IP-QoS Benchmarking in Gigabit Networks

1
IP-QoS Benchmarking in Gigabit Networks

• Andrea Di Donato
• University College London (UCL)
• Gnew 2004 CERN, Geneva

2
Introduction

• The deployment of IP-QoS in the Differentiated
  Services (DiffServ) framework both in the access
  and in the core networks requires an in depth
  knowledge of the performance of two of the
  currently most deployed router cards technology
  1GE and POS_OC-48.
• The evaluation of their performance is based on
  how precisely a minimum bandwidth guarantee can
  be allocated to an aggregate of data traffic
  under interface congestion and not it
  constitutes the foundations for the deployment of
  more complex IP QoS solutions.

3
Introduction (cont.)

• The QoS model we use is based on the
  Differentiated Services (DiffServ) model for IP
  networks.
• The traffic entering the network device is marked
  by the sending host using a single Differentiated
  Services Code Point (DSCP). For each one of these
  code-points there is assigned a different
  behaviour aggregate or class.
• This work studies the QoS performance of this two
  technologies provided by Cisco, Juniper and
  Procket.
• UDP CBR traffic is used as TCP is not
  controllable.

4
Test-bed (generic)
Generic Router
Cisco 7609 Procket 8801 Juniper M10
OC-48 Or GE

• Two classes only are configured nominally BE and
  LBE. They differ in the percentage of the port
  capacity (Mbps) allocated. The QOS configuration
  is kept simple on purpose.
• LBE is used here with the broadest meaning
  possible which is that of an IP class whose BW
  allocation is complementary to 100 with that of
  BE (should be OtherThanBEOTBE)
• The BW allocation set chosen for the tests was
  mainly the following sequence of couples
• BE-LBE (99-1, 98-2, 97-3, 96-4, 97-3, 95-5,
  94-6, 93-7, 92-8, 91-9, 90-10, 85-15, 80-20,
  75-25, 70-30, 65-35, 60-40, 55-45, 50-50)
• We refer to the sequence above as the BW
  allocation couples axis with the axis direction
  going from 99-1 to 50-50.

5
PCs and traffic

• Three PCs (Supermicro 6022P-6 Dual Intel Xeon)
  were attached to the two routers. Each PC had an
  Intel PRO/1000 XT Server Gigethernet adapter
  (e1000 v4.4.12-k1)
• The PCs were running Linux kernel version 2.4.20.
  
• Iperf version 1.6.5 (13 Jan 2003) pthreads
  was the application-level tool used to inject UDP
  CBR traffic

6
PCs benchmark

• To achieve line rate from the PCs, we required a
  packet size quite close to the Ethernet MTU.
• We chose a packet size of 1470 bytes for our
  tests. The maximum achieved UDP-level throughput
  at this packet size for the PCs plugged
  back-to-back is 955Mbps (line rate !!)

7
Metric atomic

• Received Throughput Analysis
• Link Utilisation
• sum of the per-class received throughput (Mb/s)
• BW allocation error analysis
• Absolute Error
• WhatClassXgets WhatClassXshouldGet (Mb/s)
• Relative Error
• AbsoluteError100/WhatClassXshouldGet ()
• This double metric is required since a good link
  utilisation is only necessary, not also
  sufficient, in order to have an equally good BW
  allocation precision.

8
Metric (cont.)

• WhatClassXshouldGet - algorithm

Is the Interface Congested?
LEGENDA CL3 capacity Spare BW not
allocated all allocated exp expected X
one of the two classes involved
no
XexpXsent Yexp Ysent

yes
No (X and Y both over-subscribed)
Is class X Under-subscribed?
XexpXallXall/(XallYall)spare
YexpYallYall/(XallYall)spare

yes

• Compatible with
• wfq (Cisco,Juniper)
• dwrr (Procket)

XexpXsent Yexp C - Xsent

9
Metric some notes
Offered load axis
Port not Congested
Port Congested BE undersubscribed (BE leftover
BW to LBE) and LBE oversubscribed
Port Congested Both BE and LBE oversubscribed
10
Test Metric (cont.)

• Accuracy
• Typically, the accuracy in allocating BW to a
  class is fixed to be around 95 which in turn
  means that 2.5 is the maximum error acceptable
  in module.
• We refer to the region inside which this bound
  is validated as the operating region.

11
Composite metric (cont.)

• In order to bound an operating region over the
  BW allocation couples axis, a new metric is
  introduced to synthesize the per-BW-allocation
  throughput performance along the different port
  congestion levels
• NEW METRIC (M-L/B_REWS or MAX algebra error)
• Definition
• The Maximum value that the LBE and BE Relative
  Errors taken With Sign (M-LREWS/M-BREWS) assume
  all over the port congestion level axis.
• M-L/B_REWS allows the evaluation of the BW
  scheduler based solely on its worst performance
  over the offered_load/port_congestion_level axis.
• We refer to the M-L/B_REWS-defined operating
  region as the max operating region, this
  highlighting that the method used to bound it was
  that of computing the max algebra for the
  relative errors along the port congestion levels.
  
• Thus, an accuracy of 95 in the allocation of the
  BW is equivalent to have the M-LREWS/M-BREWS lt
  -2.5.

12
Composite metric (cont.)

• In order to further investigate the max
  operating region over the BW allocation couples
  axis bounded by the M-L/B_REWS metric (previous
  slide), a new metric is introduced to quantify
  how spread the per-BW-allocation error is over
  the port congestion level axis
• NEW METRIC (A-AL/BRE or AVG. algebra Error)
• Definition
• The Average of the Absolute values of LBE/BE
  Relative Errors which we refer to as A-ALRE and
  A-ABRE respectively.
• The absolute values are used to avoid that the
  average of algebraic values could lead to a
  misleading 0 errors as the effect of the
  cross-neutralisation of opposite polarised
  relative errors of the same order of magnitude
• We refer to the so defined operating region as
  the avg operating region, this highlighting
  that the method used to bound it was that of
  computing the AVG algebra for the relative
  errors.
• The main difference between the MAX-based and the
  Averaged-based metrics is that the latter takes
  account of the errors all over the offered_load
  / port_congestion_level axis and not just of
  the maximum over such axis. This allows
  quantifying how spread the error is over the port
  congestion level axis.

13
Cisco 7609 OC-48

• This card is a POS OC-48 v2 to whom Cisco refers
  to as OSM-1OC48-POS-SS.
• The encapsulation used is PPP.
• Cisco designed an engineering code specific for
  the scheduler of this card and included it on the
  major release 12.1(19)E which has been available
  since May/03. The tests we performed used the IOS
  version just mentioned.
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos is 2318 Mbps
  The card is therefore congested up to
  (9573)/2318123.8 of its Capacity.

14
Cisco 7609 OC-48/GE-WANv2 qos-configuration
sample

• mls qos in the global configuration mode is
  needed to enable QoS on the supervisor engine
• mls qos trust dscp issued in the input and
  output interfaces is there to avoid cards to
  reset the dscp code of packets entering or
  leaving the output and input interfaces
  respectively. This configuration line is of
  particular importance if Catalyst ports are used
  in the input (not in the output as they dont
  support L3 CBWFQ) as they naturally tend to reset
  to 0 the dscp code. This happens since the legacy
  L2 COS-based QoS is the default QoS for the
  catalyst ports as the 7600 is a router built on
  top of the native Catalyst switch .
• As an architectural note, Parallel Express
  Forwarding (PXF) is present on each OSM (Optical
  Service Module) or card and is capable of CBWFQ,
  thus permitting the QoS processing directly on
  the card.

• !
• class-map match-any BE
• match ip dscp 0
• class-map match-any LBE
• match ip dscp 8
• !
• policy-map GNEW2004
• class BE
• bandwidth percent X
• class LBE
• bandwidth percent Y
• !
• mls qos
• !
• interface input
• mls qos trust dscp
• !
• interface output
• service-policy output GNEW2004

15
Cisco 7609 OC-48 results

• The link utilization is pretty poor for some of
  the BW allocations and this is sufficient to have
  bad BW allocation precision.
• Both BE and LBE relative errors are therefore
  presented in the next slide with the purpose of
• Quantifying the per-class bw allocation error
• Seeing whether the errors are localized in one or
  more port congestion level zones
• Seeing the dynamic of the error in each zone (if
  any, see point 2) as a function of the BW
  allocated when a value of the port congestion
  level is fixed.

16
Cisco 7609 OC-48 results (cont.)

• The error BE presents (right plot) is lt 2 and
  therefore negligible.
• The error is concentrated on LBE and presents
  positive polarity which suggests, along with the
  negligible BE error and with the poor link
  utilization, that the schedulers issue is the
  inability in allocating the BE leftover BW to LBE
  under a certain range of port congestion levels.
• The errors (both the MAX and the AVG over the
  port congestion level axis) decrease
  monotonically with the increase of the BW
  allocation couple axis, therefore suggesting a
  well defined operating region. The max operating
  region is shown in the next slide.

17
Cisco 7609 OC-48 results (cont.)

• In order to determine with precision the
  operating region, the MAX LBE relative error with
  sign (M-LREWS) is presented below.
• The error oscillates a bit along the value of
  2.5, thus making the definition of the max
  operating region difficult.
• A conservative maxoperating region for this
  card is from the value of 50-49 to that of 75-24
  for the BW allocation couples.
• The same max operating region would range from
  50-49 to 93-6 if the precision was 88 instead of
  95.

18
Cisco 7609 GE-WANv2

• This card is a GE-WAN v2 to whom Cisco refers to
  as OSM-24GE-WAN.
• The tests we propose make use of the 12.1(19)E
  IOS version, the same as for the OC-48 card test.
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos, is 957 Mbps
  The card is therefore congested up to
  (9572)/957 200 of its capacity.

19
Cisco 7609 GE-WANv2 results

• Again, as for the OC-48, the link Utilization is
  pretty poor and this is sufficient to have bad BW
  allocation precision. Both BE and LBE relative
  errors are therefore presented in the next slide
  with the purpose of
• detecting whether the BW allocation errors are
  localized in one or more port congestion level
  zones
• Seeing the error dynamic in each zone (if more
  than 1...see point 1)
• Seeing the dynamic of the errors as a function of
  the BW allocated when a value of the port
  congestion level is fixed.

20
Cisco 7609 GE-WANv2 results (cont.)

1. The above figures clearly show how The BE
   relative error is negligible (lt2.5) while that
   of LBE is not.
2. LBE relative error doesnt even show a monotone
   decrease of the error per BW allocation couple
   and per port congestion level, this suggesting a
   non well defined operating region
3. The MAX LBE relative error with sign analysis is
   therefore necessary (next slide) to work out
   where the boundary of the operating region is. We
   do not expect it to be monotone (see point 2)

21
Cisco 7609 GE-WANv2 results (cont.)

• The max operating region for this card is from
  the value of 55-44 to that of 70-29 for the BW
  allocation couples.
• The non monotonicity doesnt affect the operating
  region evaluation

22
Juniper M10 OC48

• The IOS used was Junos 5.3R2.4.
• The card version is 1xSTM-16 SDH, SMSR REV 05
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos is 2338 Mbps.
  The card is therefore congested up to
  (9573)/23382871/2338 122 of its capacity.

23
Juniper M10 OC48/GE configuration sample

• output
• scheduler-map MAP-UCL
• unit 0
• classifiers
• dscp UCL-classifier
• scheduler-maps
• MAP-UCL
• forwarding-class LBE scheduler
  sch-LBE
• forwarding-class best-effort
  scheduler sch-BE
• schedulers
• sch-BE
• transmit-rate percent X

• class-of-service
• classifiers
• dscp UCL-classifier
• forwarding-class LBE
• loss-priority low code-points
  cs1
• forwarding-class best-effort
• loss-priority low code-points
  000000
• forwarding-classes
• queue 2 LBE
• queue 0 best-effort
• interfaces
• input

24
Juniper M10 OC48/GE configuration sample (cont.)

• Juniper has a priority queuing mechanism which is
  not a strict priority mechanism.
• The queue weight ensures the queue is provided a
  given minimum amount of bandwidth which is
  proportional to the weight. As long as this
  minimum has not been served, the queue is said to
  have a positive credit. Once this minimum
  amount is reached, the queue has a negative
  credit.
• A queue can have either a high or a low
  priority. A queue having a high priority will
  be served before any queue having a low
  priority.
• For each packet, the WRR algorithm strictly
  follows this queue service order
• High priority, positive credit queues
• Low priority, positive credit queues
• High priority, negative credit queues
• Low priority, negative credit queues.
• The following explanation tries to clarify the
  WRR mechanism.
• The positive credit ensures that a given queue is
  provided a minimum bandwidth according to the
  configured weight (for both high and low priority
  queue). On the other hand, negative credit queues
  are served only if one positive credit queue has
  not used its whole dedicated bandwidth and no
  more packets are present in a positive credited
  queue.

25
Juniper M10 OC48 results

• With the exception of a couple of glitches due to
  poor host performance, all the utilisation curves
  overlap with the ideal one for both Test 1 (2BE
  1LBE) and Test 2 (1BE 2LBE)
• Since a good link utilisation is not sufficient
  to have a good BW allocation precision, the
  per-BW allocation couple relative errors for both
  BE and LBE are presented in the next slide
  against different levels of port congestion.

26
Juniper M10 OC48 results (cont.)

• Apart from some glitches due to poor pc
  performances (measurement background noise), both
  BE and LBE error is negligible.
• The operating region (through the M-LREW metric)
  is shown in the next slide.

27
Juniper M10 OC48 results (cont.)

• apart from the shown glitch, the whole BW
  allocation set is a max operating region as
  expected.

28
Juniper M10 GE

• The IOS used was the same as for the OC-48 test -
  Junos 5.3R2.4.
• the card version is 1x G/E, 1000 BASE-SX REV 01
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos is 957 Mbps.
  The card is therefore congested up to
  (9572)/957 200 of its capacity.

29
Juniper M10 GE results

• Again, due to the very good performance in terms
  of link Utilisation, we need to see
• the relative BE and LBE BW allocation precision
  errors in order to see
• whether there are errors and, if any, their
  magnitude and dynamics along the BW allocation
  couples and along the port congestion levels
  regions.

30
Juniper M10 GE results (cont.)

• BE error is negligible and mainly negative while
  the LBE error is mainly positive and is not
  negligible.
• The LBE error decreases monotonically with the
  increase of the bandwidth allocation couples,
  this suggesting that the MAX LBE Relative error
  is monotone as well, as shown in the next slide.

31
Juniper M10 GE results (cont.)

• The interpolated max operating region over
  the BW allocation couples ranges from 50-49 to
  70-29.

32
Procket 8801 OC48

• The System Release Version used is the
  2.3.0.180-B
• The Kernel Version used is the 2.3.0.1-P
  PowerPC
• The card version is the 4-PORT OC-48c POS SR.
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos is 2337 Mbps.
  The card is therefore congested up to
  (9573)/23372871/2337 122 of its capacity.

33
Procket 8801 OC48/GE configuration sample

• !
• qos
• class BE
• dscp 0
• class LBE
• dscp 8
• service-profile GNEW2004
• class BE
• class LBE
• queuing-discipline dwrr (BEX, LBEY,
  default1)
• !
• interface output
• qos-service GNEW2004
• !

34
Procket 8801 OC48 results
The link utilization is perfect and both BE and
LBE show negligible errors (lt1). The interesting
thing is that such errors appear from 80-19
towards 50-49 for both classes and that BE is
actually positive while LBE is negative. The
exact opposite error polarization if compared
with the typical errors the other manufacturers
show. The max operating region is not shown
since it is evident that the whole BW allocation
axis is a max. operating region!!!
35
Procket 8801 1GE

• The System Release Version used is the
  2.3.0.180-B
• The Kernel Version used is the 2.3.0.1-P
  PowerPC
• The card version is the 10-PORT 1000BASE-SX.
• The iperf UDP-payload-level capacity C of the
  link, which is obtained by congesting the
  interface and not configuring qos is 957 Mbps.
  The card is therefore congested up to
  (9573)/957300 of its capacity.
• It is worth noticing that this card is congested
  up to 300 (test1) of its capacity which is 100
  more congested than the maximum congestion
  experienced by both GE Juniper and GE-WAN Cisco.

36
Procket 8801 1GE results
The Link Utilisation is perfect. The BE
relative errors are negligible and the LBE ones
quickly tend to become negligible. The MAX LBE
relative error with sign (M-LREWS) plotted
against the BW allocation couples is presented in
the next slide.
37
Procket 8801 1GE results (cont.)

• Apart from 98-1 and 96-3 all other couples show
  an error of less than 1. A conservative max
  operating region though ranges from 95-4 to 50-49
  over of the whole BW allocation couples axis.

38
Comparative analysis

• As already highlighted, the majority of the
  errors are localised on LBE and therefore its
  relative error will be used to compare the
  performances of different routers.
• In order to bound the operating region over the
  BW allocation couples axis, the M-LREWS (Max LBE
  Relative Error With Sign) metric is used for both
  GE and OC-48 and for all the three router
  manufacturer involved.
• The so-defined operating region is the max.
  operating region.
• The M-LREWS metric allows the evaluation of the
  BW scheduler based solely on its worst
  performance over the port congestion level axis
• This is of extreme importance since the bounded
  value for the precision in the allocation of BW
  that the manufacturers refer to can be correctly
  associated to the worst case scenario out of the
  whole offered load axis. It is therefore correct
  to say that an accuracy of 95 in the allocation
  of the BW is equivalent to have the
  M-LREWS/M-BREWS lt -2.5
• In order, then, to further investigate the
  per-BW-allocation performance of a card over
  different card congestion levels, the AALRE
  (Average Absolute LBE Relative Error) metric is
  also presented
• This metric allows quantifying how spread the
  error is over the port congestion level axis.
• We refer to the so-defined operating region as
  the avg operating region, this highlighting
  that the method used to bound it was that of
  computing the AVG algebra for the relative
  errors.
• The absolute values are used to avoid that the
  average of algebraic values could lead to a
  misleading 0 errors as the effect of the
  cross-neutralisation of opposite polarised
  relative errors of the same order of magnitude.

39
Comparative analysis OC-48 M-LREWS

• In order to work out which operating region
  applies to the different manufacturers, a zoom
  over the abscissa region where all the three
  curves are close to the value of 2.5 is
  presented in the next slide

40
Comparative analysis OC-48 M-LREWS (cont.)

• Apart from a glitch showed by Juniper
  (measurement background noise), the entire BW
  allocation couples axis is a max operating
  region for both Juniper and Procket with the
  latter performing slightly better.
• It is difficult to determine a max operating
  region for Cisco as the error is not
  monotonically falling but it is oscillating
  around the value 2.5. As a consequence, a
  conservative max operating region over the BW
  allocation couple axis ranges from 75-24 to 50-49
  (31.5).

41
Comparative analysis 1GE (M-LREWS)

• With the target accuracy fixed to the canonical
  95
• Cisco max operating region, out of the whole BW
  allocation couples axis, ranges from 70-29 to
  55-44 (21).
• Juniper max operating region, which is
  linearly interpolated out of the values obtained,
  ranges from 70-29 to 50-49 (26.3) although its
  performance is better than the Cisco one
  throughout most of the bw allocation couples
  axis.
• Procket max operating region ranges from 95-4
  included to 50-49 (73.6).
• Its worth highlighting that the Procket card was
  congested up to 300 of its capacity while only
  200 was the maximum congestion that Cisco GE-WAN
  and Juniper GE experienced during the test.

42
Comparative analysis OC-48 A-ALRE

• Cisco OC-48 operating region averaged over the
  whole port congestion levels axis (avg
  operating region) ranges from 94 5 included to
  50-49 (68.42) .
• It is worth noticing how the average lowers the
  values but also acts, in this case, as a low pass
  filter whose effect is that of smoothing out the
  oscillations that led before to a conservative
  evaluation of the Cisco max operating region
  and that was the main reason for such a poor
  performance evaluation.
• The Cisco avg operating region is, in fact,
  much better than the max operating region which
  ranges from 75-24 to 50-49 (31.5) .
• The plot in the next slide zooms on Juniper and
  Procket in order to compare their performance

43
Comparative analysis OC-48 A-ALRE (cont.)

• The zoom shows how the error is negligible for
  both although Procket shows again slightly better
  performance.
• The whole BW allocation axis is a AVG
  operating region for both Juniper and procket

44
Comparative analysis 1GE A-ALRE

• Again, the average performance of both Cisco
  GE-WAN and Juniper M10 GE are much better than
  their relative max performance proving that the
  error is not spread along the offered_load/port_co
  ngestion_levels axis
• Cisco average operating region ranges from
  75-24 included to 55-44 which is 26.3 of the BW
  allocation couple axis
• In order to work out the avg operating region
  for both Juniper and Procket, a zoom is needed
  and is presented in the next slide

45
Comparative analysis 1GE A-ALRE (cont.)

• The Procket average operating region ranges
  from 97-2 included to 50-49 (78) while the
  Juniper interpolated average operating region
  ranges from 91-8 included to 50-49 (52.6) .

46
Comparative analysis Survey and percentage
improvement OC-48
A comparison based on both errors is provided.
The relative table along with the computation of
the percentage improvement (delta ?) in passing
from the max to the avg operating region is
presented for both OC-48 (this slide) and GE
(next slide). The Cisco 117.2 improvement
indicates that the per-allocation LBE relative
error is rather localised over the OC-48
congestion level axis
OC-48 Cisco Juniper Procket
Max op region 75-25 to 50-50. 6/19 31.5 99-1 to 50-50 100 99-1 to 50-50 100
Avg op region 94 6 to 50-50 13/1968.42 ?117.2 // //
47
Comparative analysis Survey and percentage
improvement 1GE
1GE Cisco Juniper Procket
Max op region 70-30 to 55-45 4/19 21 70-30 to 50-50 5/19 26.3 95-5 to 50-50 14/19 73.6
Avg op region 75-25 to 55-45 5/1926.3 ?6 91-9 to 50-50 10/1952.6 ?100 !!!! 97-3 to 50-50 15/1978 ?6
What is of particular interest is the improvement
delta of 100 that Juniper experiences in passing
from the max to the avg operating region if
compared to the much poorer 6 delta improvement
that Cisco shows. This suggests that the
per-BW-allocation Cisco LBE relative error is
much more spread and therefore serious all over
the GE port congestion level axis if compared
with that Juniper shows which is instead much
more localised on fewer GE port congestion
levels.
48
Conclusions

• We benchmarked both OC-48 and GE cards for each
  single router manufacturer by looking at the
• Link Utilisation
• How the BE and LBE Relative errors change all
  over the BW allocation set axis and with an
  increasing level of port congestion.
• This study highlighted how
• A good Link Utilisation is only necessary but not
  also sufficient to have a precise BW allocation
• The study of the error dynamic per BW allocation
  couple and per port congestion level is thus
  necessary in order to evaluate if and where
  errors in the allocation of the minimum
  guaranteed BW are.
• We chose to evaluate the performance of the cards
  based on an accuracy in the allocation of the BW
  of 95.
• This is equivalent to have the maximum LBE
  relative error with sign (M-LREWS) lt - 2.5 for
  this reason it is called max operating region
  (over the BW allocation couples).
• BE error is not taken into account as it is
  always almost negligible for any of the
  manufacturers card under test.
• This result suggests that the main problem these
  cards encounter is that they are unable to
  reallocate the BE left-over BW to LBE with a
  narrower operating region available as a
  consequence.

49
Conclusions (cont.)

• The major outcome of the tests is that
• Procket shows the best performance for both
  cards. The OC-48 result is even perfect.
• Cisco has got the worst performances of all three
  manufacturers and for both cards.
• Juniper is very close in performance to Procket
  for the OC-48 but is very close to Cisco for the
  GE card. (see next point for further evaluation
  of the GE performances between Cisco and juniper)
• A further analysis of the GE performance based on
  the percentage improvement (delta ?) in passing
  from the max to the avg operating region
  shows how 100 and 6 are the delta improvements
  experienced by Juniper and Cisco respectively.
• This suggests that the per-BW-allocation Cisco
  LBE relative error is much more spread and
  therefore serious all over the GE port congestion
  level axis if compared with that of Juniper which
  is instead much more localised on fewer port
  congestion levels.

50
Conclusions (cont.)

• It is clear, for the three manufacturers, that
  the QoS implementation in the OC-48 line cards
  presents a much more precise formulation than
  that found for the GigE line cards. This suggests
  that raw speed may not be the main issue in the
  design of good bandwidth schedulers
• It is however true that for the tests of the GigE
  line cards the level of over-commitment was
  greater than for the equivalent OC-48 line card
  tests, i.e. 3 1Gpbs over a 1Gbps link as
  opposed to 3 1Gbps over a 2.5Gbps link. This
  may be of significance but the test environment
  was the same for all line cards tested.
• The fact that SONET employs a synchronous serial
  transmission while GigE uses an asynchronous
  serial transmission may also be of significance
  to these results
• Finally, SONET is a much more mature technology
  operating at Gigabit rates in comparison with
  GigE and this may contribute in some way to the
  results presented

51
Thanks.