Transcript Slide 1

ATM : CONGESTION
CONTROL
Group 7
29 DECEMBER 2004
ZAILINA AI’SHAH ZAINAL BAHREN
WET 020199
-------------------------------------------------- INTRODUCTION -
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
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.
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.
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
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.
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.
SITI NUR ASLINDA MOHAMAD AYOB
WET 020162
--------------------------------------------- TYPES OF CONGESTION CONTROL -
Types of Congestion Control
• There are two fundamental approaches to
•
congestion control: reactive approaches and
preventive approaches
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
• 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)
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
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!
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
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)
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.
Host
computer
The bucket holds
token
One token is added
to the bucket every
T
networks
networks
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
5
4 32
1
+
To
Network
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
5
43 2
+
1
To
Network
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
5
4 3
+
2
1
To
Network
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
5
4
+
3
2 1
To
Network
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
5
X
4
+
3
2 1
To
Network
Token Bucket (Cont)
Incoming Tokens
Incoming Packets
XX
5
4
+
3
2
1
To
Network
Token Bucket (Cont)
• Tokens go into bucket at constant rate
• Each packet must consume one token
• Packets not discarded if bucket full
Leaky Bucket vs Token Bucket
Leaky Bucket (LB)
discards packets.
Token Bucket (TB)
discards tokens.
With LB, a packet can be With TB, a packet can
transmitted if the bucket is only be transmitted if
not full.
there are enough tokens
to cover its length in
bytes.
LB sends the packets at
TB allows for large bursts
an average rate.
to be sent faster by
speeding up the output.
LB does not allow saving, TB allows saving up
a constant rate is
tokens (permissions) to
maintained
send large bursts.
NORBAITY HARIANI MUHAMMAD
WET 020115
------------------------------------------ CAUSES & EFFECTS -
Causes of Congestion
1. Exhaustion of buffer space
2. Deadlock
1. 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
Exhaustion of Buffer Space
(Cont’d)
50 Mbps
Router
50 Mbps
50 Mbps
50 Mbps
Buffer
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
Types of Deadlock
1. Store and Forward Lockup
 Direct Store and Forward Lockup
 Indirect Store and Forward Lockup
2. Reassembly Lockup
Effects of Congestion
Performance of Degradation

Multiple packet loss

Low link utilization (low throughput)

High queuing delay

Congestion collapse
SITI NORHAMNIDA KAMARUZAMAN
WET 020160
-------------------------------------------- FUNCTIONS & PROCEDURES -
• To meet the objectives of traffic control and
•
1.
2.
3.
4.
5.
6.
7.
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
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.
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.
3. 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.
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.
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.
• This function is mainly concerned with the
management of virtual paths in order to
meet QoS requirements.
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.
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.
WAN NORFAZLINA WAN MUSTAFFA KAMAL
WET 020189
----------------------------------------------------- CRITERIA & SUMMARY -
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 endsystem complexity while maximizing
network utilization
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
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)
2. Fairness
• In shared environment, the throughput
for a source depends upon the demands
by the other sources.
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
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
Summary
• Congestion control is important in high
speed networks. Due to larger bandwidthdistance 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.
• 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