Equitel E451 Uživatelský manuál stáhnout pdf (Strana 28)

October 2013 – Ed.1.0

EQUITEL E400 Family – Control and communications protocol

Page 25

receivers to distinguish the packet content. These field values are normalized by IANA

(7)

. For the audio

encoded in G.711-A the value for this field is 8.

Sequence: 16 bits number identifying each packet consecutively. It is used by the receiver to reorder the

packets if necessary.

TimeStamp: This is a 32-bit time stamp indicating the transmission moment of each packet since the

session initiation. The units in which time is measured depend on the type of content. For G.711-A the

units are increments of 125 µs. Therefore, in each transmitted RTP packet we have to increase this data in

160 with respect to the previous one.

SSRC (Synchronization Source Identifier): Number of 32 bits used to synchronize different RTP streams if

necessary. In our case it can be any number since there is no synchronization of any kind.

Next we attach the data forming the audio packet content in its natural order; that is, the first is the older

one and the last one is the most recent.

This way the RTP packet will have a 172 bytes length.

This packet is sent using the UDP protocol.

3.2 Decoding

The decoding process is summarized in the block diagram of Figure 12.

The RTP packets received in the port 30000 are stored in a FIFO type buffer to compensate the jitter

(8)

introduced in the network. Furthermore, we can take advantage of this buffer to reorganize the packets if

necessary.

The higher the buffer capacity, the more robust will be the receiver against jitter. However, the more audio

latency will be introduced as well.

Every 20 milliseconds we extract a packet from this buffer which is decoded to obtain the corresponding PCM

samples, and they are sent to the digital-analogue converter.

The decoding is made applying the opposite process to codification we have just seen.

Figure 12. RTP reception and G711-A decoding

(7) Internet Assigned Numbers Authority.

(8) The time that one data packet takes to travel till destination is not fixed as it depends on several factors. The packets coming out from a transmitter with a constant

rhythm do not reach the receiver at the same rhythm. This fluctuation in the frequency is known as “jitter”.

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