2. Problems
Current Internet still provides best efforts service
No Guarantee of performance for real time multimedia application
Multimedia typically uses UDP
Not reliable
No congestion control
Multimedia traffic is normally subject to
Restricted available bandwidth
Delay, Delay jitter
Loss of packets
3. Control Mechanisms
Mechanisms which
Dynamically adapt the behavior of the audio application to
maximize the audio quality under the constraints of
• Restricted bandwidth
• Delay
• Packet Loss
• Jitter
present in the network at that point of time
4. Audio Compression Techniques
(codecs)
Current codecs have a diverse range in terms of
degree of compression (bitrates) and underlying
technologies
Thus the quality of an IP telephony call is highly
dependent on the codecs and their reaction to
available bandwidth, link delays and packet loss.
5. Mean Opinion Score (MOS)
Described in ITU recommendation P.800
Formal subjective measure if voice quality
Real number – Between 1 to 5
Toll Quality – Quality with MOS between 4 and
4.3
Communication Quality – Between 3.5 and 4
Lower bound for acceptability of a speech – 3.5
MOS has been determined for every codec under
the ideal condition of no loss.
7. Bandwidth Constraint
End to End available bandwidth – the maximum
rate that the path can provide to a flow
Depends upon the utilization of various links in the
path in presence of cross traffic
Less than or equal to capacity of the path – The
maximum rate a path can provide to a flow, wwhen
there is no other traffic in the path
8. Bandwidth Constraint (contd..)
In underutilized network we can use high bitrate codecs
which will consume more bandwidth
but will generate high quality
But switch to low bitrate codecs when available
bandwidth gets tighter
It is possible to mix multiple codecs in a certain ratio for
bandwidth optimization ensuring that the audio quality
provided is optimum for the user.
9. Delay Constraint
Delay of the path
Propagation delay of individual links
Queuing delay at individual hops/routers
Delay inherent to the codec
T(codec) = T(enc.) + T(dec.) + T(LA)
Total delay must be under the constraint of tolerable
Mouth-to-Ear (M2E) delay
The time that elapses between the moment the talker utters the
words and the moment the listener hears them
Must be under 400 ms (ITU recommendation G.114 & G.131)
10. The LP Problem
Maximizes the audio quality under the constraint of available bandwidth and link delay
Maximizes MOS (z) = c1x1 + c2x2 + ….. + cnxn
Subject to;
b1x1 + b2x2 + ….. + bnxn <= B
/*bandwidth constraint*/
d1x1 + d2x2 + ….. + dnxn <= D
/*delay constraint*/
c1x1 + c2x2 + ….. + cnxn <= 4.3
/*max possible MOS attainable by codec*/
c1x1 + c2x2 + ….. + cnxn >= 3.5
/*lower bound of acceptable MOS score*/
x1 + x2 + ….. + xn= 1
/*total of all percentage*/
11. The LP Problem (contd..)
Where
x1, x2 ….., xn = percentage of each codec in transmission mixing
c1, c2 ….., cn = MOS value for each codec
b1, b2 ….., bn = bitrates for each codec
d1, d2 ….., dn = (packet size in bytes)*(encoding/decoding delay
to create/decode 1 byte)
B = Available bandwidth
D = 400 ms (link one way delay)
12. Implementation
Codec
Bitrate
(kbps)
MOS
Delay for Packet
1 byte
size
(ms)
(bytes)
PCM µ-law
64
4.3
0.50
200
G.721
32
4.0
1.00
200
GSM fullrate
13
3.7
2.42
198
G.728
8
4.0
2.50
200
G.723.1
5.6
3.9
6.00
210
The delay values for each codec has been determined based on existing literature and
experiment.
13. Implementation (contd..)
The formulation of this particular linear programming problem is
thus:
Maximizes MOS (z) = 4.3x1 + 4.0x2 + 3.7x3 + 4.0x4 + 3.9x5
Subject to;
64x1 + 32x2 + 13x3 + 8x4 + 5.6x5 <= B
0.1x1 + 0.2x2 + 0.48x3 + 0.5x4 + 1.26x5 <= D
4.3x1 + 4x2 + 3.7x3 + 4x4 + 3.9x5 <= 4.3
4.3x1 + 4x2 + 3.7x3 + 4x4 + 3.9x5 >= 3.5
x1 + x2 + x3 + x4 + x5 = 1
Xi >= 0 where i = 1,2,3,4,5