Communication Networks

1
Communication Networks

• Recitation 10
• QoS

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Quality of Service What is it?
Multimedia applications network audio and video
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Traffic Shaping The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky
bucket with packets.
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Leaky Bucket example

• A source generates data in terms of bursts 3 MB
  bursts lasting 2 msec once every 100 msec.
• The network offers a bandwidth of 60 MB/sec.
• The leaky bucket has a capacity of 4 MB. How does
  the output look like?
• Input 0-2 msec 1500 MB/sec 100-102 msec 1500
  MB/sec 200-202 msec 1500 MB/sec
• Output 0-50 msec 60 MB/sec 100-150 msec 60
  MB/sec .

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Leaky Bucket CNTD.

• What should be the capacity of the leaky bucket
  to avoid loss?
• During the burst, data inflow is at the rate of
  1.5 MB/msec and the outflow is at the rate of
  0.06 MB/msec.
• So accumulation is at the rate of 1.44 MB/msec.
  So at the end of 2 msec, there will be an
  accumulation of 2.88 MB. This is the minimum
  leaky bucket capacity to avoid buffer overflow
  and hence data loss.

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The Token Bucket Algorithm
5-34
(a) Before. (b) After.
Token bucket allows some burstiness (up to the
number of token the bucket can hold)
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Token Bucket simple example

• 2 tokens of size 100 bytes added each second to
  the token bucket of capacity 500 bytes
• Avg. rate 200 bytes/sec, burst size 500 bytes
• Packets bigger than 500 bytes will never be sent
• Peak rate is unbounded i.e., 500 bytes of burst
  can be transmitted arbitrarily fast

8
Token Bucket example

• Bucket capacity 1 MB
• Token arrival rate 2 MB/sec
• Network capacity 10 MB/sec
• Application produces 0.5 MB burst every 250 msec
  For 3 seconds
• The bucket is full of tokens

9
Token Bucket example CNTD.

• Initially, output can be at the rate of 10 MB/s.
  But how long can the bucket sustain this?
• First, 1MB can be sent
• From then on, for X seconds, the token input rate
  is 2MB/s, the traffic rate is 10MB/s
• 1 2X 10X ? 8X 1 ? X 1/8 sec 125 ms
• The bucket can transmit 1.25 MB in this time gt
  0.5MB the application produces
• Output 0-50 ms 10 MB/s
• 50-250 ms None

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Token Bucket example CNTD.

• At the end of this period, the amount of tokens
  in the bucket is
• 1MB250ms2MB/s-0.5MB1MB
• So the bucket is full again!
• Repeat for 3 seconds

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Minimum Bucket size and Token Rate

• Discarding Bucket (Policing)
• Bucket Size 0.5MB
• Token Rate 0.5MB/250ms 2MB/s
• Queueing Bucket (Shaping)
• How will the traffic look with Bucket Size
  200K?
• 0.22X10X ? X0.2/80.025s25ms
• 0-25ms 10 MB/s 0.25MB. 0.25MB left
• 0.25MB/(2MB/s) 125ms
• 25-150ms 2MB/s
• 150-250ms None
• Tokens after 100ms2MB/s0.2MB

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(s,?) Model

• Over an interval of length t the number of
  packets/bits that are admitted is less than or
  equal to (s?t).
• Composing flows (s1,?1) (s2,?2)
• Resulting flow (s1 s2,?1?2)
• What does a router need to support streams
  (s1,?1) (sk,?k)
• Buffer size B gt S si
• Rate R gt S ?i
• Admission Control (at the router)
• Can support (sk,?k) if
• Enough buffers and bandwidth
• R gt S ?i and B gt S si

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(s,?) Model example

• The line from the previous question has router
  with 4MB of buffers. How many flows of the above
  kind can it accept?
• s 0.5MB, ? 0.5MB/250ms 2MB/s
• For n flows, we require 0.5n MB buffers, 2n MB/s
  rate ? n 5.
• Each line will be served with a 0.5MB2MB/s token
  bucket

14
Random Early Detection (RED)

• Basic premise
• router should signal congestion when the queue
  first starts building up (by dropping a packet)
• but router should give flows time to reduce their
  sending rates before dropping more packets
• Note when RED is coupled with ECN, the router
  can simply mark a packet instead of dropping it
• Therefore, packet drops should be
• early dont wait for queue to overflow
• random dont drop all packets in burst, but
  space them

QoS
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RED

• FIFO scheduling
• Buffer management
• Probabilistically discard packets
• Probability is computed as a function of average
  queue length (why average?)

Discard Probability
1
0
Average Queue Length
queue_len
min_th
max_th
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RED (contd)
Discard
Discard Probability (P)
1
0
queue_len
Average Queue Length
min_th
max_th
Enqueue
Discard/Enqueue probabilistically
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RED (contd)

• Setting the discard probability P

Discard Probability
max_P
1
P
0
Average Queue Length
queue_len
min_th
max_th
avg_len
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Average vs Instantaneous Queue