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ATM : CONGESTION CONTROL Group 7 29 DECEMBER 2004.

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Presentation on theme: "ATM : CONGESTION CONTROL Group 7 29 DECEMBER 2004."— Presentation transcript:

1 ATM : CONGESTION CONTROL Group 7 29 DECEMBER 2004

2 ZAILINA AI’SHAH ZAINAL BAHREN WET INTRODUCTION -

3 How ATM works 1. ATM network uses fixed-length cells to transmit information. –The cell consists of 48 bytes of payload and 5 bytes of header (to reduce delay variance). –The flexibility needed to support variable transmission rates is provided by transmitting the necessary number of cells per unit time

4 2. ATM network is connection-oriented. It sets up virtual channel connection (VCC) going through one or more virtual paths (VP) and virtual channels (VC) before transmitting information. It sets up virtual channel connection (VCC) going through one or more virtual paths (VP) and virtual channels (VC) before transmitting information. The cells is switched according to the VP or VC identifier (VPI/VCI) value in the cell head, which is originally set at the connection setup and is translated into new VPI/VCI value while the cell passes each switch. The cells is switched according to the VP or VC identifier (VPI/VCI) value in the cell head, which is originally set at the connection setup and is translated into new VPI/VCI value while the cell passes each switch.

5 3. ATM resources such as bandwidth and buffers are shared among users, they are allocated to the user only when they have something to transmit. So the network uses statistical multiplexing to improve the effective throughput.

6 Why ATM is important? 1. Provides guaranteed Quality of Service services 2. Uses telephone technology 3. Provides for more robust transmission of information information 4. Permits us to combine different types of traffic over high capacity links over high capacity links 5. Provides more reliable & cost effective transmission of information transmission of information

7 What Is Congestion Control? - Plays an important role in the effective traffic management of ATM networks - A state of network elements in which the network can not assure the negotiated Quality of Service (QoS) to already existing connections and to new connection requests. - May happen because of unpredictable statistical fluctuations of traffic flows or a network failure.

8 Why Need Congestion Control? The assumption that statistical multiplexing can be used to improve the link utilization is that the users do not take their peak rate values simultaneously. The assumption that statistical multiplexing can be used to improve the link utilization is that the users do not take their peak rate values simultaneously. But since the traffic demands cannot be predicted, congestion is unavoidable. Whenever the total input rate is greater than the output link capacity, congestion happens. Under a congestion situation, the queue length may become very large in a short time, resulting in buffer overflow and cell loss. So congestion control is necessary to ensure that users get the negotiated QoS.

9 SITI NUR ASLINDA MOHAMAD AYOB WET TYPES OF CONGESTION CONTROL -

10 Types of Congestion Control There are two fundamental approaches to congestion control: reactive approaches and preventive approaches There are two fundamental approaches to congestion control: reactive approaches and preventive approaches Reactive: feedback-based Reactive: feedback-based –Attempt to detect congestion, or the onset of congestion, and take action to resolve the problem before things get worse –The hosts and routers respond to congestion AFTER it occurs and then attempt to stop it

11 Preventive: reservation-based Preventive: reservation-based –Prevent congestion from ever happening in the first place, by reserving resources –The hosts and routers attempt to prevent congestion BEFORE it can occur Most ATM congestion control strategies are preventive (reservation-based)

12 Preventive and Reactive Control Preventive Techniques: Preventive Techniques: - Leaky bucket & Token bucket - Leaky bucket & Token bucket - Resource reservation - Isarithmic control Reactive Techniques: Reactive Techniques: - Load shedding - Choke packets The most popular one is leaky bucket scheme

13 Leaky Bucket Algorithms Each network interface contains a leaky bucket. Each network interface contains a leaky bucket. When the host has to send a packet, the packet is thrown into the bucket. When the host has to send a packet, the packet is thrown into the bucket. The bucket leaks at a constant rate, meaning the network interface transmits packets at a constant rate. The bucket leaks at a constant rate, meaning the network interface transmits packets at a constant rate. Bursty traffic is converted to a uniform traffic by the leaky bucket. Bursty traffic is converted to a uniform traffic by the leaky bucket. In practice the bucket is a finite queue that outputs at a finite rate. In practice the bucket is a finite queue that outputs at a finite rate. The bucket has a finite buffer, danger of overflow! The bucket has a finite buffer, danger of overflow!

14 Leaky Bucket (Cont) networks Host computer Packet Unregulated flow The bucket holds packets Regulated flow Interface containing leaky bucket Faucet Leaky bucket Water Water drips out of the hole at a constant rate

15 Leaky Bucket (Cont) Data output at constant rate Data output at constant rate If bucket overflows, input data discarded If bucket overflows, input data discarded Suitable for multimedia such as TV, audio Suitable for multimedia such as TV, audio Problem: Bursty traffic (computer data) Problem: Bursty traffic (computer data)

16 Token Bucket Algorithms In regular intervals tokens are thrown into the bucket. In regular intervals tokens are thrown into the bucket. The bucket has a maximum capacity. The bucket has a maximum capacity. If there is a ready packet, a token is removed from the bucket, and the packet is sent. If there is a ready packet, a token is removed from the bucket, and the packet is sent. If there is no token in the bucket, the packet cannot be send. If there is no token in the bucket, the packet cannot be send.

17 networks Host computer The bucket holds token One token is added to the bucket every  T

18 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network

19 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network

20 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network

21 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network 12345

22 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network X

23 Token Bucket (Cont) Incoming Packets Incoming Tokens + To Network XX

24 Token Bucket (Cont) Tokens go into bucket at constant rate Tokens go into bucket at constant rate Each packet must consume one token Each packet must consume one token Packets not discarded if bucket full Packets not discarded if bucket full

25 Leaky Bucket vs Token Bucket Leaky Bucket (LB) discards packets. Token Bucket (TB) discards tokens. With LB, a packet can be transmitted if the bucket is not full. With TB, a packet can only be transmitted if there are enough tokens to cover its length in bytes. LB sends the packets at an average rate. TB allows for large bursts to be sent faster by speeding up the output. LB does not allow saving, a constant rate is maintained TB allows saving up tokens (permissions) to send large bursts.

26 NORBAITY HARIANI MUHAMMAD WET CAUSES & EFFECTS -

27 Causes of Congestion 1. Exhaustion of buffer space 2. Deadlock

28 1.Exhaustion of Buffer Space  Routers maintain packet queues  Buffers fill up if: Routers are too slow, OR Routers are too slow, OR Combined input traffic rate exceeds the outgoing Combined input traffic rate exceeds the outgoing traffic rate traffic rate  Insufficient buffer space leads to congestion

29 Exhaustion of Buffer Space (Cont ’ d) 50 Mbps Router Buffer

30 2.Deadlock  The first router cannot proceed until the second router does something, and the second router does something, and the second router cannot proceed until the first second router cannot proceed until the first router does something router does something  Both routers come to a completely halt and stay that way forever stay that way forever

31 Types of Deadlock 1. Store and Forward Lockup  Direct Store and Forward Lockup  Indirect Store and Forward Lockup 2. Reassembly Lockup

32 Effects of Congestion Performance of Degradation  Multiple packet loss  Low link utilization (low throughput)  High queuing delay  Congestion collapse

33 SITI NORHAMNIDA KAMARUZAMAN WET FUNCTIONS & PROCEDURES -

34 To meet the objectives of traffic control and congestion control in ATM networks, several functions and procedures are suggested by the ATM Forum Technical Committee. To meet the objectives of traffic control and congestion control in ATM networks, several functions and procedures are suggested by the ATM Forum Technical Committee. There are:- There are:- 1. Connection Admission Control (CAC) 2. Usage Parameter Control (UPC) 3. Priority Control 4. Traffic Shaping 5. Network Resource Management (NRM) 6. Frame Discard 7. Feedback Control

35 1.Connection Admission Control (CAC) CAC is defined as the set of actions taken by the network during the call set-up phase in order to determine whether a connection request can be accepted or should be rejected. CAC is defined as the set of actions taken by the network during the call set-up phase in order to determine whether a connection request can be accepted or should be rejected.

36 2.Usage Parameter Control (UPC) UPC is defined as the set of actions taken by the network to monitor and control traffic at the end-system access. UPC is defined as the set of actions taken by the network to monitor and control traffic at the end-system access.

37 3.Priority Control The end-system may generate traffic flows of different priority using the Cell Loss Priority (CLP) bit. The end-system may generate traffic flows of different priority using the Cell Loss Priority (CLP) bit. The network may selectively discard cells with low priority if necessary such as in congestion to protect, as far as possible, the network performance for cells with high priority. The network may selectively discard cells with low priority if necessary such as in congestion to protect, as far as possible, the network performance for cells with high priority.

38 4.Traffic Shaping Traffic shaping is a mechanism that alters the traffic characteristics of a stream of cells on a connection to achieve better network efficiency while meeting the QoS objectives, or to ensure conformance at a subsequent interface. Traffic shaping is a mechanism that alters the traffic characteristics of a stream of cells on a connection to achieve better network efficiency while meeting the QoS objectives, or to ensure conformance at a subsequent interface.

39 5.Network Resource Management (NRM) NRM is responsible for the allocation of network resources in order to separate traffic flows according to different service characteristics, to maintain network performance and to optimize resource utilization. NRM is responsible for the allocation of network resources in order to separate traffic flows according to different service characteristics, to maintain network performance and to optimize resource utilization. This function is mainly concerned with the management of virtual paths in order to meet QoS requirements. This function is mainly concerned with the management of virtual paths in order to meet QoS requirements.

40 6.Frame Discard If a congested network needs to discard cells, it may be better to drop all cells of one frame than to randomly drop cells belonging to different frames, because one cell loss may cause the retransmission of the whole frame, which may cause more traffic when congestion already happened. If a congested network needs to discard cells, it may be better to drop all cells of one frame than to randomly drop cells belonging to different frames, because one cell loss may cause the retransmission of the whole frame, which may cause more traffic when congestion already happened.

41 7.Feedback Control Are defined as the set of actions taken by the network and by the end-systems to regulate the traffic submitted on ATM connections according to the state of network elements. Are defined as the set of actions taken by the network and by the end-systems to regulate the traffic submitted on ATM connections according to the state of network elements.

42 WAN NORFAZLINA WAN MUSTAFFA KAMAL WET CRITERIA & SUMMARY -

43 What is expected from Congestion Control?? The objectives of traffic control and congestion control for ATM are: The objectives of traffic control and congestion control for ATM are: Support a set of QoS (Quality of Service) parameters and classes for all ATM services and minimize network and end- system complexity while maximizing network utilization

44 Selection Criteria To design a congestion control scheme is appropriate for ATM network and non- ATM networks as well, the following guidances are of general interest To design a congestion control scheme is appropriate for ATM network and non- ATM networks as well, the following guidances are of general interest

45 1. Scalability The scheme should not be limited to a particular range of speed, distance, number of switches or number of virtual channel (VCs). The scheme should be applicable for both Local Area Network (LAN) and Wide Area Networks (WAN) The scheme should not be limited to a particular range of speed, distance, number of switches or number of virtual channel (VCs). The scheme should be applicable for both Local Area Network (LAN) and Wide Area Networks (WAN)

46 2. Fairness In shared environment, the throughput for a source depends upon the demands by the other sources. In shared environment, the throughput for a source depends upon the demands by the other sources.

47 3. Robustness The scheme should be insensitive to minor deviations such as slight mistuning of parameters or loss of control messages. It should also isolate misbehaving users and protect other users from them The scheme should be insensitive to minor deviations such as slight mistuning of parameters or loss of control messages. It should also isolate misbehaving users and protect other users from them

48 4. Implement ability The scheme should not dictate a particular switch architecture. It also should not be too complex both in term of time or space it uses The scheme should not dictate a particular switch architecture. It also should not be too complex both in term of time or space it uses

49 Summary Congestion control is important in high speed networks. Due to larger bandwidth- distance product, the amount of data lost due to simultaneous arrivals of bursts from multiple sources can be larger. Congestion control is important in high speed networks. Due to larger bandwidth- distance product, the amount of data lost due to simultaneous arrivals of bursts from multiple sources can be larger. For the success of ATM, it is important that it provides a good traffic management by using congestion control. For the success of ATM, it is important that it provides a good traffic management by using congestion control.

50 To solve congestion control is by using the preventive and reactive techniques To solve congestion control is by using the preventive and reactive techniques Preventive Preventive - The hosts & routers attempt to prevent congestion BEFORE it can occur Reactive Reactive - The host & routers responds to congestion AFTER it occurs & then attempt to stop it


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