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LTE Random Access Procedure and Contention Resolution

LTE random access procedure is used by the UEs to initiate a data transfer. The UEs also obtain uplink timing information from the initial handshake.

This sequence diagram describes the tale of three UEs (UE-A, UE-B and UE-C) that are powered on at the same time:

  1. UEs synchronize with the downlink channel by decoding the PSS and SSS signal. The UEs are synchronized to the downlink frames after completing this procedure.
  2. The three UEs initiate the random access procedure at exactly the same time. Two of them (UE-A and UE-B) happen to pick the same preamble. This results in a resulting in a collision. UE-C picks a distinct preamble so it succeeds in the random access procedure.
  3. Contention between UE-A and UE-B is resolved in UE-A’S favor. UE-A proceeds with the RRC connection.
  4. UE-C times out and retries the random access procedure.

LTE random access procedure


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HTTPS SSL/TLS Session for SPDY Sequence Diagram

This sequence diagram covers the establishment of a SSL/TLS connection for sending Google SPDY data. The protocol flow covers:

  1. SSL/TLS initial cryptographic parameter negotiation.
  2. X.509 Certificate exchange and encryption start with elliptic curve Diffie Hellman key exchange.
  3. Master key generation and encrypted data transfer.
  4. SSL/TLS session release.
    SSL TLS Sequence Diagram for SPDY


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S1AP View of LTE Attach and EPS Bearer Setup

This sequence diagram looks at LTE Attach and EPS Bearer Setup from the S1AP vantage point. The following signaling is covered:

  1. UE Attach, authentication and security signaling
  2. Setup of two EPS Bearers (RAB id 5 and 6)
  3. Release of UE context due to inactivity
  4. Reestablishment of the UE context with a Service Request.

You can click on individual messages in the sequence diagram to see field level details.

Click to see the "S1AP View of LTE Attach & EPS Bearer Setup"  sequence diagram


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Voice Over LTE (VoLTE) Originating Call

Voice over LTE (VoLTE) is the standard for voice call setup in LTE networks. When VoLTE is deployed, phones will not need to fallback to 3G for voice calls.

VoLTE uses IMS SIP signaling to set up voice calls. The following VoLTE call flow describes the IMS call setup and release. An example of sending an SMS over IMS is also included. Sample RTP and RTCP messages are also shown in the flow.
Voice over LTE Call Flow

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The Anatomy of an INVITE Request

Originally posted on SIP Adventures:

As a high school student, I loved biology. I loved learning about the natural world and how our bodies function. The spleen was my friend — not to mention the pancreas, liver, and lungs.

Unlike many of my classmates, I didn’t mind dissecting frogs and the obligatory fetal pig. Okay, I wasn’t overly fond of the smell of formaldehyde, but it was mesmerizing cutting into the skin and exploring what lurked below.

I look upon SIP in a very similar fashion except instead of using a knife, my dissection tool of choice is Wireshark. Followers of this blog should already know that from reading my article, How to Debug SIP.   Wireshark is powerful software that not only captures SIP packets, but displays them in a very readable manner. On top of that, you can also capture and replay audio streams. If you want to call yourself a SIP professional…

View original 1,443 more words


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BGP Router Startup

The Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol. An autonomous system is a group of networks under common administrative control and routing policies.

This sequence diagram describes the sequence of messages exchanged when a new BGP router is made operational. The steps involved are:

  1. Establish TCP connections
  2. Exchange BGP Open messages.
  3. Start periodic exchange of Keepalive messages.
  4. Exchange routing information with the BGP Update message.

BGP Router Startup


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OSPF Router Startup and Link State Update

OSPF (Open Shortest Path First) is a routing protocol that is used as an interior gateway protocol in large enterprises.

Here is a sequence diagram that describes the message exchange that takes place when a new OSPF router comes online. You can click on any message in the flow to see full message contents.

Covered sequence:

  1. OSPF hello message exchange
  2. Master-slave resolution
  3. Router database synchronization
  4. Link state update
OSPF router startup and link status update

OSPF router startup and link status update

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