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ATM : CONGESTION CONTROL

ATM : CONGESTION CONTROL. Group 7 29 DECEMBER 2004. ZAILINA AI’SHAH ZAINAL BAHREN WET 020199 -------------------------------------------------- - INTRODUCTION -. How ATM works. ATM network uses fixed-length cells to transmit information.

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ATM : CONGESTION CONTROL

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  1. ATM : CONGESTION CONTROL Group 7 29 DECEMBER 2004

  2. ZAILINA AI’SHAH ZAINAL BAHREN WET 020199 -------------------------------------------------- - INTRODUCTION -

  3. How ATM works • 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. • 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 4. Permits us to combine different types of traffic over high capacity links 5. Provides more reliable & cost effective 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. 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 020162 --------------------------------------------- - TYPES OF CONGESTION CONTROL -

  10. Types of Congestion Control • There are two fundamental approaches to congestion control: reactive approaches and preventiveapproaches • 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 • 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: - Leaky bucket & Token bucket - Resource reservation - Isarithmic control • 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. • 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. • 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. • The bucket has a finite buffer, danger of overflow!

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

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

  16. Token Bucket Algorithms • In regular intervals tokens are thrown into the bucket. • 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 no token in the bucket, the packet cannot be send.

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

  18. Token Bucket (Cont) Incoming Tokens Incoming Packets To Network + 4 3 2 1 5

  19. Token Bucket (Cont) Incoming Tokens Incoming Packets To Network + 1 5 4 3 2

  20. Token Bucket (Cont) Incoming Tokens Incoming Packets To Network + 5 4 3 1 2

  21. Token Bucket (Cont) Incoming Tokens Incoming Packets To Network + 5 4 3 2 1

  22. Token Bucket (Cont) Incoming Tokens Incoming Packets X To Network + 1 3 2 5 4

  23. Token Bucket (Cont) Incoming Tokens Incoming Packets X X To Network + 3 2 1 5 4

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

  25. Leaky Bucket vs Token Bucket

  26. NORBAITY HARIANI MUHAMMAD WET 020115 ------------------------------------------ - CAUSES & EFFECTS -

  27. Causes of Congestion • Exhaustion of buffer space • Deadlock

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

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

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

  31. Types of Deadlock • Store and Forward Lockup • Direct Store and Forward Lockup • Indirect Store and Forward Lockup • 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 020160 -------------------------------------------- - 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. • There are:- • Connection Admission Control (CAC) • Usage Parameter Control (UPC) • Priority Control • Traffic Shaping • Network Resource Management (NRM) • Frame Discard • Feedback Control

  35. 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.

  36. 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.

  37. Priority Control • 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.

  38. 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.

  39. 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. • This function is mainly concerned with the management of virtual paths in order to meet QoS requirements.

  40. 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.

  41. 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.

  42. WAN NORFAZLINA WAN MUSTAFFA KAMAL WET 020189 ----------------------------------------------------- - CRITERIA & SUMMARY -

  43. What is expected from Congestion Control?? • 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

  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)

  46. 2. Fairness • 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

  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

  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. • 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 • Preventive - The hosts & routers attempt to prevent congestion BEFORE it can occur • Reactive - The host & routers responds to congestion AFTER it occurs & then attempt to stop it

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