Please explain. I marked it in a red circle.

It's marked in red color

It's rounded in red color.I got this from the book Computer networking a top down approach.Pg NUM 241. Section 3.4 Rdt 3.0

  • 1
    I don't think the symbol below the line is a delta.
    – Ron Trunk
    Nov 4, 2017 at 20:07
  • Ya it's not Delta but still I don't had a suitable key in keyboard for it Nov 5, 2017 at 2:16
  • The Λ character is a Lambda (U+039B) that indicates the time period over which you are evaluating the expression.
    – Ron Maupin
    Nov 5, 2017 at 16:51
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Dec 21, 2020 at 16:19

1 Answer 1


The capital lambda 'Λ' is the "output alphabet" -- it's part of the notation for finite state machines, and is the set of possible outputs. The trigger events of the transition come from the "input alphabet" (normally capital sigma 'Σ'). Transition events are listed above the line, the actions underneath.

See for example http://www.dcs.ed.ac.uk/home/mic/FiniteStateMachines1-slides.pdf

Or, on the subject of reliable data transmission http://www.nyu.edu/classes/jcf/CSCI-GA.2262-001_fa13/slides/session6/ReliableDataTransfer.pdf

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.