Acer ISDN 128 Surf USB Bedienungsanleitung PDF herunterladen (Seite 3)

Firstly, VoIP doesn't use TCP because it is too heavy for real time applications, so instead a

UDP (datagram) is used.

Secondly, UDP has no control over the order in which packets arrive at the destination or

how long it takes them to get there (datagram concept). Both of these are very important to

overall voice quality (how well you can understand what the other person is saying) and

conversation quality (how easy it is to carry out a conversation). RTP solves the problem

enabling the receiver to put the packets back into the correct order and not wait too long for

packets that have either lost their way or are taking too long to arrive (we don't need every

single voice packet, but we need a continuous flow of many of them and ordered).

1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

V=2

P X CC M PT sequence number

Time stamp

Synchronization source (SSRC) identifier

Contributing source (CSRC) identifiers

......

Tab. 3 - Payload of Real Time Transport Protocol

Where:

• V indicates the version of RTP used

• P indicates the padding, a byte not used at bottom packet to reach the parity packet

dimension

• X is the presence of the header extension

• CC field is the number of CSRC identifiers following the fixed header. CSRC field are

used, for example, in conference case.

• M is a marker bit

• PT payload type

• For a complete description of RTP protocol and all its applications see relative RFCs

1889 and 1890

2.3.2 RSVP (Resource ReSerVation Protocol)

There are also other protocols used in VoIP, like RSVP, that can manage Quality of Service

(QoS). RSVP is a signaling protocol that requests a certain amount of bandwidth and latency in

every network hop that supports it. For detailed info about RSVP see the RFC 2205.

2.3.3 ITU-T H.323 signaling protocol

H.323 protocol is not a separate protocol. H.323 covers plenty of protocol for transmit

voice or video over IP on different transport networks like ATM, ISDN, PSTN. See below tab.

1 2 3 4 5 6 7 8 ... 36 37

Keine Kommentare

Acer ISDN 128 Surf USB Bedienungsanleitung Seite 3