VoIP Flashcards
What is a Voice over IP Telephone
a Voice over IP telephone is more of a computer than a telephone. It has to be a LAN device, it has to be an IP device, it has to have a codec for digitizing the speech and compressing it, it has to run protocols like SIP to set up phone calls, it might have to do authentication to register with a Soft Switch, there are a lot of pieces of software that is running inside of that telephone.
What “computer” functions must a VoIP phone perform?
Since it is VoIP, this computer that looks like a telephone also has to have a IP protocol stack. It also has to have a DHCP client (Dynamic Host Configuration Protocol). This is a little software program that lets the phone go off over the LAN to a DHCP Server and get an IP address assigned to it. It also has to support protocols like UDP.
Short answer: IP, DHCP and LAN functions
Which of the following are the minimum requirements on a VoIP telephone?
The G.711 Codec & SIP Client
What component produces coded digitized speech?
Codec
How are IP packets moved from one router to another?
Carried in Ethernet MAC frames (Media Access Control)
How much speech is typically contained in an IP packet?
20 Milliseconds
What is the difference between SIP and a softswitch?
SIP is a Protocol. A softswitch is a call management system
What is the main purpose of a SIP?
To communicate the IP address of telephones to each other
What is a key attribute of a video server?
Movies stored in Random Access Memory so multiple people can watch at different start times
What protocol is between RTP (Real-Time Transport Protocol) and IP (Internet Protocol)?
UDP (User Datagram Protocol)
What does RTP (Real-Time Transport Protocol) do?
Adds a time stamp
What does “IP dial tone” mean?
A service that allows you to send an IP packet to any destination on the network, like telephone calls.
The telephone network and the internet being the same thing.
Telephone service becomes a helper application to set up a Voice over IP phone call.
Once there is IP dial tone, what happens to the World Wide Web?
It is simply a helper application DNS (Domain Name Server) to look up web server addresses
What technique is first used to represent speech inside a telephone?
Analog
What is a LAN (Local Area Network)?
Connects IP telephones to a router in-building
Which of the following is a function of a gateway?
Convert between DS0 (Digital Service Level 0) and RTP/IP
What is a media server?
Video server, Web server, Email server
what is a WAN (Wide Area Network)?
Connects to buildings
Which of the following does computer-computer VoIP over the internet provide?
Free calling
What performs the voice digitization and reconstruction function when doing computer to computer VoIP over the internet?
The sound card in the computer
What part of the network from calling party to called party does the VSP (Voice over IP Service Provider) provide?
Internet to LEC (Local Exchange Carrier)
What two essential functions does the VSP provide for internet to phone VoIP?
Gateway and softswitch
Why is phone to phone over the internet service cheaper than POTS?
Avoids switched access charges at the originating end
Avoids fees and taxes leveled on POTS
Service provider is not providing the long-distance network
What is a managed IP network?
An IP packet communication service with Service Level Agreements backing it up, i.e. guaranteed network quality
What part of the network from calling party to called party with POTS does a managed-IP telephony service provider provide?
Calling party to LEC
What is most of SIP trunking traffic?
VoIP
What does “native” communication mean?
The data is not reformatted
What functions does a VoIP-enabled PBX provide?
Softswitch and gateway
What would be a primary reason to use on a VoIP-enabled PBX?
Existing investment
Which of the following is an advantage of a softswitch (non-PBX) system for PBX replacement?
Leverage investment in PBX operations
Possibility of geographic diversity
Possibility of standards-based interfaces
What is necessary as part of the design using a softswitch product in a medium or large business?
Two softswitches, call-state-mirroring and automatic redirection when a failure occurs
Which of the following is an advantage of a Hosted PBX solution?
Scalability and flexibility
Professional implementation, security, reliability
Backup and redundancy
What is communicated from the VoIP phones to the Hosted PBX?
Formatted text messages coded into ASCII (American Standard Code for Information Interchange)
How can a VoIP system use existing PBX trunks?
Put a gateway between the two
What are the five facets of communications supported by SIP?
Locating the called party
Determine and report availability of called party
Call progress “tones”
Enable call type negotiation
Manage calls
What would be part of a call type negotiation?
Media type
Which of the following is associated with Personal Communication Services?
Separation of wires and services
Which protocol carries the SIP message as a payload?
UDP (User Datagram Protocol)
Which protocol’s data is carried as a payload in a SIP message?
SDP (Session Description Protocol)
What is an “address of record”?
Your public SIP address
What does URI stand for?
Uniform Resource Identifier
What is a proxy?
A thing that acts for something else
What does REGISTER do?
Updates your contact URIs
Which of the following functions does an outbound SIP proxy perform?
Returns call progress messages
Sends INVITE messages
Resolves URIs
How does the outbound proxy determine what IP address to send the INVITE message to?
Looks up the server portion of the address of record in the DNS (Domain Name Server)
Which message does the caller send to complete call setup?
ACK (Acknowledgment)
What message does the called party’s terminal return to indicate it accepts a call INVITE?
180 RINGING
Which message carries the IP address or contact URI of the called party?
180 RINGING
The correct sequence of messages is?
INVITE, TRYING, RINGING, OK, ACK
Which are learned during a SIP message exchange?
The IP addresses of phones and media port number
What is the difference between a softswitch and a SIP server?
A SIP server is part of a softswitch
What function does a softswitch do that SIP does not?
AAA (Authentication, Authorization, and Accounting)
What is “call disposition”?
What happens to an incoming phone call
What is “media” in a VoIP phone call?
Voice
Video
Music
The location database associates what?
Address of Record and URIs
Analog-Digital conversation is performed by?
An integrated circuit
Coding is performed by?
A software program
An integrated circuit
What is a hard constraint on voice quality for POTS?
The voiceband frequency restrictions
Which are factors affecting voice quality?
Coding
Delay
Jitter
Packet Loss
Which codec provides near toll quality with moderate delay?
G.729
Why is buffering used in network elements?
Allows temporary storage of data waiting to be processed
What is the maximum recommended delay on VoIP systems?
150 Milliseconds
What does a jitter buffer do?
Increases delay
What does jitter mean?
Variable delay
What is the principal purpose of RTP (Real-Time Transport Protocol)?
Time stamp
What protocol carries the RTP Protocol Data Units?
Transport Protocol
What does TCP (Transmission Control Protocol) do that UDP (User Datagram Protocol) does not do?
Retransmission
Which protocol is used to carry RTP (Real-Time Transport Protocol) Protocol Data Units?
UDP (User Datagram Protocol)
What causes packets to be lost?
Buffer overwriting
What causes buffer overwriting?
Transient faster input than output
What creates the analog of the voice that is digitized and packetized?
A microphone
What is segmentation?
Creating fixed-sized blocks of bits from the codec output
What are buffers?
Memory like RAM
What device is currently required to connect to a LEC?
Gateway
What is the essential function that VSPs (VoIP Server Providers) like Vonage perform?
Connect to the internet to the PSTN
How is a phone call communicated with carrier-carrier IP interconnect?
Digitized voice in IP packets
What is a function of a Session Border Controller?
Controlling outbound calls
Address resolution
Network security
What is a SBC?
A device that connects a carrier and a LEC for VoIP
