Voice over WLAN
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Voice over WLAN
Voice over IP
WAN
VoIP GatewayVoIP Gateway
PBX PBXPSTN
Challenges in VoIP1. Delay
1. Absolute delay2. Jitter
2. Packet loss1. Less than 5% can be tolerated
3. Bandwidth1. Wire bound : no problem2. Wireless : always limitation
Voice over Internet
Separate signaling and transport
Transport
1. Sampling1. Speech 4KHz->sampling 8kHz2. 8bit-> 64kbps per voice channel
TransportEqualizationEcho cancellation
line echoacoustic echo
Encoding bit rateG71164 G726-32 32G729ab 8G729e 12G728 16
TransportDigit Relay
Send tonesDetect key strokes
Voice activity detectionDetect silence periods
Channel codingTCP is not used -> unsecure channelFEC is used
PacketizationCoded voice packet form a RTP packet typical
period= 20 msTone detection
The receiver detects tones (diling tones, fax,modem)
MGCP Call SetupRG Calling RG CalledCMS
12345678910 11
121314 15
161718
Message 1RQNT 1000 aaln/[email protected] 1.0N: [email protected]: 2427X: 1234abR: hd(N)S:
Comments:When you see the event asked for in R send them to Ca@the callagent.com, UDP port 2427X: Use the following requiest ID in your responseR: Look for hook-detect event. Notify me right awayS: don’t play any signal
Message 2200 1000 OK
Comments:Positive ack
Message 3NTFY 2000 aaln/[email protected] 1.0N: [email protected]: 2427X: 1234abO: hdComments:User picks up the phoneO: is indicating the event
Message 4200 2000 OK
Comments:Positive Ack
Message 5RQNT 1001 aaln/[email protected] 1.0N: [email protected]: 2427X: 1234abR: hu(N),[0-9] (D)S:dlD: (XXXXXXXX/0T/00T)
Comments:The CMS is instructing the RG what to do nextR: Look for hook-up event. Notify me right away or dialled digits [0-9] accumulate according to DS: Play dial toneD: Match digits any 9 digits or 0 and short time-out or 00 and short time-out
Message 6200 1001 OK
Comments:
Message 7NTFY 2001 aaln/[email protected] MGCP 1.0N: [email protected]: 2427X: 1234abO: 3,0,1,5,5,5,1,2,1,2,
Comments:The user dials
Message 8200 2001 OK
Comments:
Message 9CRCX 1002 aaln/[email protected] MGCP 1.0L: p:20, a:PCMU, s:off, e:onC: abcde1234M: recvonlyX: 1234abR: hu(N), hf(N)S:
Comments:Use packetization period 20 ms, use the PCMU codec (G711 mu-law); nor silence detection; do echo cancellation.C: use this string as call identityM: prepare to receive but don’t send anything yetX,R,S: look for hang-up or hook flash event and report, don’t play any signal
Message 7NTFY 2001 aaln/[email protected] MGCP 1.0N: [email protected]: 2427X: 1234abO: 3,0,1,5,5,5,1,2,1,2,
Comments:The user dials
Message 9RQNT 1000 aaln/[email protected] 1.0N: [email protected]: 2427X: 1234abR: hd(N)S:
Comments:When you see the event asked for in R send them to Ca@the callagent.com, UDP port 2427X: Use the following requiest ID in your responseR: Look for hook-detect event. Notify me right awayS: don’t play any signal
Message 10200 2002 OKI: abcd1234v=-0O=-25678 753849 IN IP4 192.164.3.11s=-C=IN IP4 192.168.3.11T=0 0M= audio 30000 RTP/AVP 0
Comments:RG acknowledges that a media connection is set upc= is the address where the called party shall send its mediam= is the UDP port to be used for media
Message 11CRCX 1003 aaln/[email protected] 1.0L: p:20, a: PCMU, s:off, e:onC: abcde1234M: sendrecvX: 1234abR: hd(N)S:rgv=-0O=-25678 753849 IN IP4 192.164.3.11s=-C=IN IP4 192.168.3.11T=0 0M= audio 30000 RTP/AVP 0
Comments:Set-up of the connection to called party sideLook for off-hook event Ring signalNote the SDP from the calling party
Message 12200 1003 OKI: def1234v=-0o=-1111 23245 IN IP4 192.168.3.12s=-c=IN IP4 192.168.3.12t=0 0m= audio 40000 RTP/AVP 0
Comments:
Message 13MDCX 1004 aaln/[email protected] MGCP 1.0I: abcd1234M: recvonlyX: 1234aeR: hu(N), hf(N)S:rtv=-0o=-1111 23245 IN IP4 192.168.3.12 s=-c=IN IP4 192.168.3.11t=0 0m= audio 40000 RTP/AVP 0
Comments:The CMS knows that the called party is ready and alerts the callerHe plays the ring tone, Gives the media session description to the callerHook on and hook flash shall be reported I: is the call identifier
Message 14200 1004 OK
Comments:
Message 15NTFY 5001 aaln/[email protected] MGCP 1.0N: [email protected]: 2427X: 1234abO: hd
Comments:The called party picks upThis is reported to the CMS
Message 16200 5001 OK
Comments:
Message 17MDCX 1005 aaln/[email protected] MGCP 1.0I: abcd1234M: sendrecvX: 1234aeR: hu(N), hf(N)S:
Comments:The CMS tells the caller to stop playing the ring tone (s and to enable bidirectional media
Message 18200 1005 OK
Comments:Now the caller acks the message and the call is set up.
Wireless Local Area Network
The Physical layer
MAC Layer
PHY Layer
PLCP Sub layer
PMD Sub layer
Direct Sequence Spread Spectrum – DSSS
•Physical Layer Convergence Procedure PLCP•PLCP protocol data unit
PLCPpreamble
PLCPheader MPDU
Sync128 bits
SDF16 bit F3Ah
Signal8 bit
Service Length16 bit µs
CRC16 bit
Architecture
ScramblingBand spreading
Modulo-2adder
TransmitMask Filter
DBPSKDQPSK
Modulator
PPDU
11 bitBarker word
De-spread correlator
DBPSKDQPSK
De-modulatorDe-scrambling
Clock recoveryClock
PPDU
802.11 MAC frame structure
General format
FrameControl
Duration/ID
Address1
Address2
Address3
SequenceControl
Address4
MAC Header Frame Body FCS
Frame ControlLength 16 bits
Protocolversion Type Subtype To
DSFromDS
MoreFrags Retry Power
Mgt WEP Other
2 2 4 1 1 1 1 1
MoreData
1 1 1
Frame types
Information for updating NAV
FrameControl
Duration/ID
Address1
Address2
Address3
SequenceControl
Address4
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