Voice over WLAN

1
Voice over WLAN
2
Voice over IP
WAN
VoIP Gateway
VoIP Gateway
PBX
PBX
PSTN
3
Challenges in VoIP

1. Delay
2. Absolute delay
3. Jitter
4. Packet loss
5. Less than 5 can be tolerated
6. Bandwidth
7. Wire bound no problem
8. Wireless always limitation

4
Voice over Internet

• Separate signaling and transport

5
Transport

1. Sampling
2. Speech 4KHz-gtsampling 8kHz
3. 8bit-gt 64kbps per voice channel

6
Transport

• Equalization
• Echo cancellation
• line echo
• acoustic echo
• Encoding bit rate
• G711 64
• G726-32 32
• G729ab 8
• G729e 12
• G728 16

7
Transport

• Digit Relay
• Send tones
• Detect key strokes
• Voice activity detection
• Detect silence periods
• Channel coding
• TCP is not used -gt unsecure channel
• FEC is used
• Packetization
• Coded voice packet form a RTP packet typical
  period 20 ms
• Tone detection
• The receiver detects tones (diling tones,
  fax,modem)

8
MGCP Call Setup
CMS
RG Calling
RG Called
9
Message 1

• RQNT 1000 aaln/1_at_my.mta.com
• MGCP 1.0
• N ca_at_the.callagent.com
• Com 2427
• X 1234ab
• R hd(N)
• S

Comments When you see the event asked for in R
send them to Ca_at_the callagent.com, UDP port
2427 X Use the following requiest ID in your
response R Look for hook-detect event. Notify me
right away S dont play any signal
10
Message 2

• 200 1000 OK

Comments Positive ack
11
Message 3

• NTFY 2000 aaln/1_at_my.mta.com
• MGCP 1.0
• N ca_at_the.callagent.com 2427
• X 1234ab
• O hd

Comments User picks up the phone O is
indicating the event
12
Message 4

• 200 2000 OK

Comments Positive Ack
13
Message 5

• RQNT 1001 aaln/1_at_my.mta.com
• MGCP 1.0
• N ca_at_the.callagent.com 2427
• X 1234ab
• R hu(N),0-9 (D)
• Sdl
• D (XXXXXXXX/0T/00T)

Comments The CMS is instructing the RG what to
do next R Look for hook-up event. Notify me
right away or dialled digits 0-9 accumulate
according to D S Play dial tone D Match digits
any 9 digits or 0 and short time-out or 00 and
short time-out
14
Message 6

• 200 1001 OK

Comments
15
Message 7

• NTFY 2001 aaln/1_at_my.mta.com MGCP 1.0
• N ca_at_the.callagent.com 2427
• X 1234ab
• O 3,0,1,5,5,5,1,2,1,2,

Comments The user dials
16
Message 8

• 200 2001 OK

Comments
17
Message 9

• CRCX 1002 aaln/1_at_my.mta.com MGCP 1.0
• L p20, aPCMU, soff, eon
• C abcde1234
• M recvonly
• X 1234ab
• R 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
identity M prepare to receive but dont send
anything yet X,R,S look for hang-up or hook
flash event and report, dont play any signal
18
Message 7

• NTFY 2001 aaln/1_at_my.mta.com MGCP 1.0
• N ca_at_the.callagent.com 2427
• X 1234ab
• O 3,0,1,5,5,5,1,2,1,2,

Comments The user dials
19
Message 9

• RQNT 1000 aaln/1_at_my.mta.com
• MGCP 1.0
• N ca_at_the.callagent.com
• Com 2427
• X 1234ab
• R hd(N)
• S

Comments When you see the event asked for in R
send them to Ca_at_the callagent.com, UDP port
2427 X Use the following requiest ID in your
response R Look for hook-detect event. Notify me
right away S dont play any signal
20
Message 10

• 200 2002 OK
• I abcd1234
• v-0
• O-25678 753849 IN IP4 192.164.3.11
• s-
• CIN IP4 192.168.3.11
• T0 0
• M audio 30000 RTP/AVP 0

Comments RG acknowledges that a media connection
is set up c is the address where the called
party shall send its media m is the UDP port to
be used for media
21
Message 11

• CRCX 1003 aaln/1_at_my.mta.com
• MGCP 1.0
• L p20, a PCMU, soff, eon
• C abcde1234
• M sendrecv
• X 1234ab
• R hd(N)
• Srg
• v-0
• O-25678 753849 IN IP4 192.164.3.11
• s-
• CIN IP4 192.168.3.11
• T0 0
• M audio 30000 RTP/AVP 0

Comments Set-up of the connection to called
party side Look for off-hook event Ring
signal Note the SDP from the calling party
22
Message 12

• 200 1003 OK
• I def1234
• v-0
• o-1111 23245 IN IP4 192.168.3.12
• s-
• cIN IP4 192.168.3.12
• t0 0
• m audio 40000 RTP/AVP 0

Comments
23
Message 13

• MDCX 1004 aaln/1_at_my.mta.com MGCP 1.0
• I abcd1234
• M recvonly
• X 1234ae
• R hu(N), hf(N)
• Srt
• v-0
• o-1111 23245 IN IP4 192.168.3.12
• s-
• cIN IP4 192.168.3.11
• t0 0
• m audio 40000 RTP/AVP 0

Comments The CMS knows that the called party is
ready and alerts the caller He plays the ring
tone, Gives the media session description to the
caller Hook on and hook flash shall be reported
I is the call identifier
24
Message 14

• 200 1004 OK

Comments
25
Message 15

• NTFY 5001 aaln/1_at_your.mta.com MGCP 1.0
• N ca_at_the.callagent.com 2427
• X 1234ab
• O hd

Comments The called party picks up This is
reported to the CMS
26
Message 16

• 200 5001 OK

Comments
27
Message 17

• MDCX 1005 aaln/1_at_my.mta.com MGCP 1.0
• I abcd1234
• M sendrecv
• X 1234ae
• R hu(N), hf(N)
• S

Comments The CMS tells the caller to stop
playing the ring tone (s? and to enable
bidirectional media
28
Message 18

• 200 1005 OK

Comments Now the caller acks the message and the
call is set up.
29
Wireless Local Area Network
30
The Physical layer
MAC Layer
PHY Layer
PLCP Sub layer
PMD Sub layer
31
Direct Sequence Spread Spectrum DSSS

• Physical Layer Convergence Procedure PLCP
• PLCP protocol data unit

PLCP preamble
PLCP header
MPDU
Sync 128 bits
SDF 16 bit F3Ah
Signal 8 bit
Service
Length 16 bit µs
CRC 16 bit
32
Architecture
PPDU
Scrambling
Band spreading Modulo-2 adder
Transmit Mask Filter
DBPSK DQPSK Modulator
11 bit Barker word
DBPSK DQPSK De-modulator
PPDU
De-spread correlator
De-scrambling
Clock
Clock recovery
33
802.11 MAC frame structure
34
General format
MAC Header
Frame Body
FCS
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4
35
Frame Control

• Length 16 bits

Protocol version
Type
Subtype
To DS
From DS
More Frags
Retry
Power Mgt
WEP
Other
More Data
2
2
4
1
1
1
1
1
1
1
1
36
Frame types
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4

• Information for updating NAV