VoIP (I & II) Flashcards
Public Switched Telephone Network (PSTN) definition
This is an aggregate of all Circuit-Switched Networks around the world. Allows for one telecom network to connect to another and this is where tolls are assessed for calls between two different telecom providers. Contains all of the wiring and infrastructure for telecommunications which includes cables laid in the Atlantic ocean that connects the US and Europe
Circuit Switched definition
Circuit Switching is the traditional telephony discussed so far, there is a dedicated line line for the user to complete their call. Original a manual process but changed to automated switching
Packet Switched definition
Packet Switching has no dedicated line and messages are encapsulated in packets. Calls are made using Internet Protocols with the Telecommunications Infrastructure. Packet switching allows multiple users to communicate on the same line simultaneously which lowers the cost of communication for telecom providers which they then pass on to their subscribers and therefore it is more scalable the circuit switching.
Private Branch Exchange (PBX) definition
Allows an organisation to manage its own incoming/outgoing calls as well as facilitate internal calls and can combine landline & VoIP
Advantage of PBX
The advantage for a company to use a PBX is that they can save money by not requiring a direct line to each telecom provider for every telephone in the organisation. A PBX can facilitate both landline and VoIP calls.
Transmission Control Protocol (TCP)
Layer 4 protocol (Transport Layer) designed to work with IP. Connection orientated protocol and can eliminate many of the problems
associated with data transmission. It can
* Can establish a session
* Can corrects errors
* Can retransmit packets
* Can reassemble packets
User Datagram Protocol
Connectionless communication protocol that has limited packet recovery functionality and operates at the Transport Layer. UDP contains no reliability or error-recovery abilities however this protocol has less network overhead because it headers contain fewer bytes than TCP and delivers packets faster. UDP is used for video and for VoIP e.g., Skype. Speed is important but reliability less so and therefore loss of packets is not deemed an issue.
Advantages of VoIP
Cost
Efficiency
Information and metadata can be stored longer
Information can be filtered and searched more easily
Challenges of VoIP
Interference / call clarity
Emergy Services cannot identify location as easily
In a disaster VoIP might not work
H.323 definition
H.323 was introduced by the ITU Telecommunication Standardisation sector (ITU-T Specification) in 1996. This protocol defines audio/visual communication sessions on a packet network. Especially re-known for its use in videoconferencing. It was the first VoIP Standard to Adopt IETF Real-time Transport Protocol (RTP). It specifies the transfer of AV over an IP Network. A Gateway is used to connect to different networks on this Gateway and provides a connection between a H.323 network and another network (ISDN or
PSTN).
Elements of H.323
Gatekeeper - An H.323 Gateway device is used on a VoIP Network and enables address translation & network access control for H.323 Endpoints.
A Gatekeeper utilises the concept of a zone which is a logical collection of H.323 nodes such as, Gateways & Terminals. There is one
active Gatekeeper per zone
Gateway - Gateways are used to connect Enterprise VoIP Networks to other Traditional Telephony Systems e.g., a PBX or a Public Land & Mobile Network (PLMN).
Multipoint Control Unit (MCU) - A MCU is aka a video transcoding server. It sits at the core of any video conferencing system. Connects users to a video conferencing
system that resides within the same network. It is generally used on a H.323 VoIP network. It connects all devices that wish to be part of a video conference.
Session Initiation Protocol (SIP) Definition
The protocol is comprised of Initiation, Modification & Termination of Interactive Multimedia Sessions. It is used with audio/video conferencing and chat sessions, game sessions, instant messaging, file transfer and Fax over IP.
Differences H.323 and SIP
- H.323 is based on telephony whilst SIP is based on the use of the Internet
- H.323 is recommended by ITU and SIP was designed by IETF
- H.323 uses an alias which is a Gatekeeper map which means a host or a telephone number, whereas SIP uses a URL. SIP is
compatible with H.323. - H.323 is an application protocol that is used for VoIP communications that is used for audio and video conferencing, but it
is not used for file sharing, online gaming and similar communications.
Network problems of VoIP
Delay / latency
Jitter
Packet Loss
Voice Activation Detection (VAD)
Voice Quality Problems
Noise
Voice Distortion
