UMTS - SCTP

Stream Control Transmission Protocol (SCTP) is a reliable transport protocol that runs on top of a packet service without potentially unreliable connection such as IP. It was developed specifically for applications and signaling offers recognized without unduplicated transfer error datagrams (messages). Detection of data corruption, data loss and data duplication is performed using checksums and sequence numbers.

A selective retransmission mechanism is applied to correct the loss or corruption of data. The decisive difference is TCP multi-homing and the concept of multiple streams in a connection. Where in a TCP flow is called a sequence of bytes, a SCTP stream represents a sequence of messages. SCTP tries to combine the advantages of UDP and TCP, but avoid their drawbacks; it is defined in IETF RFC 4960.

SCTP is used on several network internal control plane interfaces, with these SCTP applications −

S1-MME: between eNodeB and MME
SBc: between MME and SBc.
S6a: between MME and HSS
S6d: between SGSN and HSS
SGs: between MSC/VLR and MME
S13: between MME and EIR

S1 Application Protocol

Two categories of procedures across S1-MME exist: UE associated and non-associated UE. Furthermore two classes of messages are defined: Class1 is with the class 2 is answered. Class 1 and related procedures initiator/response messages are listed in the table below; procedures for Class 2 message names are largely identical to the procedure names, and the table below (second table) lists only these.

Elementary Procedure	Initiating Message	Response(if successful)
Handover preparation	Handover required	Handover command
Hand resource allocation	Handover req.	Path switch req. ack.
Patch Switch Request	Path Switch Req.	Path switch ack.
Handover Cancellation	Handover cancel	Handover cancel ack.
E-RAB Setup	E-RAB setup Req.	E-RAB setup Resp.
E-RAB modify	E-RAB modify req.	E-RAB modify Resp.
E-RAB release	E-RAB command release	E-RAB command Resp.
Initial context setup	Initial context setup req.	Initial context setup Resp.
Reset	Reset	Reset Ack.
S1 setup	S1 setup req.	S1 setup Resp.
UE context release	UE context release command	UE context release complete
UE context modification	UE context modification req.	UE context modification resp.
eNodeB configuration	ENB configuration update	ENB configuration update ack.
MME Configuration	MME configuration update	MME configuration update ack.
Write- Replace warning	Write-Replace warning req.	Write-Replace warning resp.

Elementary Procedure

Handover Notification
E-RAB release indication
Paging
Initial UE message
Downlink NAS transport
Uplink NAS transport
NAS non delivery indication
Deactivate Trace
Trace start
Trace failure indication
Location reporting failure indication
Location reporting control
Location report
Cell Traffic Trace
Error indication
UE Context release request
Downlink S1 CDMA2000 tunneling
uplink S1 CDMA2000 tunneling
UE capability info indication
eNodeB status transfer
MME status transfer
Overload Start
Overload Stop
eNodeB direct information Transfer
MME direct information transfer
eNodeB configuration transfer
MME configuration

X2 Application Protocol

X2 application protocol has much in common with the S1-AP; same categorization in class 1 and class 2 messages is made. The setup message is much smaller, corresponding to the specialized function of X2.

Procedure	Initiating Message	Class	Response(if successful)
Handover preparation	Handover req.	1	Handover req. ack.
Reset	Reset req.	1	Reset resp.
X2 setup	X2 setup	1	X2 setup resp.
eNodeB Configuration update	ENB Configuration update	1	ENB Configuration update ack.
Resource Status Reporting Initiation	Resource Status req.	1	Resource Status resp.
Load Indication	Load Information	2
Handover Cancel	Handover Cancel	2
SN status transfer	SN status transfer	2
UE context release	UE context release	2
Resource Status	Resource Status	2
Reporting	update
Error Indication	Error Indication	2