Voice over WLAN Voice over IP WAN Vo

Voice over WLAN

Voice over IP WAN Vo. IP Gateway PBX Vo. IP Gateway PSTN PBX

Challenges in Vo. IP 1. Delay 1. Absolute delay 2. Jitter 2. Packet loss 1. Less than 5% can be tolerated 3. Bandwidth 1. Wire bound : no problem 2. Wireless : always limitation

Voice over Internet Separate signaling and transport

Transport 1. Sampling 1. Speech 4 KHz->sampling 8 k. Hz 2. 8 bit-> 64 kbps per voice channel

Transport Equalization Echo cancellation line echo acoustic echo Encoding bit rate G 711 64 G 726 -32 32 G 729 ab 8 G 729 e 12 G 728 16

Transport Digit Relay Send tones Detect key strokes Voice activity detection Detect silence periods Channel coding TCP is not used -> 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)

MGCP Call Setup RG Calling CMS RG Called 1 2 3 4 5 6 7 8 9 10 13 14 17 18 11 12 15 16

Message 1 RQNT 1000 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com Com: 2427 X: 1234 ab R: hd(N) S: Comments: When you see the event asked for in R send them to Ca@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: don’t play any signal

Message 2 200 1000 OK Comments: Positive ack

Message 3 NTFY 2000 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com: 2427 X: 1234 ab O: hd Comments: User picks up the phone O: is indicating the event

Message 4 2000 OK Comments: Positive Ack

Message 5 RQNT 1001 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com: 2427 X: 1234 ab R: hu(N), [0 -9] (D) S: dl D: (XXXX/0 T/00 T) 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

Message 6 200 1001 OK Comments:

Message 7 NTFY 2001 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com: 2427 X: 1234 ab O: 3, 0, 1, 5, 5, 5, 1, 2, Comments: The user dials

Message 8 2001 OK Comments:

Message 9 CRCX 1002 aaln/1@my. mta. com MGCP 1. 0 L: p: 20, a: PCMU, s: off, e: on C: abcde 1234 M: recvonly X: 1234 ab R: hu(N), hf(N) S: Comments: Use packetization period 20 ms, use the PCMU codec (G 711 mulaw); nor silence detection; do echo cancellation. C: use this string as call identity M: prepare to receive but don’t send anything yet X, R, S: look for hang-up or hook flash event and report, don’t play any signal

Message 7 NTFY 2001 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com: 2427 X: 1234 ab O: 3, 0, 1, 5, 5, 5, 1, 2, Comments: The user dials

Message 9 RQNT 1000 aaln/1@my. mta. com MGCP 1. 0 N: ca@the. callagent. com Com: 2427 X: 1234 ab R: hd(N) S: Comments: When you see the event asked for in R send them to Ca@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: don’t play any signal

Message 10 2002 OK I: abcd 1234 v=-0 O=-25678 753849 IN IP 4 192. 164. 3. 11 s=C=IN IP 4 192. 168. 3. 11 T=0 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

Message 11 CRCX 1003 aaln/1@my. mta. com MGCP 1. 0 L: p: 20, a: PCMU, s: off, e: on C: abcde 1234 M: sendrecv X: 1234 ab R: hd(N) S: rg v=-0 O=-25678 753849 IN IP 4 192. 164. 3. 11 s=C=IN IP 4 192. 168. 3. 11 T=0 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

Message 12 200 1003 OK I: def 1234 v=-0 o=-1111 23245 IN IP 4 192. 168. 3. 12 s=c=IN IP 4 192. 168. 3. 12 t=0 0 m= audio 40000 RTP/AVP 0 Comments:

Message 13 MDCX 1004 aaln/1@my. mta. com MGCP 1. 0 I: abcd 1234 M: recvonly X: 1234 ae R: hu(N), hf(N) S: rt v=-0 o=-1111 23245 IN IP 4 192. 168. 3. 12 s=c=IN IP 4 192. 168. 3. 11 t=0 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

Message 14 200 1004 OK Comments:

Message 15 NTFY 5001 aaln/1@your. mta. com MGCP 1. 0 N: ca@the. callagent. com: 2427 X: 1234 ab O: hd Comments: The called party picks up This is reported to the CMS

Message 16 200 5001 OK Comments:

Message 17 MDCX 1005 aaln/1@my. mta. com MGCP 1. 0 I: abcd 1234 M: sendrecv X: 1234 ae R: hu(N), hf(N) S: Comments: The CMS tells the caller to stop playing the ring tone (s and to enable bidirectional media

Message 18 200 1005 OK Comments: Now the caller acks the message and the call is set up.

Wireless Local Area Network

The Physical layer MAC Layer PLCP Sub layer PHY Layer PMD Sub layer

Direct Sequence Spread Spectrum – DSSS • Physical Layer Convergence Procedure PLCP • PLCP protocol data unit PLCP preamble Sync 128 bits PLCP header SDF 16 bit F 3 Ah MPDU Signal Service Length 8 bit 16 bit µs CRC 16 bit

Architecture PPDU Scrambling Band spreading Modulo-2 adder Transmit Mask Filter DBPSK DQPSK Modulator 11 bit Barker word De-spread correlator DBPSK DQPSK De-modulator Clock recovery Clock De-scrambling PPDU

802. 11 MAC frame structure

General format MAC Header Frame Control Frame Body FCS Duration/ Address Sequence Address ID 1 2 3 Control 4

Frame Control Length 16 bits Protocol To From More Power More Type Subtype Retry version DS DS Frags Mgt Data 2 2 4 1 1 1 WEP Other 1 1

Frame types Frame Control Duration/ Address Sequence Address ID 1 2 3 Control 4 Information for updating NAV