Voice Over IP
Birla Institute Of Technology,India
Pramod Jindal BE266/04
Gaurav Kumar BE222/04
Akansha Nag BE17/03
Dr R K LAL
Current state of internet
Simulations of existing algorithms
Limitations and motivation
Design and simulation of proposed algorithm with OPNET
How does VOIP work?
• Analogue speech is first converted into 1s and 0s.
• Digital signal is then broken up into packets.
• Packets are sent over the Internet and re-assembled at
destination and converted back into audio.
Private IP Network
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IP PacketIP Packet
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Current State of Internet
Uses best-effort service model
No guarantee of timeliness or delivery
No service discrimination
Bandwidth and network congestion problems
VOIP QOS Requirements
VoIP is a time-critical, loss-critical application
150 ms max end-to-end delay
Slight loss (lesser than 1%) is tolerable but leads to deterioration of
voice quality. Steps need to be taken for correction
However , delay requirement is more critical because a lost packet
is better than a late packet
Packets entering a router, are first classified based on their e.g.
source address/port, destination address/port. After that, they are
forwarded to the output interface of the router, where they
experience a predefined scheduling mechanism.
The packet scheduler is responsible for the order in which the
packets of the various queues are dequeued and transmitted in the
FIFO (First in First out)
PQ (Priority queue)
WFQ (Weighted fair Queueing)
Proposed Algorithm :
PQ :real time
WFQ : other traffic with dynamically assigned weights
Isolation and sharing:
Allow sharing of common resources in a controlled way
In circuit switched network full isolation, no sharing
Under-utilization of resources
Either deterministic (guaranteed service) or statistical
Deterministic bounds give the best isolation
Statistical delay bound allow more efficient sharing, less isolation
When there is a contention for resources, BW must be allocated fairly to
all competing flows.
Packet Scheduling Algorithm
First In First Out (FIFO) :The first packet that arrives at a router is the first
packet to be transmitted . If a packet arrives and the queue (buffer space) is
full, then the router discards (drops) that packet.
FIFO offers high cost-efficiency and no versatility.
Priority Queuing (PQ) :
Each packet is given a Priority , in the IP Type of Service (ToS) field.
Multiple FIFO queues implemented, one for each priority class, still
managed in a FIFO manner. Packet are dequeued on basis of priority
May lead to STARVATION. If a source sends too much traffic with highest
priority, may occupy up to 100% of the bandwidth.
Packet Scheduling Algorithm(cont)
Weighted Fair Queuing (WFQ) : Separate queue for each flow is
maintained by the router. The router then services these queues in a round
robin manner. It compares the weight for each sub-queue with the
We have implemented and evaluated existing Packet Schedulers
(FIFO, WFQ, PQ) at router level. Through these Simulations we
have shown the relative advantages of FIFO , PQ ,WFQ algorithms
• Packet Dropped
• FIFO has busty nature and drops significant number of packets.
It may work for Best efford traffic but not for real time
• WFQ gives near optimum performance
• FIFO: No bounded delay ,PQ : starvation
In Packet Networks a dropped packet is better than a late packet.
Through these promising results , we wish to model an algorithm that
has all the best features of these .We use PQ for real time traffic
and WFQ for other types of traffic along with dynamically assigned
Current Limitations and
Undoubtedly WFQ and at times PQ scores well among the existing
algorithms for scheduling however WFQ scheduling has some limitations.
In WFQ schemes, every new packet arrival is stamped with a tag and
packets are transmitted in an increasing order of tags. The complexity of
WFQ arises from the following sources, of which the second is present in
all WFQ schemes.
a) Tracking the System State: For the computation of tags to be stamped on
new arrivals, all WFQ schemes have to track (with time) the state of the
system (called “virtual time” or “system potential”). Each of these events
invoke moderate amount of processing.
b) Tag Sorting: In all WFQ schemes, before scheduling any packet
transmission, it is required to determine which session among has the
packet with the smallest tag. Such a tag sorting requirement may become a
bottleneck at high transmission speeds. Complexity for the same can be
With these promising results we wish to model a packet scheduler using PQ
for real time traffic and WFQ for other traffic types along with dynamically
A model of our proposed algorithm is something like this, serving different
classes of traffic
TCL1-Highly Interactive Traffic
TCL2-Lesser Interactive Traffic
TCL3-Non real time traffic
TCL4-Best Effort Traffic
Since real time traffic is de-queued using PQ instead of WFQ,optimum
performance is reached for services like voice. Also limitations of WFQ have
been eliminated on VOICE traffic
While other services should not starve they are served by WFQ, whose
weights are assigned dynamically depending on traffic pattern , which is
given near fair bandwidth
It must provide
Fair Bandwidth share
lesser sorting overhead
Queuing Delay for TCL1 < 8msec, for TCL2 < 20msec, TCL3 < 50msec and
for TCL4 < 80 msec
Simulation Results cont….
Proposed Algorithm is able to implement differentiated service giving a fair
pie of resources to different traffic types
Instead of using a single packet scheduler ,proposed algorithm uses two
types of scheduler treating each traffic class separately
Since real time Voice traffic is scheduled by PQ instead of WFQ, the sorting
overhead for voice packets for WFQ has been avoided
The delay encountered by each traffic class in our simulations falls pretty
much in the tolerable limits as per ITU-T recommended QOS requirements
Advantage is taken of the fact that VOIP is insensitive to dropped packets
( upto 1%) to make a tradeoff between bounded delay and packet loss!
• Our simulation results verify the design and correctness of proposed
• However still better results can be obtained if we could really vary the
weights of WFQ packets depending on traffic pattern
• We faced problems in JIT allocations of weights which would not only
provide bounded end to end delay but also fair bandwidth share at times
when traffic pattern goes unexpectedly bursty!
• There is still enough of scope in the betterment of Router Design
3)Lee Breslau, et al., “Advances in Network Simulation, IEEE
Computer, Vol. 33, No. 5, pp.59-67, May 2000.
4)Uyless black, Voice over IP, Prentice Hall PTR, 1999.
5)P.J. Kuhn, C.D. Pack and R.A. Skoog, “Common Channel
Signaling Networks: Past, Present, Future,” IEEE Journal on
Selected Areas in Communications, pp. 381-393, April 1994.
6)Sheldon M. Ross, Introduction to Probability Models, 7th ed.,
Academic Press, Feb. 2000.