Voip

VOIP: COMPUTER NETWORKS TERM PAPER PRESENTATION SYED WALEED SHAH WALEED UR REHMAN

What is VOIP?
 Voice over IP (VoIP) is a transmission technology for transfer of

voice communications over IP networks such as the Internet or other packet-switched networks.
 Its a communication service including :

 voice, voice-messaging(voice mail) applications that are transported via the

Internet, rather than the public switched telephone network (PSTN) that are typical analog telephone lines.
 Integrated VoIP services also allow you to make and receive calls to and

from traditional landline numbers.

VOIP: The Process
 The basic steps involved in originating an Internet telephone

call are:
 conversion of the analog voice signal to digital format  placing them into Internet protocol (IP) packets  transmission over the Internet  the process is reversed at the receiving end.

Why Use VOIP?
 VoIP can turn a standard Internet connection into a way

to place free phone calls.

 We are actually bypassing the phone company and its

charges entirely.

Comparison of : circuit switched phone system and VOIP:
 Circuit switched traditional phone system:  dedicated resources that are mostly wasted.  overhead of establishing a dedicated path:
 dedicated channels on every medium in that path  creating a connection state in every intermediate switching node being

traversed.
 the dedicated bandwidth is never shared and is used by a single call through out

the duration of a conversation
 inefficient.  dedicated resources makes CS very expensive.

Circuit Switched phone system
 limitations of using the same signaling rate on both ends.  in terms of resilience complex algorithms are needed if the dedicated path

breaks.  might result in call being dropped.
 in the current digital infrastructure merging analog CS with the digital

equipment also poses a great challenge.
  since the circuit is continuously open resources are wasted since most of

the time there is nothing to send

VOIP: Packet Switched
 VOIP uses packet switching:  No continuous circuit data is transmitted when required.  No channels or other resources dedicated.  Overhead of establishing a circuit and switch configuration is eliminated

switches just forward the packets.  Virtual Circuit/Datagram  The data medium is shared between packets TDM.  bandwidth of a single CS call can be utilized by many VoIP calls.  The network routes the packets along the least congested and cheapest lines
 Resilience: nodes or link failures will cause packets to take alternative routes

and the call might be interrupted but will not be dropped.

How VOIP Works?
 VoIP systems employ session control protocols to control the set-up and

tear-down of calls as well as audio codecs which encode speech allowing transmission over an IP network.
 What Are Codecs?  Codecs normally work at the application layer.  VoIP softwares normally employ codecs.  Codecs use advanced algorithms to help sample, sort, compress and packetize

audio data.

CODECS?
 Codecs simply code/digitize the analog voice signals using PCM ,that might further

be compressed depending on the voice quality required and sampling frequency can be varied accordingly  the PCM data is then framed and sent in an ip packet. on the receiver's side PCM samples are torn off the ip packets and reconstructed into analog voice again.
 some implementations rely on:
 narrowband and compressed speech  high quality stereo codecs.

 use of codecs largely depends on the connection the available bandwidth and the quality of speech required

Forms Of VOIP: Making VOIP possible!
 ATA -- (analog telephone adaptor).
 The ATA is a device allows you to connect a standard phone to your computer

or your Internet connection for use with VoIP.  The ATA is an analog-to-digital converter.  It takes the analog signal from your traditional phone and converts it into digital data for transmission over the Internet calls.
 IP Phones –
 These special phones look just like normal phones. IP phones have an RJ-45

Ethernet connector. IP phones connect directly to your router and have all the hardware and software necessary right onboard to handle the IP call.

Forms Of VOIP:
 Computer-to-computer –
 This is certainly the easiest way to use VoIP.
 There are several companies like Skype/Google/yahoo offering free or very low-

cost software that you can use for this type of VoIP. All you need is the software, a microphone, speakers, a sound card and an Internet connection.

 The main attraction is that we are charged for the internet connection, not for the time or the distance of our call.

Making a VOIP CALL
 You pick up the receiver, which sends a signal to the ATA.

 The ATA receives the signal and sends a dial tone. meaning a connection to the “Internet”.
 The dialed phone number data is sent in the form of a request to your VoIP Company’s call processor(soft switch).  Soft switch maps, the phone number to an “IP address”.  The soft switch connects the two devices on either ends.  Receiver phone rings! 

Internet infrastructure routes the voice carrying packets.  Each system must use the same protocol to communicate.  The systems implement two virtual channels, one for each direction, as part of the session.  When hanged up ATA sends a signal to the soft switch connecting the call, terminating the session

Routing in VOIP: The issues
 in VoIP each user has a phone number that is fixed  since users connect through internet they at any given time have random

ips.
 PSTN call forwarding/routing is area dependant(country /city/ area codes)

but current ip addresses of a VoIP users is hard to locate.
 The challenge with VoIP is that IP-based networks don't understand phone

numbers. They look for IP addresses.

Call Processor & Soft switch
 Soft switch and Central Call Processor  The central call processor is a piece of hardware running a specialized  


 

mapping program called a soft switch. The soft switch connects VoIP users. It Must Know: Where the network's endpoint is What phone number is associated with that endpoint The endpoint's current IP address .

 Once the soft switch needs to find out the current ip address of a dialed

user. then packet is left at the mercy of ip network to route the packet to the desired host .

Features
 VoIP can be heavily integrated with other digital services .  Caller ID  Call waiting  Call transfer  call filtering options available.

 Forward the call to a particular number
 Send the call directly to voice mail  Give the caller a busy signal

 Tele conferencing:
 The VoIP service can be merged with video conferencing to allow the people

tele presence, with people talking face to face.

Requirements & Issues
 The issues concerning Packet switched networks also affect call quality:  VoIP requires a constant flow of packets so that the reconstructed voice

signals are replica of sampled original voice signals.
 Bandwidth:

 VOIP needs bandwidth so that voice conversations are intelligible by both

parties  BW variations can also cause distortion of the call.
 codecs  improper sampling can cause Voice cutoffs, echoing and can reduce

intelligibility of voice.

Issues:
 Latency  the greater the distance between calling parties, the greater the latency.

Greater the latency less is understandability and quality of conversation. Latency can also be caused due to heavy traffic congestion in the network
 Jitter
 jitter can also affect speech quality; jitter is the variable latency between

packets.  Jitter is more common in IP-based speech because the path for voice packets across the network may not always follow the same route. The buffers commonly used in IP networks can also increase jitter.

 Packet loss  packet loss or discard "throw away" a "lot of speech" as opposed to an

uncompressed sound wave.  The more highly compressed the voice packet, the greater the amount of conversation lost when a packet is discarded  Phone conversations can become distorted, garbled or lost because of transmission errors.
 Threats  new threats, scams, and attacks unique to IP telephony are now emerging.  VoIP is also susceptible to worms, viruses and hacking  A call can be affected by processor drain. Quality loss will become

immediately evident. In a worst case scenario, your system could crash in the middle of an important call.

PSTN VOIP Integration
 COM COs can seriously reduce the bandwidth usage.  By routing thousands of phone calls through a circuit switch and into

an IP gateway.  Once the call is received by a gateway on the other side of the call, it's decompressed, reassembled and routed to a local circuit switch.
 On a user level a PSTN user’s call can be routed to an ip gateway

through circuit switching where devices such as Soft switches can route the call to the intended VOIP through a packet switched network by digitizing the Circuit switched analog data.

Disadvantages of Using VOIP
 Complex internet infrastructure prone to errors.  Power issues: phantom power VS wall Power.

 Emergency calls: hard to locate caller based on ip address.
 PSTN + VoIP =resilience.  VoIP=no Backup?

Benefits:
 Lower costs: charged for internet access. not distance /time  Single network: No separation b/w data & voice networks.  Multiplexing(TDM)The ability to transmit more than one telephone

call over a single net line.  Secure calls using security protocols . Data can be encrypted and authenticated.
 Location independence. Only a sufficiently fast and stable Internet

connection is needed to get a connection from anywhere to a VoIP provider