Ingeniero
Ed Lally CHLUG meeting April 2007
1
Agenda
Terminology The
PSTN and VOIP About Asterisk About FreePBX About Trixbox Demonstration
2
Terminology
VOIP:
Voice Over Internet Protocol PSTN: Public Switched Telephone Network POTS: Plain Old Telephone Service TDM: Time-division Multiplexing PBX: Private Branch Exchange IVR: InteractiveVoice Response ACD: Automated Call Distribution DID: Direct Inward Dial
3
Terminology
SIP:
Session Initiation Protocol RTP: Real-time Transfer Protocol IAX: Inter-Asterisk eXchange Protocol μLaw: Codec for speech used by PSTN aLaw: Similar codec used in Europe
4
The PSTN and VOIP
The
public phone network has been designed to carry voice calls reliably in realtime.
PSTN
is a circuit-switched network
Each
call gets a dedicated circuit for the duration The circuit directly links the two endpoints No inherent latency, delay, or dropped sound
Bandwidth
is about 300-3500Hz (good for voice) Calls can be multiplexed for more efficient transmission (e.g., ISDN at 2 channels or T1 lines at 24 channels)
5
The PSTN and VOIP
Thepublic IP network (aka the Internet) has been designed to carry data reliably.
Internet
Each
is a packet-switched network
connection shares the network with other packets Different packets may take different paths to the same destination Packets may arrive late, in the wrong order, or not at all
Bandwidth
is variable based on physical connection layer, time of day, etc.
6The PSTN and VOIP
VOIP attempts to send voice over the public IP network.
Voice channels are encoded and packetized for transmission Receiver must reassemble packets and decode them Many implement a “jitter buffer” to compensate for small network delays Connections use Real-Time Transfer Protocol to reduce delivery time Tradeoff is that sender doesn’t know if packetshave been received VOIP packets can leverage QOS (Quality of Service) flags if the traversed networks support them
7
About Asterisk
Officially,
Asterisk is an Open Source hybrid TDM and packet voice PBX and IVR platform with ACD functionality. Mark Spencer, Asterisk Inventor
8
About Asterisk
Unofficially, Asterisk is quite possibly the most powerful, flexible, andextensible piece of integrated telecommunications software available. Its name comes from the asterisk symbol, *, which in UNIX (including Linux) and DOS environments represents a wildcard, matching any filename. Similarly, Asterisk the PBX is designed to interface any piece of telephony hardware or software with any telephony application, seamlessly and consistently. Mark Spencer, Asterisk Inventor
9Why Use Asterisk?
Allows
you to implement a phone system at low cost
Software is F/OSS Can use commodity hardware Compare to Nortel’s Business Communications Manager: combines decade-old technology for $515K
Implement
the system you want
Extremely flexible configuration Can be extended through code or scripts
10
Why Use Asterisk?
Transition
Use
Canto VOIP at your pace
analog cards to connect to PSTN
use anything from POTS to BRI to Channel Bank
Use
ATAs (e.g., Linksys or Sipura adapters) to connect regular phones Use soft phones for pure VOIP Mix-and-match
11
Why Not Use Asterisk?
If
you want a pure VOIP solution, you need to shop around for VOIP providers. Support among VOIP providers varies. SIPdifficulties with firewalls and NAT Fax support not fully baked-in
12
Asterisk Concepts
Channel: The medium over which a call arrives or leaves (e.g., analog line, SIP connection, etc.) Dialplan: The configuration that describes how to handle all calls Context: A block of instructions in the dialplan that describes how to handle calls on a particular channel Extension: An...
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