Why SCTP (Stream Control Transmission Protocol)

New Trends & Protocols in Voice Network:

Understanding Transport Layer (4) Protocols is vital in understanding how data is transmitted from one device to another successfully. Traditionally these protocols were used for data transfer. Last decade has seen a new trend emerging of sending voice over PS (Packet Switched) networks opposed to using traditional CS (Circuit Switched) networks. VoIP (voice over IP) is one such prime example. Leaving merits & demerits of Voice over CS or PS aside, we will focus on new protocols emerging in this area, especially SCTP. Let’s start with few important basics:

IETF: The Internet Engineering Task Force is an international body based in USA working on protocols and development work relating to internet.

ITU: international Telecommunication Union, based in Europe, controls developments and protocols / standards relating to communication, primarily for voice.

3GPP: Part of ITU, it focuses on GSM, 2G, 3G and 4G development and protocols.         

Layer 3 Protocol: IP Internet Protocol is a connection-less one, and hence lacks reliability. To supplement, Reliability is provided by layer 4 protocols mentioned below.

Layer 4 Protocols: TCP / UDP / SCTP (Stream Control Transmission Protocol): These are all transport layer (4) protocols. They are responsible for end to end reliable delivery of data. All are connection oriented, i.e. need connection or session association with pier before working (otherwise who will they ask for acknowledgement??)

Read detailed Discussion on TCP & UDP Here.  

UDP & TCP were designed for Data (bits):

UDP is packet (chunks of data) oriented. A fixed sized data chunks received from the application and then forwarded in UDP packets. UDP is un-acknowledged (may be one acknowledgement after 10 packs). It is light weight and hence used where good speed is important than reliability, e.g. voice.

TCP is bytes or stream oriented. A stream of byes of any size received and then divided into TCP packets and sent. TCP is made more reliable using Acknowledgements. So it is good for file transfer because file transfer does not tolerate any loss.

 So Why SCTP? It is a relatively new protocol developed in 2000. It was designed as Message Oriented (opposed to bit oriented). It gives importance to the order of messages also. Hence it is suitable for message oriented applications, e.g. SS7 Signaling. It also considers session/path loss, which is required for a voice connection. In short SCTP was developed specifically for carrying signaling in VoIP applications.

Who Did it? SCTP was designed by IETF’s subcommittee responsible for SIGTRAN (SS7 Signaling Transport) Protocols. IETF is more IP & internet oriented & has experience in this area, and hence bias towards this area.

Problems: The fact that IETF developed SCTP created two problems:

1-    IETF lacked experience in voice Signaling. SS7 Singling protocols were produced by ITU. So SS7 & SCTP did not fit well.

2-    IETF lacked experience in Voice networks. Therefore SCTP was not liked (preferred) by 3GPP/ITU but they were in need of something in this area.

There was a need for something to carry SS7 Signaling over IP as VoIP was getting popular. IP & voice networks were getting merged.

So what was the Solution, Read it Here: