Networks 07

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Networks 07

• Voice Networks

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TDM

• Traditional POTS is based on the idea of time
  division multiplexing (TDM)
• It offers each voice call a guaranteed bandwidth
  of 64kbps
• Phone circuits are based on multiples of 64kbps
• PRI 31 x 64kbps 1984kbps
• STM-1 80 PRIs 2480 x 64kbps 155.52Mbps

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FXO FXS

• A foreign exchange office (FXO) is any device
  that behaves like a telephone (generates
  on-hook/off-hook)
• A foreign exchange station (FXS) is a device that
  behaves like an exchange (supplies power, sends
  dial tone, generates ringing voltage)

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Convergence

• Voice and data are converging
• Is there a difference?
• If so, has there ever been a difference?
• What does convergence mean?

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Net-heads vs Bell-heads

• Net-heads
• Think in terms of network bandwidth
• Come from an environment where unreliability is
  the norm
• Are usually innovative
• Bell-heads
• Think in terms of voice channels
• Come from an environment where five nines is the
  norm
• Are usually conservative

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Voice over IP

• Use existing bandwidth more effectively by
  compressing data
• Remove arbitrary bandwidth restrictions
• Reduce costs
• Location independent
• Integration

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Call Quality

• How do we assign a quantitative measurement to
  something as subjective as call quality

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Mean Opinion Score

• There are some standard phrases used to calculate
  mean opinion score
• You will have to be very quiet.
• There was nothing to be seen.
• They worshipped wooden idols.
• I want a minute with the inspector.
• Did he need any money?
• Does this mean MOS is language-dependent?

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VoIP Codecs

• Do we trade quality for bandwidth savings?

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Networks for Voice

• Voice (in fact any real-time multimedia) creates
  specific demands on networks
• Specifically
• Latency
• Jitter
• Bandwidth
• Packet Loss
• Quality of Service considerations

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Bandwidth for Voice

• We designing a voice network we dont guess how
  much bandwidth is required, we work it out
• Voice traffic is measured in Erlang
• one Erlang refers to a single resource being in
  continuous use, or two channels being at fifty
  percent use, and so on, pro rata.
• Erlangs are calculated based the probability of
  blocking in the busy hour
• We can convert Erlang to network bandwidth if we
  know the codecs in use
• Must take into account packet overheads

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Quality of Service

• How do we guarantee network quality?
• Traffic shaping
• Prioritisation, traffic classes
• Bandwidth reservation
• Congestion avoidance
• 802.1p traffic classes
• Or buy more bandwidth?

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Voice Protocols

• There are two predominant voice over IP standards
  in the telco world
• H.323 (ITU)
• SIP (IETF)
• SIP is starting to take market share
• What about Skype?

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SIP

• The Session Initiation Protocol (SIP) is an
  application-layer control (signaling) protocol
  for creating, modifying, and terminating sessions
  with one or more participants
• Transport independent, although usually UDP TCP
• Text-based protocol

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SIP Elements

• Endpoint something to make and receive calls, a
  telephone
• SIP proxies help route calls and provide
  services, much like a PBX
• SIP registrars handle authentication and
  authorisation

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SIP Sessions
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VoIP Problems

• Reliability
• Faxes, modems (why?)
• Phone numbering
• Security
• Caller ID (CID, also CNID, CLI)
• E.911

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Asterisk

• Open source IP PBX
• Supports most of the traditional PBX functions
• You can run your own PBX at home
• If you have an analogue telephony adapter (ATA)
  you can plug it into a traditional phone line
• FXO and FXS cards are available
• Are net-heads taking over?

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Dial Plans

• macro-phone
• exten gt s,1,Dial(SIP/ARG1IAX2/ARG1,30)
• exten gt s,n,Goto(DIALSTATUS,1)
• exten gt _s-.,1,Goto(s-NOANSWER,1)
• exten gt ANSWER,1,Hangup
• exten gt CANCEL,1,Hangup
• exten gt NOANSWER,1,Voicemail(MACRO_EXTEN,u)
• exten gt BUSY,1,Voicemail(MACRO_EXTEN,b)
• exten gt CONGESTION,1,Voicemail(MACRO_EXTEN,b)
• exten gt CHANUNAVAIL,1,Voicemail(MACRO_EXTEN,u)
  
• exten gt a,1,VoicemailMain(MACRO_EXTEN)
• Real Extensions
• stations
• exten gt 1001,1,Macro(phone,1001)
• exten gt 1002,1,Macro(phone,1002)
• exten gt 1003,1,Macro(phone,1003)

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