telecom • networking • design

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TCP fast retransmit and recovery

TCP Slow Start and Congestion Avoidance lower the data throughput drastically when segment loss is detected. Fast Retransmit and Fast Recovery have been designed to speed up the recovery of the connection, without compromising its congestion avoidance characteristics.

Fast Retransmit and Recovery detect a segment loss via duplicate acknowledgements. When a segment is lost, TCP at the receiver will keep sending ack segments indicating the next expected sequence number. This sequence number would correspond to the lost segment. If only one segment is lost, TCP will keep generating acks for the following segments. This will result in the transmitter getting duplicate acks (i.e. acks with the same ack sequence number)

Click here for TCP fast retransmit and recovery sequence diagrams

TCP fast retransmit

The transmitter acts on duplicate acks and retransmits the packet, without waiting for the segment timer expiry

TCP fast recovery

TCP maintains the current data flow

Click here for TCP fast retransmit and recovery sequence diagrams

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Long Term Evolution (LTE) Tutorials

Here are a few hand picked links to LTE tutorials. Click here for the complete list.

LTE video tutorial

LTE video presentations

LTE physical layer

OFDM and SC-FDMA Signal Chains

LTE link layer design

data flow through PDCP, RLC, MAC and PHY layers of LTE

This article describes the LTE link-layer protocols, which abstract the physical layer and adapt its characteristics to match the requirements of higher layer protocols.The LTE link-layer protocols are optimized for low delay and low overhead and are simpler than their counterparts in UTRAN. The state -of-the-art LTE protocol design is the result of a careful crosslayer approach where the protocols interact with each other efficiently. This article provides a thorough overview of this protocol stack, including the sub-layers and corresponding interactions in between them, in a manner that is more intuitive than in the respective 3GPP specifications.

Introduction to LTE Architecture


This article provides an overview of the LTE radio interface, together with a more in-depth description of its features such as spectrum flexibility, multi-antenna transmission, and inter-cell interference control. The performance of LTE and some of its key features is illustrated with simulation results.

This article provides a high-level overview of LTE and some of its key components: spectrum flexibility, multi-antenna transmission, and ICIC. Numerical simulations are used to show the performance of the first release of LTE, as well as assess the benefit of the key features. Indeed these contribute strongly to LTE meeting its performance targets. An outlook of the evolution of LTE toward LTE-Advanced and full IMT-Advanced capabilities complete the article. Clearly, LTE offers highly competitive performance and provides a good foundation for further evolution.

LTE Protocol Stack

Click here for a more LTE tutorials that cover the entire spectrum of LTE development.