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Introduction to Git with Scott Chacon of GitHub

This talk introduces the Git Version Control System by looking at what Git is doing when you run the commands you need to do basic version control with it. The talk covers how to use Git to do the basics, while seeing how it differs from Subversion, what staging and committing actually looks like, how it stores it’s data, how it branches and merges so nicely and how it talks to a server when pushing and fetching.

The video then describes how to look at your history with log in interesting ways. This should help Git newbies get acquainted with the popular VCS and other Git users get a glimpse of what’s happening under the hood.


IMS User to IMS User Call Flow

Call from an IMS user to another IMS user

A detailed IMS subscriber to IMS subscriber is presented here. The preconditions for this call flow are:

  • The calling IMS subscriber is currently roaming outside the home network.
  • The called user is registered in the home network.
  • The caller and the called user require resource reservation before the call can be setup.

The major steps in the call flow are:

  • IMS routing of the initial SIP INVITE.
  • IMS routing of the first response to the INVITE.
  • Resource allocation via PDP context activation.

IMS (IP Multimedia Subsystem) user calls another IMS user

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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

Hierarchical State Machine

In this article, we will be highlighting the advantages of hierarchical state machine design over conventional state machine design.

In conventional state machine design, all states are considered at the same level. The design does not capture the commonality that exists among states. In real life, many states handle most  messages in similar fashion and differ only in handling of few key messages. Even when the actual handling differs, there is still some commonality. Hierarchical state machine design captures the commonality by organizing the states as a hierarchy. The states at the higher level in hierarchy perform the common message handling, while the lower level states inherit the commonality from higher level ones and perform the state specific functions.

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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.