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Voice over IP Fundamentals

M. ArvaiNEC Senior Technical Eng.

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Agenda• Switching Telephony Network Fundamentals

o Overview o Enterprise Telephonyo Telephony Signaling o PBX Features

 • VoIP and IP Telephony Technology

o Data Network Basicso VoIP Basicso VoIP Compressiono VoIP Signalingo Networking

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Telephony Network

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Privately Owned Switches

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Basic Call Setup

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Digital versus Analog Connections

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E1 Channel Associated Signaling

E1 framing and signaling, 30 of the 32 available channels, or time slots, are used for voice and data 

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Q.Sig

• The QSIG (Q Signaling) protocol is based on the standard and provides signaling for private integrated services network exchange devices.

• QSIG is implemented on PRI interfaces only. By using QSIG PRI signaling, a Router can route incoming voice calls from a PINX across a WAN to a peer Router, which can then transport the signaling and voice packets to a second PINX

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VoIP Advantages

• Flexibility The sophisticated functionality of IP networks allows organizations to be flexible in the types of applications and services they provide to their customers and users.

• Advanced featureso Advanced call routing o Unified messaging o Integrated information systems o Long-distance toll bypass o Encryption o Customer relationship

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VoIP Protocols

• H.323 An ITU standard protocol for interactive conferencing. H.323 was originally designed for multimedia in a connectionless environment, such as a LAN.

• Media Gateway Control Protocol (MGCP) defines a protocol to control VoIP gateways that are connected to external call-control devices, referred to as call agents

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VoIP Protocols

• Session Initiation Protocol (SIP) A detailed protocol that specifies the commands and responses to set up and tear down calls. SIP also details features such as security, proxy, and transport (TCP or User Datagram Protocol [UDP]) services. SIP defines end-to-end call signaling between devices.It also adopts a modified form of the URL-addressing scheme used within e-mail that is based on Simple Mail Transfer Protocol (SMTP).

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VoIP Protocols

• Real-Time Transport Protocol (RTP) An IETF standard media-streaming protocol. RTP carries the voice payload across the network. RTP provides sequence numbers and time stamps for the orderly processing of voice packets

• RTP Control Protocol (RTCP) Provides out-of-band control information for an RTP flow

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Issues and Solutions for VoIP

• Latencyo Increase bandwidth.o Choose a different CODEC type.o Fragment data packets.o Prioritize voice packets.

• Bandwidtho Calculate bandwidth requirements, including voice,

payload, overhead, and data.• Jitter

o Use dejitter buffers.

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VoIP Network Architectures

• Centralized Network Architectures

IP Net.

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VoIP Network Architectures

• Distributed Network Architectures

IP Net.

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Comparing Network Architectures

• Configuration o The centralized call control model provides superior

control of the configuration and maintenance of the dial plan and endpoint database. It simplifies the introduction of new features and supplementary services. The centralized call control model also provides a convenient location for the collection and dissemination of call detail records (CDRs).

o The distributed model requires distributed administration of the configuration and management of endpoints. This approach complicates the administration of a dial plan. Distributed call control simplifies the deployment of additional endpoints while making new features and supplementary services difficult to implement.

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Comparing Network Architectures

• Security Centralized call control requires that endpoints be known to a central authority. This approach avoids or reduces security concerns. The autonomy of endpoints in the distributed model elevates security concerns.

• Efficiency Centralized call control fails to take full advantage of call routing intelligence that resides in the endpoints. It also consumes bandwidth through the interaction of the call agent and its endpoints.

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Comparing Network Architectures

• Reliability o The centralized model has two points of vulnerability:

single point of failure and contention. It places high demands on the availability of the underlying data network, possibly requiring a fault-tolerant WAN design.

o The distributed call control model minimizes the dependence on shared common control components and network resources. This approach reduces exposure to single points of failure and contention for network resources.

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Building Scalable Dial Plans

• Numbering plan : A numbering plan identifies each VoIP endpoint and application in the network with a unique telephone number.

• Dial plan : is a key element of an IP telephony system and an integral part of all call-processing agents. Primary functions of a dial plan include:o Endpoint addressing o Path selection o Calling privileges o Digit manipulation o Call coverage o Overlapping number processing

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Building Scalable Dial Plans

Hierarchical Numbering Plans A hierarchical design has the following advantages:

o Simplified provisioning Refers to the ability to easily add new groups and modify existing groups

o Simplified routing Keeps local calls local and uses a specialized number, such as an area code, for long-distance calls

o Summarization Establishes groups of numbers in a specific geographical area or functional group

o Scalability Provides additional high-level number groupso Management Controls number groups from a single point in

the overall network

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Building Scalable Dial Plans

The challenges faced with numbering plan integration include the following:

• Varying number lengths • Specialized services • Voice mail • Necessity of prefixes or area codes • International dialing consideration

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Signaling and Call Control

• In the traditional telephony network, a voice call consists of two paths:o An audio path carrying the voiceo A signaling path carrying administrative information

such as call setup, teardown messages, call status, and call-progress signals.

• VoIP call leg requires two paths:o A protocol stack that includes RTP, which provides the

audio call lego One or more call control models that provide the

signaling path

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Signaling and Call Control

Call Control Models

o H.323 describes the architecture to support multimedia communications over networks without quality of service (QoS) guarantees.

o Session initiation protocol (SIP) is an Internet Engineering Task Force (IETF) RFC 3261 call control model for creating, modifying, and terminating multimedia sessions or calls.

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Signaling and Call ControlCall Setup

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Optimizing Voice QualityFactors that Affect Voice Quality

o Fidelity The bandwidth of the transmission medium almost always limits the total bandwidth of the spoken voice.

o Echo A result of electrical impedance mismatches in the transmission path.

o Jitter Variation in the arrival of coded speech packets at the far end of a VoIP network.

o Packet drops The discarding of voice packets.

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Optimizing Voice QualityFactors that Affect Voice Quality

o Delay The time between the spoken voice and the arrival of the electronically delivered voice at the far end.

o Sidetone The purposeful design of the telephone that allows the speaker to hear the spoken audio in the earpiece.

o Background noise The low-volume audio that is heard from the far-end connection.

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Thank YouQuestions ?

arvahi @ nec-unified.ir