North South University
ETE 605(2): IP Telephony
Project Report on
“VOIP on MPLS Network- Technology”
Md. Mazharul Islam Khan
Dr. Moshiour Rahman
Department of computer science and Engineering
North South University
Over View 3
Multiprotocol Level Switching (MPLS) 4
Why does MPLS matter for VOIP 6
How MPLS- Enables VOIP analysis 7
What will MPLS do for VOIP Network 8
Voice Over MPLS / Diffserv Architechere 8
Challenges with VOIP 10
Multi-protocol Label Switching (MPLS) has often been presented as a technology that
will help service providers deliver better performance and availability, including specific
benefits for demanding applications such as voice and video. In recent years, many
service providers have implemented MPLS technology in their core networks. Now, with
MPLS backbones in place, these carriers are offering MPLS-based services to enterprise
customers in support of voice over IP (VoIP) and video applications. The carriers are also
aggressively advertising the benefits of MPLS, claiming large improvements in
performance and availability when compared with using the public Internet.
Figure1: Basic Terminology of VOIP over MPLS Network
As shown in Figure 1: CNA was deployed at three of the customer’s branch offices.
These three offices are linked to parallel network fabrics by MPLS and Internet links at
T1 speeds. Through its monitoring and assessing mechanisms, CNA evaluated the quality
of voice communication between Branch Site 1 and Branch Site 2 and between Branch
Site 1 and Branch Site 3, 2004. The assessment was done using CNA configured to use
its Voice Application Model.
CNA Application Models follow a five-stage methodology, shown in Figure 2. From the
bottom, the five steps include:
1. The measurement of low-level network quantities such as latency, loss, and jitter for
each available path between locations.
2. The computation of transport delays from the raw scores or the impact on applications
in general, not taking specific application sensitivities into consideration.
3. The computation of application delay, specifically the mouth-to-ear delay that affects
VoIP. Note that high jitter would result in high application delay; similarly, sustained loss
results in lost speech and unintelligibility, leading to increased application delay
.4. The determination of an application quality metric, using a ranking from zero to five
stars, similar to movie ratings. For VoIP, this step effectively equates to a Mean Opinion
Score (MOS) calculation for the network: the larger the jitter or the greater the sustained
loss, the lower the MOS rating.
5. Finally, the time periods where the voice quality is determined to be unacceptable are
logged as “bad minutes” for that path.
Using this methodology, kept a cumulative count of all “bad minutes” observed on the
various MPLS and Internet path choices between the sites in question
Multi-protocol Label Switching (MPLS)
MPLS is a standards-approved technology for speeding up network traffic flow and
making it easier to manage. In addition to moving traffic faster overall, MPLS is flexible,
fast, cost efficient and allows for network segmentation. While MPLS technology has
been around for several years, businesses are now taking advantage of service provider
offerings and beginning their own corporate implementations. It's a good idea for
network architects and engineers to get a head start on the technology.
Multiprotocol Label Switching (MPLS) is a standards-approved technology for speeding
up network traffic flow and making it easier to manage. MPLS involves setting up a
specific path for a given sequence of packets, identified by a label put in each packet,
thus saving the time needed for a router to look up the address to the next node to forward
the packet to. MPLS is called multi protocol because it works with the Internet Protocol
With reference to the standard model for a network (the Open Systems Interconnection,
or OSI model), MPLS allows most packets to be forwarded at the layer 2 (switching)
level rather than at the layer 3 (routing) level. In addition to moving traffic faster overall,
MPLS makes it easy to manage a network for quality of service (QoS). For these reasons,
the technique is expected to be readily adopted as networks begin to carry more and
different mixtures of traffic.
Figure 4: Multi protocol Level Switching Architecture
Like any significant business decision, a number of qualifying factors usually drive a
potential migration to MPLS. Several common reasons are:
• Converged services capabilities (voice, video, data).
• Any-to-any connectivity without the high cost of individual circuits.
• Advanced features for ingress and egress routing policies (load sharing, policy
• Secure flexibility of adding future businesses and partners (multiple VPN
• Circuit consolidation (frame, T-X, ATM).
Why does MPLS matter for VOIP?
Multi-protocol label switching (MPLS) enables a common IP-based network to be used
for all network services and for multiple customers of a network operator. It allows IP
networks to carry voice, data and video traffic with differentiated service-level
performance parameters. MPLS also enables Voice over Internet protocol (VOIP)
services over IP networks, so that a network operator can offer private networking
services to multiple customers on a shared infrastructure. Although MPLS may be used
with non-IP networks, it is IP networking; and more specifically data, voice, video
services over IP networks; that makes MPLS an attractive and growing technology.
MPLS is used to ensure that all packets in a particular flow take the same route over a
backbone. Deployed by many telcos and service providers, MPLS can enable traffic
engineering to deliver the quality of service (QoS) required to support real-time voice and
video as well as service level agreements (SLAs) that guarantee bandwidth. An MPLS
network ingress element attaches labels to IP packets. This label instructs the routers and
switches in the network where to forward the packets based on reestablished IP routing
information. Label switched paths (LSP) are defined in routing tables, and are used to
send tagged packets on specific paths through the network. LSPs represent a new type of
virtual paths for segregating traffic in an IP network. MPLS has been applied to
implementing Virtual Private Networks, which is a key revenue generator for service
providers offering enterprise services.
engineering, and other techniques.
Figure 6: Call routing through MPLS
How MPLS-enabled VOIP analysis
The application of MPLS to IP networks adds a new challenge, but also a new
opportunity, for VoIP analysis and testing. The challenge is that VoIP traffic from
multiple different virtual networks and service class tiers are mixed on common physical
links. The opportunity is that MPLS provides a means to separate this traffic for targeted
analysis. It simply requires theright tools to do this.
The Network Analyzer comprises many MPLS capabilities that not only enable targeted
VoIP analysis over MPLS in specific domains (VPNs, LSPs, service tiers, etc), but also
enable analysis of underlying network layers including MPLS networks. When MPLS is
used in DiffServ architectures to provide prioritization for different services, diagnosing
performance problems can be significantly expedited by focusing the analysis domain on
specific LSPs. Using the Network Analyzer, one can see VoIP performance for all traffic
on a link, and for traffic in specific domains. These domains can be based on MPLS To
analyze in a specific domain, one first applies a capture filter for a specific LSP. A filter
can be applied for up to 6 values of labels in a label stack. One can further filter on
specific values for Class of Service (CoS). This enables one to then analyze VoIP for that
LSP, for targeted analysis
of performance for that service tier. One can view traffic performance, for both VoIP and
non-VoIP traffic, per LSP and CoS.
MPLS Quality of Service
QoS on an MPLS backbone is used to provide predictable, guaranteed performance
metrics required to transport real time and mission critical traffic. The providers have an
overall QoS architecture that is used to deliver a subset of QoS services to each customer.
Since MPLS also supports reservation of Layer 2 resources, MPLS can deliver finely
grained quality of service, much in the same manner as ATM and Frame Relay. DiffServ
can support up to 64 classes while the MPLS shim label supports up to 8 classes. --
MPLS Resource Center QoS for IP/MPLS Networks by Santiago Alvarez is a practical
guide that will help you facilitate the design, deployment, and operation of QoS using
Cisco IOS Software and Cisco IOS XR Software. The book provides a thorough
explanation of the technology behind MPLS QoS and related technologies, including the
different design options you can use to build an MPLS network with strict performance
Figure 7: QoS Over MPLS Solutions
What will MPLS do for VOIP network?
MPLS based VOIP ( which offers exclusive and interconnectivity using Internet protocol
to computers or Local Area Networks across the PSTN) reduce customer-networking
complexity, costs and do away with the requirement of in-house WAN specialists.
Rather than setting up and managing individual point-to-point circuits between each
office using pair of Leased Lines, MPLS VOIP customers will need only one connection
from each office router to a Edge Router. MPLS VOIP is a technology that allows a
Service Provider to have complete control over parameters that are critical to offering its
customers service guarantees with regard to a high level of QOS e.g. bandwidth,
throughputs, latencies, jitters and availability. Many Company has tied up with various
Networking solution providers to provide end-to-end solution to its valued customers,
including Customer End (CE) routers and other networking components.
Voice over MPLS /Diffserv Architecture:
It is observed that the premium service (EF) of Diffserv is not enough alone to ensure the
quality of service. Some research results have shown the failure of EF PHB in meeting
the delay and jitter targets if an EF packet arrives on an output link on which a large BE
packet is already in the middle of being transmitted. Therefore it is important that some
type of prior reservation be made for transmission of real-time voice across a domain.
MPLS provides the mechanism that can be used to separate the BE traffic path from EF
and AF traffic path and to install explicitly routed LSP's (Label Switched Paths). We
have set up voice over MPLS/Diffserv using NS-2 simulator and its associated modules.
We are in the process of designing rules and specifications for an architecture to support
voice in an MPLS and Diffserv domain. These rules will dictate the formation of LSP's
carrying voice calls and aggregation of LSP's into virtual trunks across the domain. The
specifications are supposed to standardize the treatment of voice calls in the IP networks
and to make it easy to perform traffic engineering in a domain that deals with voice calls.
additional ports to the VOIP gateway and the voice data (in data form) is processed easily
by the PC. A Gateway interconnects the Voice source to the network through the local
exchange, forwards the voice to the destined subscriber across the Internet, and sends the
incoming calls to the corresponding extension via the PXB. VOIP delivers real-time and
two way synchronous voice traffic over the Internet or Intranet.
The IP telephony provides a number of benefits as compared to the Public Switched
Telephone Network (PSTN) such as: integration of voice, data and fax, sound grading,
video telephony, unified messaging, low-cost voice calls, real-time billing, remote
teleworking, enhanced teleconferencing, etc. It may face many technical challenges such
as: loss, delay, and jitter. Internet telephony has caught the world's attention despite
the inferior quality for many of these connections. Many companies have introduced
products that improve and commercialize the technology. New protocols such as Diffserv
and MPLS are being introduced with additional features like QoS, reliability and traffic
engineering that improve the performance of voice transmission over the IP network.
Unlike data, VOIP is more sensitive to delay than loss, thus sufficient bandwidth must be
guaranteed to the voice application. Some protocols that decrease voice transmission
delay by giving high priority to the voice than data traffic have been introduced.
Resource reSerVation Protocol (RSVP) provides for the routers to reserve the required
bandwidth for the voice connections, and Real-time Transport Protocol (RTP) uses
Figure 8: The Voice architecture over MPLS / Diffserv domain
Challenges with VOIP
Voice is a real-time service. It must be delivered with minimal delay (150 milliseconds
end-to-end is a common recommendation) and it must be reproduced with a constant bit
stream on the egress network or endpoint. Due to the delay requirements, IP
retransmissions are not allowed. Therefore, packets that are dropped on the network, or
late packets dropped by a jitter buffer, are not saved or reproduced. IP’s best effort
delivery and non-deterministic routing introduce delay and more importantly variance in
delay, also known as packet jitter, in the voice transmission. VOIP packets may be lost
due to packets dropped in router queues or by the jitter buffer.
MPLS enables service providers to deliver new services governed by specific SLAs and
COS. These services comprise the triple play: offering real-time voice and video along
with data on a common network. It is the real-time services like voice that will generate
the most revenue. The ability to keep these services running at quality levels that meet
customer expectations is crucial to retaining customers and realizing revenues. While
MPLS introduces new challenges to diagnosing and troubleshooting service-level
problems, advanced tools like the Agilest Network Analyzer makes this job simple and
fast for next generation network engineers and technicians.
These results suggest that an MPLS service is not the panacea for VoIP. MPLS service is
in fact essentially comparable to Internet service. Both provide good base connectivity,
but by themselves neither can deliver the quality and availability required for business-
quality voice communication.
To discover how can reduce the impact of bad minutes on VOIP communication over
both MPLS and Internet links, please refer to Performance of Virtualized MPLS Internet
Infrastructure in Delivering VOIP Service.