voip the detail

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VoIPThe DetailAn in depth look at how VoIP works
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Introduction

VoIP is the generic name for voice over the Internet Protocol. It comes inmany flavours including Hosted VoIP. Hosted VoIP is a generic name for tele-phone services where the switchboard is located in the Internet asopposed to within the building with the rest of the phone systems.

Phones

We are going to consider how telephone calls are made over the internetusing an internet based switchboard.

A telephone call is made between two or more people. At each end of the

telephone call you must have a telephone or a device capable of acting like aphone such as a computer (to state the obvious). I am going to describe thenetwork in terms of these phones but wherever you see the word phone youcould substitute it for a computer based soft phone or an adapter.

These phones are specially designed for this use and are called by differentnames such as IP Phones (IP stands for Internet Protocol) or SIP phones (SIPstands for Session Initiation Protocol). These IP phones are phonesspecially made to work with the Internet. They are far more intelligent thanstandard old fashioned analogue hand sets.

Internet Addresses

Each device connected to the Internet must have a unique code numberwhich identifies it to the network. So if the network gets a message it knowswhere to send it. This code is known as an IP address. (IP stands forInternet Protocol.) There are two types of IP addresses dynamic and static.

When you plug your IP phone into a network socket the network gives it a dy-namic IP address. The part of the network that actually allocates the

address is usually a device called a router this is an electronic device whichroutes network traffic to the correct destination. The address will expire if youunplug the phone.

When you get a new phone it has to know about itself. A VoIP company likeBirchills will already have loaded an Internet Address for it to visit and findthe information it needs. This process is called provisioning. It tells the phoneits extension number, passwords, its model number and mostimportantly the address of the Switchboard in The Sky so it knows where toconnect to and where to send information.
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Protocol Stacks and Packets

So now your phone is connected to the Internet and has a unique address.How does it 'talk' to the other devices on the network?

When you pick up your handset the phone sends out a message to thenetwork and you hear a dial tone. What has actually happened is quitecomplex. You have invoked a SIP session. So lets try and explain that.

Protocols, Protocols

Protocols are a series of computer based rules which are applied to data. Ifprotocols are linked so that they apply one after another to a message then

they are called a protocol stack.

Each device that connects to the Internet uses a protocol stack which allows itto send out the messages in a format that the other devices attached to thenetwork will understand. The messages are sent out broken down into smallpieces called packets.

What happens is that any data is broken down into a stream of packets andthese packets are passed down the protocol stack with each layer in the stackoperating on the packet and passing it along to the next layer. When thepacket reaches the bottom of the stack at the local device it is passed alongthe network to the address defined within it. This means that a packet onceprocessed by the phone heads off into the network to the next serveraddress.

When incoming packets are received they start at the bottom of the stack andwork up.

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TCP/IP Protocol Stack

The protocol stack used on the Internet is referred to as the TCP/IP protocol

stack because of the two major communication protocols used. The TCP/IPstack has 4 layers:

1) The Application Protocols LayerThese protocols are specific to the application. In our case this is where thetelephony protocols SIP and RTP is RTP is located. There will be a lot moreabout SIP and RTP later.

2) Transmission Control Protocol TCPThe TCP layer converts the data into a format which can be sent over the

Internet by the IP layer. Due to network congestion, traffic load balancing, orother unpredictable network behaviour, IP packets can be lost or delivered outof order. TCP detects these problems, requests retransmission of lost packets,rearranges out-of-order packets, and even helps minimise network congestionto reduce the occurrence of the other problems.

3) Internet Protocol Layer IPThis layer directs the packets to a specific address.

4) Hardware Layer

This layer converts outgoing binary packet data to network signals andreceives incoming network signals and converts them back into packets. Thepacket is then passed back up the stack.

Session Initiation Protocol - SIP

SIP is the protocol that establishes the call. It finds the phone to which thecall is to be made and then negotiates what protocols will be used during thecall and then sets up the session.

Because the SIP supports name mapping and redirection services, it makes itpossible for users to initiate and receive communications and services fromany location, and for networks to identify the users wherever they are.

SIP works in concert with several other protocols and is only involved in thesignalling portion of a communication session

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SIP Agreement

SIP reaches agreement as to which protocols to use for the call including howthe data is encoded. The protocol adopted for data encoding is referred to asa codec. There are several codecs available and SIP has to agree the codecs.By default our system will try and use the G711a Codec, although SIP maychange that if the receiving end does not agree.

SIP "sessions" are packet streams of the Real-time Transport Protocol (RTP).RTP is the carrier for the actual voice or video content itself.

Real-time Transport Protocol - RTP

There are two parts to RTP: Data Transfer Protocol and an associatedControl Protocol.

The RTP data transfer protocol manages delivery of real-time data betweenend systems.

The RTP Control Protocol (RTCP) provides reception quality feedback,participant identification and synchronisation between media streams. RTCPruns alongside RTP, providing periodic reporting of this information. While theRTP data packets are sent every few milliseconds, the control protocol oper-ates on the scale of seconds.

How does this all come together?

We can see how this works by taking a look at a simple example of user 'A'trying to call user 'B'. A picks up his phone which sends a SIP invitationrequest to the Switchboard in the Sky via the internet.

The Switchboard In The Sky passes on the invitation and becomes anintermediary in the process. So if A requests a protocol that B cant support

the Switchboard can translate as it goes.

The invitation includes a full description of the session including all of themedia streams that A wants to use. B replies to the invitation, but includes adescription of any modifications that he wants to make. This has to happenfor compatibility as B might not support all of the features that A asked for.

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Session

After this negotiation is completed, the session is created and then A and Bcan communicate. The voice is digitised and then cut up into packets. The

packets pass through the various protocol stacks and out onto the Internet,where they are routed via the various servers and routers including theSwitchboard In The Sky.

They are sent on their way until eventually they arrive at the destinationphone B. They then work their way back up the protocol stack on the phonerecombining into a message stream and then back into analogue at thespeaker end of the handset. Meanwhile B is busy sending out packets towardA as the conversation continues.

At the end of the call, either side can send a disconnect, terminating thesession.

All of this process is automatic. For example, when A calls B, if B picks up hisphone, the phone automatically handles the media negotiation process. WhenB puts the phone down, the disconnect is automatically sent.

Its Not Really A Switchboard Is It?

No, its a server running a standard operating system. On top of the systemserver based software runs handling all of the packet routing and requests tothe various other servers that make up the Internet.

How Does This Connect in To The NormalTelephone Network?

The normal phone network is called the PSTN network (Public SwitchedTelephone Network).

There are providers who provide gateways to the PSTN and so when yourequest a PSTN number the Switchboard in the Sky bounces the request on tothe gateway and a session is established with the gateway acting as anintermediary.

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