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SCTP connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) is a relatively new transport layer in the IP Protocol Stack. In an LTE network, SCTP is used to provide guaranteed message delivery between the MME and eNodeB.

SCTP belongs to the SIGTRAN protocol family and has been used as the transport layer for carrying telecom signaling over IP. SCTP provides a reliable transport service that operates at message level (unlike TCP that provides a byte stream
interface with no message boundaries).

SCTP allows multiple message streams to be exchanged on a single SCTP connection. Data from multiple streams can be sent in a single SCTP message as chunks. Selective acknowledgements are supported at individual chunk level.

SCTP protocol allows dynamic configuration of the IP addresses. Similar to SS7, SCTP can be switched over from one link to another. SCTP supports a make-before-break changeover, i.e. the packet stream is moved before removing the link that needs to be taken out of service.

In this sequence diagram we will be examining some of the features of SCTP.

  1. SCTP Connection establishment.
  2. SCTP data exchange and selective acknowledgement.
  3. Addition of a new IP address to an SCTP connection.
  4. Switching over to the new IP address.
  5. Removing the old IP address.
  6. SCTP connection release

SCTP Connection Setup, Reconfiguration and Release

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LTE Attach and Default Bearer Setup Sequence Diagram

LTE Attach and Default Bearer Setup

Click to open the PDF sequence diagram describing LTE attach and default bearer setup

This call flow describes the signaling involved in LTE attach and default bearer setup. The sequence diagram covers the following phases:

  1. Random Access Procedure: The UE uses a slotted aloha procedure to access the eNodeB. The UE sends a preamble on a random access channel, the network replies with corrected timing and resource assignments.
  2. RRC Connection Establishment: The UE and the eNodeB then interact to establish a Signaling Radio Bearer (SRB). The SRB will be used for the signaling between the eNodeB and the UE.
  3. Attach and Authentication: The UE now registers with the Core Network. Session contexts are setup at the MME and the Serving Gateway. This step also results in the authentication of the UE and the Network.
  4. Default Radio Bearer Setup: Finally, the default bearer for data transfer is established. Default bearer session is established at the UE, eNodeB, MME, Serving GW and PDN Gateway. User data sessions is exchanged once the default bearer is setup.

The sequence diagram also contains links to access further details about the messages. Click on the messages with blue title for details about the message.

RRC Connection, LTE Attach and Default Radio Bearer Setup

 


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LTE RRC Connection Setup

lte-rrc-connection

Establishing an RRC connection in LTE typically involves the following steps:

  1. UE initiates a session to the eNodeB using a random RA-RNTI.
  2. eNodeB assigns a C-RNTI and sends timing adjustment to the UE.
  3. UE adjusts the the timing and requests an RRC Connection Setup via the UL-SCH channel.
  4. The eNodeB sends the RRC Connection Setup to the UE on the DL-SCH channel. This message sets up the SRB. DRB may also be established at this point.
  5. Finally, the UE acknowledges the message with RRC Connection Complete message.

The messages in the RRC Connection Setup are described in the following presentation.

LTE RRC Connection Setup Messaging


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LTE Attach and Default Bearer Setup

LTE reduces the latency in attach and PDP context activation. Attach and default bearer context setup take place almost in parallel.

The parallel procedures result in more complicated signaling messages. The messages involved in these procedures are explained here.

 

LTE Attach and Default Bearer Setup


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LTE security: encryption and integrity protection call flow

LTE key hierarchy

LTE security is based on a shared secret key K between the USIM and the HSS. The UE, the eNodeB and the MME derive keys for encryption and integrity protection from K.

The derived keys are used for:

  • NAS encryption and integrity protection
  • RRC encryption and integrity protection
  • User plane encryption

The key derivation and the key exchange call flow is described in the following link:

LTE security procedure: authentication, encryption and integrity protection


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ROHC – Robust Header Compression

ROHC improves the throughput on the link by compressing the TCP, UDP, IP and RTP headers to a a few bytes. This is particularly important for voice over IP as in absence of ROHC, the headers would have consumed more bandwidth than the voice channel being carried.

ROHC is envisioned as an extensible framework for robust and efficient header compression over highly error–prone links with long round–trip times. This design is motivated by the large bit error rates (typically on the order of 10−4 – 10−2) and long round trip times (typically 100–200 msec) of cellular networks. The design of ROHC is based on the experiences from the header compression schemes reviewed above. In particular, ROHC incorporates elements from ROCCO and Adaptive Header Compression (ACE) which may be viewed as a preliminary form of ROHC.

ROHC