Intro to VoIP Flashcards
Brief History of the Telephone
1875 - Alexander Graham Bell transmits sound
1876 - “Mr Watson, come her! I want to see you!
1885 - AT&T founded
1919 - First Rotary Telephone
1946 - Area Codes Established
1961 - Touch tone Phone released to public
1963 - Push button Telephone
1973 - First Handheld Cellphone Call
1982 - Caller Id
1984 - New AT&T formed
1991 - GSM Created
History Cont’d
Dr Martin Cooper and John Mithcell - Motorola 1973 - 1st mob phone
1G tech initially - Analog tech that only supported voice calls
2G (1991) (GSM) - Allowed SMS MMS
->2.5
->2.75
3G Data/Video Calling/ Access to Internet
4G (2008) - Video Conferencing
5G - Being rolled out
Briefly describe the telephone switchboard
Introduced in late 1890s
Switchboard operator
Manuall connected a caller with intended recipient
Plugged into a jack into the intended port
Many phased out in the 60s
Almon Brown Strowger invented the Automated Switchboard
1891 - Patent for first Automatic telephone exhange
Automated switchboards were introduced
Long distance calls still req operator
Describe Public Switched Telephone Network (PSTN)
Aggregate of all Circuit Switched Networks
Telephony Infrastructure
Trunk Lines and Calls
Assess Tolls
AKA Plain Old Telephone System (POTS)
Circuit Switched vs Packet Switched
Circuit Switching - Traditional telephony - Dedicated Line - Automated Swithcing
Packet Switching - No dedicated line - Message in Packets - Internet Protocols used with telecom infrastructure
Allows multiple users to communicate on the same line simultaneously (lowers cost of comms dramatically)
Describe the Private Branch Exchange (PBX)
Manages Incoming/Outgoing Calls - Landline and VoIP
Hardware\Software
Telephones
Adapters
Switches
Hubs
Routers
This system enables a company to manage incoming and outgoing calls, as well as facilitate internal calls.
Cost effective - No req for direct line to provider
(Image) - PBX connects to public switch tel network and PBX routes calls to appropriate telephone or landline connected devices like a fax machine
TCP - Designed to work with IP
Layer 4 protocol (Transport Layer)
Connection-oriented comms-
can establish a session
can correct errors
can retransmit packets
can reassemble packets
Built of top of IP (Hence TCP/IP)
User Datagram Protocol (UDP)
Connectionless comm protocol
Limited packet recovery
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 VoIP (Skype)
If a packet is lost during transmission it’s not really important (but speed is)
CALEA (Communications Assistance for Law enforcement Act)
Advances in telecomms made it v difficult for LE to carry out surveillance.
Title 3 Wiretap.
VoIP not covered by CALEA
Telecomm companies had to redesign much of their structure to become compliant with CALEA and provide improved surveillance for LE
Describe VoIP
Communication on a Best Efforts Basis
UDP - No Error Checking - Trade off Vs TCP worth it
PSTN - Public Switch Telephone Network
- Voice Signals -> Digital Signals
-Pulse Code Modulation (PCM) - On a IP network, voice signals are digitised by telephone handset or their device using digital signal processors (DSP) and then inserted into an IP packet.
- Circuit/ Path determined during initial call set up
IP Network - Does not need a handset (Comp for comms)
- Voice Signals Digitised at source device - Packets from multiple subscribers are sent simultaneously.
- Digital Signal Processors (DSP)
- No dedicated path for packets
- Eliminate Local Exchanges and tolls - initial reason for introducing VoIP - Low cost and multiple comms on same circuit.
PSTN - Dedicated Line = Signaling System 7 (SS7)
- With an IP network, call signaling shared network with resources with data traffic
VoIP advantages
Cost
Bandwidth
-64kbps in both directions vs 10kbps(ip network, this is all that’s req
-Silence Suppression - monitors for speech (packets that contain no voice signals are suppressed and not sent to receiver, saving bandwidth)
Conference/Video Calling
Instant Messaging
File Sharing
Investigations
Compared to trad telephony, massive advs for digital forensic investigators - digital comms iwth identifying info and metadata can be stored for a long period by scaling providers and then later requested by subpoena by enforcement warrant.
Data from this info can be filtered and searched in a way diff to PSTN and analog signals
Final Thoughts
IP Networks Connected to Traditional Telephone (PSTN) (Must interact with)
Skype users can request a local telephone number (USA to +353)
Google Voice
-Unlimited Calls and Text over WiFi
Select a local telephone number
-Smartphone/Tablet/PC or Mac (Web)
VoIP Hardware - Boxes available to connect to PSTN users
Challenges of VoIP
Interference/Call Clarity - Echo/ Delay
Emergency Services - challenges with identifying location
Enhanced 911 - FCC E911 rules
Limitations
Route Calls to Local Public Safety Answering Point (PSAP)
Disaster
-Hurricanes
