be considered as insulatedfrom the lower levels having more varying packet loss ratio. 2.4 SIMA service chain As a conclusion for this chapter the total servicechain of SIMA is outlined in Fig 1. The user input to the SIMA network consistsof charge(C -> X Ecu), actualtraffic sent into the network (T), and tVnrt selection (the only traffic or quality parameterused with SIMA). The network may inform the user of the offered servicetariffs by announcingthe NBR (or as the user may know the chargingfunction, he can selectdirectly a proper NBR). The main output of the network is the actual QoS of the flow, which dependsdirectly on the network performance.Note that there is a (one-way) connectionfrom the chargeof the flow to the actual QoS of the flow via NBR, the SIMA control and network performance.Therefore, althoughthere is no predefined exact relation betweencharging and QoS, the user may optimize the charging of the flow by trying firstly a low charge, and then doubling the chargeuntil the quality level is sufficient. Another important feature is that the trafhc control information is conveyedpurely by the SIMA packetsor cells, which means that there is no needto have any traffic control information transportedbetween different network nodes, Finally, as there is no packet or cell discarding based on a separatetraffic flow, packets or cells are discardedonly if the total load exceedsthe link capacity.
User
charging
X Ecu + tariff function I
input network
to
traffic control
performance
C --->
p -------------->
-
=> NBR for the flow -, + actual offered traffic of the flow + => =t,nrt SIMA packets + SIMA traffic control + aggregate traffic process + network capacity
Q network performance
Fig. 1. Service chain of SIMA. C is the user’s readinessto pay, I is information given by the network, T is actual traffic sentinto the network, P is the parametersneededto control the flow, andQis thequalityexperiencedbytheuser. 3. Motivation This chapter gives several argumentsfor the use SIMA service especially for the future Internet (you may
skip this chapterif you are alreadyconvincedof the usefulnessof SIMA). 3.1 Current service schemes The SIMA service is intendedto give a reasonableservice conceptfor ordinary Internet userswhile offering quality and fairness. The main motivation of the SIMA service concept is the apparent unsuitability of current Internet service conceptsto entirely meet theserequirements.The starting point of the development of Internet services is the current best-effort service model. The service chain of best-effort service is presentedin Fig. 2. The well-known problems of best-effort service are that there is no relation between quality and charging, and that there is no way to offer high quality (small packet loss or small delay) for those flows that need thesefeatures.The prevalent approachto solve this problem is to design guaranteedservice classes,eachof which hascertain quality features.A simplified service chain of this approachis presentedin Fig. 3.
5
user
traffic control
performance
C
Q network performance
Fig. 2. Servicechain of a best-effortservice User
input network
P ---)
requested traffic + quality parameters
T
traffic control
to
---------------
+
I
tariff table
I
X ECU
=> charging
=> service +
class
traffic descript.
=>
‘flow control" + offered traffic
.---------------->
I
Ql
= 6 = Int(x) if 0 < x < 6 (1) 0 if x 5.66 NBR(i) the packet (or cell) gets the lowest priority (O), and if MBR(i,j) < 0.17 NBR(i) the packet (or cell) gets the highest NBR-priority (6). Priority 7 is reserved for those connections that use a network service with guaranteedbandwidth and quality. The accepting and discarding of packets (or cells) inside a SlMA network is entirely basedon the priorities. Since the bit rate of every connection may change significantly in several time scales, the operator must apply an averaging measuringprinciple to determine the instantaneouscell rate of each connection. The approach presented in this chapter is applicable, but any measuring scheme which gives a feasible approximation of the instantaneousbit rate can be used. The measuring approachis based on the wellknown principle of exponential moving average.In addition, we assumethat ATM is used at the transport layer (ATM is also used in all the simulations presentedlater). If we supposethat the moving averageis calculated at every time slot, the measuredload generatedby a connection (i) at the instant of transmission of j:th cell is: ,Oi,j = (1 - a)Ni.J pi,j-l + a
(2)
where N;,j is the distant betweenj:th and (j-1):th cells in time slots and cxis a parameterwhich defines the time scale of measurement.Formula (2) is obtained by assumingthat the estimation for the instantaneous load is updatedat every time slot, but all calculations are performed only at the arrival instant of a cell, The following starting values can be used: psO= 0 and A$,,= C/NBRi.In order to obtain an exact steady state WlUe fOr COINtAUt bit rate connectionsthe following conversion between load (pu) and measuredbit rate (MBR~Jshall be applied:
10
(3) In 1-z Pi,j 1 ( where C is the link capacity bit/s] at the user/networkinterface. The proper value for parameterc( dependson the buffer capacityreservedfor the service class used by the connection. With real-time services (with small delay variation) the buffer should be small, and thus the value of Q must be quite high. On the contrary, when using a non-real-time service the user may want to sendburstsof cells without high cell loss ratio. As a consequencea must be much smaller (or the averaging period shouldbe much longer). As an interim approachthe following approximation might be applicable:
CZ=5JK,,
(4)
where K. is the buffer capacity in cells reservedfor the setvIce class n. The difference between actual and measuredbit ratesis illustrated in Fig. 5. * = actual
bit
rate measured bit measured bit rate
o = real-time
******o*o*o*o*
x = nrt
rate
00
0 0
0
****o*o*o*o*o*o*o*o*o
0
00
0 3 xx Kx
0 0
XXXXXXOXXXXXXX
xxx XXX
00 XxXxXx******o*o*o*o*o*o*o*o*o
xxxxXXXXXXXXxxxxX 0 00
***o*o*o
o------------------------------------------------------------------
time Fig. 5. The difference betweenactual bit rate, measuredbit rate for a real-time flow and measuredbit rate for a non-real-timeflow.
4.2. Scheduling and buffering unit The key issue in the implementation of the SIMA service in a high capacity core network is the packet or cell discarding system before the actual buffeting shown in Fig. 6. At any instant there is an acceptedlevel of priority (PL-a): if an incoming packet or cell hasthe sameor higher priority, it is accepted,otherwise it is discarded.The calculation of PL-a is basedon the buffer occupancylevels of the real-time buffer (M-it) and non-real-time buffer (M-nrt). In practice, the calculation of PL-a can be based on the use of a precalculatedtable (and therefore,thereis no significant real-time processingeffort). All the packets or cells which have beenacceptedin the schedulingunit are situatedeither in the real-time or non-real-time buffer (the scheduling algorithm can guaranteethat there is no cell loss in actual buffers). Both buffers may apply the First In First Out (PIFO) principle. In order to obtain a small delay and delay variation, the real-time buffer should be relatively small (e.g., 10 kbyte). All packets (or cells) in the realtime buffer shall be transmitted before any packet (or cell) in the non-real-time buffer. It should be emphasizedthat the delay priority of real-time flows has no effect on the packet loss ratios. The non-realtime buffer should be much larger (e.g., 1 Mbytes) becauseof the packet scale fluctuations in typical nonreal-time traffic processes.Moreover, large buffers make it possible to offer reasonableservice for those flows that are capableof adjusting their bit rate. 11
It should be emphasizedthat the function of eachschedulingandbuffering tit (SBIJ) is independentof all other SBU’s; all the tasks of SBU are performed basedon the information of incoming cells (or packets), and moreover, all the necessaryfunction for the implementation are describedin Fig. 6. Thus, due to the autonomousproperty of switching units and the mmecessityof resourcereservation,the managementof the SIMA network is very straightforward.
PL-a = F(Mrt,Mnrt)
PL-a packet or cells in
V / \ /
\
no
-x---> / PL< \---->/ \ PL-a/ \ / \ /
xxxxxxxxxI--/--- open only or cell) M-rt = 0
V Fig. 6. A packet (or cell) schedulingandbuffering tit 4.3 Interoperability
,-x-->
packet or cells out
if
(SBIJ) for a SIMA network node
with ordinary ATM networks and services
The main obstacle of SIMA service might be the incompatibility problems with the ordinary ATM technology. First of all, SIMA servicerequires3 bits in eachATM cell for the determination of cell priority (or 2 additional bits if the current cell loss priority, CLP, bit in the cell headeris used). In addition, one bit is needed due to the sorting of real-time and non-real-time connections.An approach in which the retitime/non-real-time bit is included in every cell, may further simplify the implementation of SBU units and core network nodesas the tits do not needto keeprecordof everyconnection. One possibility is to use the current GenericFlow Control (GFC)-field with 4 bits in the cell header.In this caseall the 3 priority bits and rt/nrt-bit can be situatedin the cell header.If this is not possible, the required bits (2, 3 or 4 depending on the use of CLP bit and the statasof rt/nrt bit) shall be situated outside the currentcell header. Another compatibility question is how the SIMA service is working with ordinary ATM network using guaranteedconnections.As the ordinary ATM servicesare offered in parallel with SIMA service, it shall be possible to transmit ordinary ATM connectionsin a SIMA network. In this casethe operatorshall have an UPC device for each ordinary ATM connection(or possibly for eachvirtual path). All cells with CLP=O in theseconnections are marked with the highest priority (7). If the operatorwants to mark excessivecells as CLP=l cells, thosecells shall be marked with lower priority in SIMA network, for example, with priority 2. There should be a simple connection admission control (CAC) method probably based on peak rate allocation. Peak rate allocation is supposedto be sufficient in practice becausethe chargeof a conventional ATM connection with priority 7 will be essentially higher than the charge of SIMA connection with the samebit rate and with priority 4. 5. Performance
aspects
The main difference in performance evaluation between SIMA and conventional ATM services is the priority levels. Therefore, the focus of this chapter is to illustrate the QoS and throughput at different 12
priority levels. The first question is how large is the quality difference betweenadjacentpriorities. It should be rememberedthat the price is doubled if the user wants to obtain one degreehigher priority for every cell without changingthe real bit rate. In consequence,the QoS should be improved so much that large amounts of usersare willing to pay the additional charge. 5.1. Performance
evaluation with independent traffic sources
In this chapterit is assumedthat there are many identical traffic sourceswhich generatetraffic independent of the current or previous load condition in the network The following traftic parametersare assumed:the link capacity C = 1, peak bit rate = 0.1, the ON probability at the burst (or packet) scale = 0.2, and the averageburst duration = 1000 time slots (i.e., the averagepacket size = 100 cells). In addition we are supposingthat there is an upper on/off layer which is used to model the random processof connections.It is assumedthat both the averageon-period and off-period of this layer are 100 000 time slots. The real time buffer contains 200 cell locations and non-real-timebuffer 5000 cell locations. By using the equation (4) for the time scale parametera we obtain: an = 0.025 a nrt = 0.001 In this example, eight different connectiontypes are assumed:four axmection types are real-time ones and four are non-real-time ones. Also, four different NBR values are assumedas: 0.2, 0.1, 0.05, and 0.025. The priorities correspondingto these NBR values, with maximum MBR = 0.1, are 3.4, 5 and 6, respectively. It should be noted, however, that not all cells are assignedtheseexact priorities, and that especially with nonreal-time connections, many cells obtain better priority values because of the effects of the averaging measuringprinciple. The distribution of cells haying different priority levels, representedas percentages,is presentedin Table 3. Table 3. The percentageof cells of different priority levels priority level
real (simulated) percentageof offered cells 14.0
percentagebased on peak rates
24.3
25
23.5
25
21.5
25
16.8
25
0
In Fig. 7, there is shown a graph illustrating the relationship of averagecell loss ratio, P,,, as a function of priority level for four specific load levels, r. In the caseof load = 0.80 the cell loss ratios for real-time and non-real-time cells are indicated by dotted and broken lines, respectively. The figure shows that the differencein cell loss ratio betweenreal-time andnon-real-timecells is insignificant,
13
lE+OO yI m-01 lE-02 +r=O.88
lE-03 liz-04 lE-0.5 lE-06 1E07 lE-08 I 2
4
3 Priority level
Fig. 7. Average cell loss ratio vs. priority level for load levels r = 0.72,0.80,0.88,0.96. In caseof load = 0.80 the cell loss ratios for real-time and non-real-timecells arepresentedby dotted andbroken lines. Given, for example, a traffic scenariowherethe operatorwants to offer a cell loss ratio of 10” to cells with priority 4, the total load can be approximately 0.75. It can be assumedthat this averagecell loss ratio is sufficient for most video applications. Give the sametraffic conditions, priority level 3, which corresponds to ploss= 10” can meet the requirementsof many voice applications,and priority 2, which correspondsto Pl0ss= 3. 10m3, is suitable for a TCPLP type of file transfer, provided that there is an adequatepacket discardingschemein place. It should be emphasized,however, that the difference in cell loss ratio betweenadjacentpriorities depends strongly on the offered traffic processand, in particular, the inherent control loops of the SIMA service. When the user perceivesan unsatisfactoryQoS, the user can, and should, changeeither the actual bit rate or the nominal bit rate of the connection.In either case,the priority distribution changesas well. Nevertheless, if this phenomenon is temporarily ignored, the basic behavior of priority distribution may be further appreciatedby making the following simplifying assumption.If it is assumedthat all traffic variations are slow as comparedto the measuringperiod and buffer size, then a well known, conventional ATM approach to approximating cell ratio may be used, with the additional requirementthat the eight NBR priority levels aretaken into account. If the loss ratio of cells with priority k is denotedby P,oss,kand the averageloss ratio of cells with priority of 0 to k is denotedby Plzss,k, then the following equation,which ignores buffering effect, providesthat:
j:z,cPr{al =ajXai-c) P*1oss.k
= P;C
P1oss.k
P; fLs,k - P;.+1fLs,k+l =
(5) for
Pi -Pi+*
14
k = 6...0
where, ai representsthe momentary bit rate level of all cells with a priority of 0 to k, pi representsthe average offered load produced by these cells, and C represents the link capacity. The probability Pr{Li = ;lj} can be calculated in a straightforwardmannerby using known convolution techniques For purposesof further illustration, we assumethe samesourcesdescribedin the beginning of this chapter (exceptthe long ON and OFF periods). Becauseof the long periods the peak rate always determinesthe cell priority. As in this case the buffers are not capableof filtering any traffic variations, the allowed load is much lower in this example than in the original case. In Fig. 8, there is illustrated in graphical form a relationship betweencell loss ratio as a function of priority level for different load levels, r. Fig. 8 showsthe cell loss ratios obtainedby application of Equation (5) for different priorities. It is assumedin Fig. 8 that the peak cell rate of eachconnection depictedby solid lines is 0.1. The peak cell rate of connection depicted by broken line is 0.2, which actually means that trafllc variations have been doubled by changing both the peak cell rate and nominal bit rate. The peak rate cell rate of connection depicted by dotted line is 0.05. As the nominal bit rate is halved, as well, the traffic variations are decreased.
Hz-02 +PCR=O.l, t=O.56 +PCR=O.l, r=O.48 *PCR=O.l, r=O.40 - 0 - PCR=O.2, r=0.24 - - A - - PCR=O.05, r=O.76
lE-03 Hz-04 1505 1E06 lE-07 lE-08
7
2
4
3
5
Priority level Fig. 8. Cell loss ratio vs. priority level for different load levels (r); solid lines: peak cell rate of eachconnection= 0.1; brokenline: peak cell rate of eachconnection= 0.2; dotted line: peak cell rate of eachconnection= 0.05. In a network that embracesthe SIMA serviceconcept, an increaseof trafhc variations has two main effects if the operatorkeepsthe QoS of priority level 4 unchanged.First, the allowed load level is decreasedin the same way as in conventional ATM, and second,the difference in cell loss ratio between adjacentpriority levels decreases.For purposes of providing a rough estimate of QoS based on Fig. 7 and 8, it may be assumedthat if priority level 4 offers a cell loss ratio of 10’. then the cell loss ratio will be approximately 1O-4to lo” with priority level 3 dependingon the overall traffic variations.The cell loss ratio with priority 5 can be supposedto be less than 1O-9unlessthe traffic variations arevery pronounced. 5.2. Packet based discarding A possible problematic issue with SIMA could be partly transmitted packets,if ATM or similar technology is used as at the~transmissionlevel. However, accordingto simulations, this is not as a significant problem as could be expected.Table 4 provides the result of a simulation study which illustrates the situation. All the 15
sourcesare basically the same:peak bit rate = 0.05, the ON probability at the burst (or packet) scale = 0.4, the ON probability at connectionscale = 0.5, and the averageburst duration = 6000 time slots Each packet contains 30 cells, and a burst contains on average10 packets In addition, at the upper on/off layer both ON and OFF periodslast on average100 000 time slots. In the cell baseddiscarding schemeall cells arehandledindividually. However, a packet is consideredto be lost always if at least one cell is lost. In the packetbaseddiscardingschemethe main decision is made at the arrival of the first cell of each packet. If that cell is accepted,every following cell belonging to the same packet gets a gain of one priority level. If a cell (either the first one or any other cell) is discarded,all the following cells are discardedas well. The drawbackof this schemeis that the node must keeptrack of each connection,although the algorithm itself is quite simple to implement. The results show that there is quite a small difference betweencell and packet based discarding schemes. Even if the averagecell loss ratio of a flow is oved20% and there is no packet baseddiscarding, the packet loss ratio could be less than 30%. In addition, the simulations indicate that although the packet based discarding is able to increasethe useful throughput (or goodput), the advantageis quite small (0.937 vs. 0.950). The reasonto these, somewhatunexpected,effects is that the priority based discarding algorithm works in a way that cell lossesconcentrateto a minority of flows. Therefore,during a loss period, each flow encountersusually either a very high loss ratio (tensof percents)or no loss at all. It seemsthat packet based discarding is avoidable, which makes possible to build very simple core nodes without any information aboutseparateflows or connections. Table 4. Cell vs. packet baseddiscardingscheme Cell baseddiscarding
Source NBR we SO l/320
Packetbaseddiscarding packet loss packet No. of aver. cell loss packetloss packet cell loss sources load probability probability goodput probability probability goodput 20
0.200
0.2225
0.2776
0.144
0.2227
0.2233
0.155
Sl
l/160
20
0.200
0.0235
0.0368
0.193
0.0257
0.0258
0.195
s2
1180
20
0.200
0.0004
0.200
0.0006
s3
l/40
40
0.400
0
0.0009 0
0.400
0
0.0006 0
0.200 0.400
100
1.000
0.0493
0.0631
0.937
0.0498
0.0499
0.950
total
5.3. User reactions to quality differences Although the previous examplesprovide illustrations betweenQoS and priority levels, it may be unfruitful to attempt to exactly determinethe allowed load or the cell loss difference between adjacentpriority levels until user reactions to different QoS and usagechargesare evaluated.In a SIMA service environment, a scheduleof chargesbasedon different QoS levels may be determined,in a certain sense,automatically. For example, if the difference in cell loss ratio betweenpriority levels 4 and 3 is very small, it can be assumed that some of the connectionswill tend to move from priority level 4 to level 3 becauseof a lower assessed charge.This changeindicates, apparently,that the cell loss ratio of priority level 4 decreasesand the cell loss ratio of priority level 3 increases.It can be reasonablyassumedthat this type of movement continues until the QoS difference correspondsto the averageuser’s expectationof a reasonablechargingstructure. Similar concernsare raised with regardto the differenceswhen chargingusersduring busy hours in contrast to idle hours. For example, it would appearreasonableto chargehigher prices during low load periods for a certain QoS and bit rate. With SIMA servicethis differenceis achievedautomatically as the user has to use better priority by means of higher NBR during busy hours in order to obtain the same QoS as during idle hours. This “supply and demand’ effect may tend to automatically even out the load betweenbusy and idle hours. In this respectthere is a fundamentaldifferencebetweenSIMA andthe conventional ATM, where the
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operator must itself plan a complicated but practical charging structure for several QoS classes, and separatelyfor idle and busy hours. 6. Conclusions Notwithstanding the complexity of conventional ATM traffic management schemes, the current ATM specifications fail to adequatelyaddressthe need of simple managementand feasible charging for future Internet and other networks with high capacity and quality requirements.Accordingly, there is a needin the communications industry for a network management architecture that is simple in concept and in its implementation, yet adequatelyaddressesthe quality of servicerequirementsto support a variety of network services, including real-time and non-real-time services. There exists a further need for a system and methodology that provides for the implementation of a simple and effective charging capability that accountsfor the use of network services.The presentSIMA service introduced in this document is capable to fuhll theseand other needswhich remain unaddressedby currenttraffic managementapproaches. The SlMA service is technically basedon three key ideas:the use of nominal bit rate concept,the use of 8 priority levels for every cell or packet, and separationof real-time and non-real-time connections at the buffer level. If a user needs a connection over an IP or ATM network, he should select a nominal bit rate which could be even a constant proportional to a monthly fee. The other decision needed before a connection establishmentis that the user shall select either a real-time or a non-real-time service class. In addition to thesetwo parametersthe user doesnot need to give any information about the properties of the connection like required bit rate or quality of service. After the connection establishment the capacity division among different comxctions is based on a priority which is determined using a ratio of the measuredbit rate and the nominal bit rate. This priority in addition to the real-time/non-real-timeseparation is sufficient information for every network nodeto properly managethe traffic in the network. Becausethere is no need for various traffic classes,traffic parametersand network services, the SIMA service makes possible a simple and efficient implementation of network nodes, a simple and fair charging scheme,and very simple traffic managementin the core, high speednetwork. In consequence,the SlMA conceptis a very promising schemefor solving the most acutetraffic control problems in Internet.
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