7. VOIP Flashcards
What is the Public Switched Telephone Network (PSTN)?
PSTN is the aggregate of all Circuit-Switched Networks
It is the telephony Infrastructure Allows for one telecom network to another.
Includes all wiring and cables incl those under the ocean (trunk lines and calls)
AKA Plain Old Telephone System (POTS)
What are Circuit Switched Networks?
Circuit switching refers to the traditional telophony.
A dedicated line for the user to make their call.
Manual then automated switching.
What are Packet Switched Netwworks?
No Dedicated Line
Messages encapsulated in Packets
Calls are made using Internet Protocols Used with Telecom Infrastructure
Allows multiple users to communicate on the same lines simultaneously. Lowers cost for telecom providers / subscribers. Theerfore more more scaleable that circuit switched networks.
What is a PBX (Private Branch Exchange)?
PBX (Private Branch Comapny) is a system that allows a compamny or organisation to manage their own incomign and outgoing calls. It also facilitates internal calls. Can facilitate landline and VoIP
A PBX consists of:
Hardware (telephones, adapters, switches, hubs, routers)
Is cost effective - don’t require a direct line for the telecom provider to every phone.
It is a PRIVATE SWITCHED NETWORK
The PBX routes calls to the specific internal device.
What is TCP (Transmission Control protocol)?
- Layer 4 protocol (Transport Layer)
- Connection-oriented communication
Can establish a session
Can corrects errors
Can retransmit packets
Can reassemble packets - Built on top of IP
What is UDP (User Datagram Protocol)?
- Connectionless communication protocol
- Limited packet recovery functionality
- Operates at the Transport Layer - No reliability or error-recovery abilities
- Less network overhead
Headers contain fewer bytes than TCP
Delivers packets faster - Used for video and for VoIP when speed and low latency is priority over some loss of packets (guaranteed delivery)
e.g. Skype / gaming / video streaming.
Describe the structure of a UDP Header
The structure of a User Datagram Protocol (UDP) packet consists of two main parts:
- The HEADER. 4 fields. Each field = 16 bits (2 bytes)
Source port (16 bit)
Destination port (16 bit)
UDP length (size of total UDP packet) - 16 bit
UDP checksum (16 bit) - The DATA (payload).
Variable in size.
What is CALEA?
Communications Assistance for Law Enforcement Act (CALEA) (47 U.S.C. § 1002).
Provides framework for companies to assist law enforcement with data requests.
Cisco Published a Lawful Intercept Configuration Guide
Cisco 7600 Guide
www.cisco.com/c/en/us/td/docs/routers/7600/ios/
12-2SR/configuration/lawful_intercept/ lawful-int–Book Wrapper/76LIch1.html
Title III Wiretap
VoIP Not Covered by CALEA
No Switches
How does CIRCUIT SWITCHED network work?
With a circuit switched tech the PSTN carries an anologue voice signal that needs to be converted to a digital signal.
Pulse Code Modulation is used to do that (PCM).
The circuit / path for the signal is determind during the initial call set up and is dedicated to that voice communication for the duration of the call.
A PSTN maintains a dedicated line called a Signalling System 7 (SS7).
Key points about VoIP
VOIP:
Communication is via UDP therefore on a best effort basis as there is no error checking.
With VoIP because it is an IP network:
- the voice signals are digitised at source device
- uses Digital Signal Processors (DSPs) to do this digitisation
- there is no dedicated path for the packets. packets from multiple subscribers are sent similtaneously.
- any internet enabled device can be used like a computer - does not have to be a handset.
- lower cost because multiple communications can occur simultaneously on same circuit and -it eliminates local exchanges and tolls.
IP network call signalling shares network resources with data traffic.
What are the advantages of VoIP?
- Cost
- Bandwidth
64 kbps in both Directions are required on circuit switched network. But VoIP requirement is only 10 kbps
Silence Suppression (means it is more efficient because it does not transmit the silent parts of the call - usually saves 40 to 50% of bandwidth) - Conference/Video Calling
- Instant Messaging
- File Sharing
- Investigations - digital comms contain identifying info and metadata. Easy to serach and analyse IP packet data.
What are the challenges of VoIP?
- Interference
- Caller locations are not given for emergency calls
BUT there are enhanced VoIP rules for 911 FCC E911 rules.
- More powered required - battery packs can run out e.g in hurricane disastr areas.
