MPustisek1998-Classes-of-service

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					                              Classes of service in ATM networks
                                       3HWHU+RPDQ-DQH]%HãWHU0DWHYåPustišek
                                              Faculty of electrical engineering
                                                  University of Ljubljana
                                          7UåDãND6,/MXEOMDQD6ORYHQLD
                                   Tel.: +386 (0)61 1768 431, Fax: +386 (0)61 264 990
                                     lt@mail-lt.fe.uni-lj.si http://www-lt.fe.uni-lj.si



Abstract                                                         -   Unspecified bit rate (UBR) type of traffic has no
                                                                     guaranties about available capacity and assured
The next article handles Classes of Service (CoS) and                quality of service. It is typical a representative of
related traffic management functions in Asynchronous                 best-effort type of traffic. UBR flows are used to
Transfer Mode (ATM) networks. Existent service                       fill up the rest of unused network capacities. It is
classes are briefly described and then analysed. In                  mainly intended for non-real-time transfer of less
order to assure agreed capacity and quality of service               important data.
traffic management in ATM network is essential.
Existing CoS require quite complicated management                CoS is used for more or less accurate specification of
and this is one of the reasons for slow introduction of          transfer requirements needed for individual services
ATM technology.                                                  and applications. At connection set-up time, precise
We are proposing simpler traffic management scheme               adjustments are made for each virtual connection:
with only two classes of service: rt-VBR and nrt-VBR             - The capacity requirements are specified by PCR
– we have come to the conclusion that those two CoS                   (Peak Cell Rate), SCR (Sustainable Cell Rate),
are enough to optimally satisfy most applications. In                 MBS (Maximum Burst Size) and MCR (Minimum
order for our proposal to work optimally, some                        Cell Rate); accordingly to selected service class.
suggestions should be followed: traffic sources should           - The quality of service is typically connected to the
make use of cell loss priority bit (CLP) and ATM                      terms CTD (Cell Transfer Delay), CDV (Cell
switches have to distinguish between low and high                     Delay Variation) and CLR (Cell Loss Ratio).
priority cells.
                                                                 One of the reasons for slow introduction of ATM
                                                                 technology is due to complicated management
Introduction                                                     functions. Those functions are tightly connected with
                                                                 existing classes of service. In the following chapter we
ATM networking technology enables transfer of                    briefly describe existing ATM traffic management
various data. Traffic flows can have either constant or          functions. This is followed by short analysis of existing
variable bit rates, can request appropriate transfer delay       ATM classes of service and problems involved with
and cell loss ratio, etc. Traffic flows are systematically       them. As a result we propose introduction of simplified
divided in so called classes of service (CoS). Currently,        traffic management scheme with reduced number of
there are four CoS available [Afd96], as shown on                service classes. Finally, most important statements are
Figure 1:                                                        repeated in the conclusions.
- Constant bit rate (CBR) emulates classical bit-
     pipes between source and destination. CBR are
     real-time traffic flows with constant bandwidth,            Traffic management in ATM networks
     low transfer delay and delay variation, and low bit
     error ratio.                                                In order to assure agreed capacity and quality of
- Variable bit rate (VBR) is divided into two                    service traffic management in ATM network is
     subclasses: real-time-VBR and non-real-time-                essential. The following set of traffic and congestion
     VBR. The main difference between those two is in            control functions is available for traffic management
     total transmission delay and delay variation.               [Afd96]:
     Rt-VBR is suitable for transfer of compressed               - CAC (Connection Admission Control),
     video or audio signals. Nrt-VBR is for example              - UPC (Usage parameter Control),
     suitable for fast transfer of bank transactions.            - SCD (Selective Cell Discard),
- Available bit rate (ABR) type of traffic was                   - Traffic Shaping,
     designed in order to assure best utilisation of             - EFCI (Explicit Forward Congestion Indication),
     available network capacities. ABR flow capacity is          - Resource Management using Virtual Paths,
     varying between agreed minimum and maximum                  - Frame Discard,
     values. It is mainly intended for transfer non-real-        - GFC (Generic Flow Control),
     time data.                                                  - ABR Flow Control.
                                                        Services


                                   Real-time                           Non real-time

                                       CBR                                     ABR

                                       VBR                                     UBR
                                         Figure 1: Classes of service in ATM networks




ATM networks can implement one or a combination of              VBR on the other hand is a problem. Users in this CoS
these functions in order to meet quality of service             are allowed to transmit regular bursts of cells, which
objectives of compliant connections. Since ATM                  have to be delivered to the destination. This implies
technology was designed especially for broadband                two problems:
communications, flow control was realised mostly                - smaller buffers can be overflowed,
through preventive methods (CAC, UPC, SCD, Traffic              - larger buffers can cause large cell delays.
Shaping,...) [Sch96] [Rop96] [Key95] [Onv93]. The               Use of statistical multiplexing is one of the attempts to
use of backward information (feedback loops) was                solve this problem.
limited (EFCI). The change occurred with introduction
of ABR (Available Bit Rate) service class. ABR uses             ABR represents the biggest problem of all. Due to
feedback traffic control loops, one for each virtual            feedback delays buffers must be able to absorb the
connection [Fah98] [Jai96] [Bon95] [Kun95]. Current             difference between actual cell rate and allowed cell rate
state of traffic control mechanisms in ATM networks             for the duration of feedback delay. Large buffers in
can be represented with following two extremes:                 term cause large cell delays. ABR is therefore not
- CBR, VBR and UBR classes of service do not                    appropriate for real-time transfer. Implementation and
     consider and do not adjust to the current network          management of ABR traffic control mechanism is quite
     conditions (except during connection set-up time –         complicated and requires a lot of processing power and
     CAC),                                                      memory. That is the reason for slow introduction of
- ABR class of service uses closed control loops of             this CoS.
     end-to-end type, one for each virtual connection.
     This enables precise adjustments to the current            ATM switch needs separate output queue for each class
     traffic conditions, with some delay.                       of service in order to assure agreed quality of service to
                                                                all connections. In fact, it even needs two queues for
                                                                VBR: one for real-time-VBR and one for non-real-
Problems with existing classes of service                       time-VBR. Each queue needs separate management
                                                                procedure. The results are complicated traffic
In this chapter we will briefly analyse existing CoS.           management functions for ATM networks.
CBR and UBR are not problematic. The first one
transmits cells at constant pace and therefore represents
no threat to fill the buffers in ATM switches. UBR can
transmit large bursts but those cells can be simply
discarded if needed.
Reduced number of service classes                                 UBR is achieved with nrt-VBR, where only PCR has to
                                                                  be specified. SCR and MCR are set to zero and all cells
Bearing in mind the conclusions from previous chapter             must have low priority (CLP=1).
we have decided to make a proposal for simplified
traffic management scheme. In fact, we have come to               ABR is the only class that can not be used in our
the conclusion that only two classes of service are               proposed traffic management scheme. Instead, we
enough to optimally satisfy most applications:                    propose introduction of Enhanced Selective Cell
- real-time-VBR                                                   Discard Mechanism (E-SCD), as shown on Figure 2.
- non-real-time-VBR                                               Let's assume situation where congestion occurs in
                                                                  output queue of switch B. Selective cell discard
This implies that ATM switches need only two                      mechanism starts discarding low-priority cells that are
separate output queues. Queue serving real-time traffic           trying to enter the congested queue. All tributaries are
will be small and will have higher priority. Queue                considered equally – SCD discards rightful share of
serving non-real-time traffic shall be as large as                low priority cells from all tributaries. Existent SCD
possible to prevent cell losses. Traffic sources should           mechanism can be improved by introducing congestion
make use of cell loss priority bit (CLP) and ATM                  notification function, as shown on Figure 2. The basic
switches have to distinguish between low and high                 idea of Enhanced SCD mechanism is to transfer SCD
priority cells.                                                   function from congested queue closer to the traffic
                                                                  sources. Cells that are discarded in ATM switch A
All VBR sources should use conformance definition 2               would so or so be discarded in ATM switch B. Early
or 3, in accordance with ATM Forum specifications                 discarding of ATM cells reduces load in ATM switch
[Afd96] – for SCR (Sustainable Cell Rate) only high               B and releases some bandwidth between A and B.
priority cells (CLP = 0) are verified for compliance and          Released bandwidth can now be used to increase traffic
for PCR (Peak Cell Rate) compliance of all cells                  flow Z.
(CLP = 1 and CLP = 0) is verified.

CBR is achieved with rt-VBR, where PCR is set to the
same value as SCR and maximum burst size is set to
zero (MBS=0). All cells should have high priority
(CLP=0).




                  ATM switch                                 ATM switch                     ATM switch
                        A                                        B                                 C
                                     congestion
                    selective cell
                                     notification                    congestion
                       discard
           X                                                                                                      X
                                            selective cell
                                               discard




                       Z                                     Y       Z                                        Y
                      X – virtual connections with origin in ATM switch A and destination in ATM switch C
                      Y – virtual connections with origin in ATM switch B and destination in ATM switch C
                      Z – virtual connections with origin in ATM switch A and destination in ATM switch B

                 Figure 2: Transfer of selective cell discard mechanism from ATM switch B to ATM switch A
                                                  – Enhanced SCD mechanism
Conclusions                                                 References

Existing classes of service in ATM networks are well
                                                            [Afd96]   The ATM Forum Technical Committee,
defined but complicated to manage. We have proposed
                                                                      Traffic Management Specification Version
simpler traffic management scheme with only two
                                                                      4.0, April 1996.
classes of service: rt-VBR and nrt-VBR. In order for
our proposal to work some limitations have to be            [Bon95]   F. Bonomi, K. W. Fendick, The Rate-
considered. It is very important to differentiate between             Based Flow Control Framework for the
low and high priority ATM cells. Applications have to                 Available Bit Rate ATM Service, IEEE
consider possible losses of low priority cells. Proposed              Network, str. 24-39, March/April 1995.
enhanced selective cell discard mechanism should be
used to increase network utilisation.                       [Fah98]   S. Fahmy, R. Jain, R. Goyal, B. Vandalore,
On the other hand, our scheme is simple to manage.                    S. Kalyanaraman, Feedback Consolidation
We have come to the conclusion that only two classes                  Algorithms for ABR Point-to-Multipoint
of service are enough to optimally satisfy most                       Connections in ATM Networks, Submitted
applications. Users can individually decide how much                  to INFOCOM'98, 1998.
bandwidth should be guarantied (CLP=0) and how
much should be luck based (CLP=1). Connection price         [Jai96]   R. Jain, Congestion Control and Traffic
should be set accordingly.                                            Management in ATM Networks: Recent
                                                                      Advances and a Survey, Computer
                                                                      Networks and ISDN Systems Journal,
                                                                      October 1996.

                                                            [Key95]   P. B. Key, Connection admission control
                                                                      in ATM networks, BT Technology
                                                                      Journal, Vol. 13, No. 3, str. 52-66, July
                                                                      1995.

                                                            [Kun95]   H. T. Kung, R. Morris, Credit-Based Flow
                                                                      Control for ATM Networks, IEEE
                                                                      Network, str. 40-48, March/April 1995.

                                                            [Onv93]   R. O. Onvural, Asynchronous Transfer
                                                                      Mode Networks: Performance Issues,
                                                                      Artech Hause, Inc., Norwood, MA, USA,
                                                                      1993.

                                                            [Rop96]   C. Roppel, R. Habermann, Usage
                                                                      Parameter Control in ATM-Netzen, Der
                                                                      Fernmelde Ingenieur, 50. Jahrgang, Heft 8,
                                                                      str. 1-32, August 1996.

                                                            [Sch96]   M. Schwartz, Broadband integrated
                                                                      networks, Prentice-Hall Inc., New Jersey,
                                                                      USA, 1996.