41902 Lecture Notes - Lecture 2: Packet Switching, Propagation Delay, Queuing Delay

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14 Jun 2018
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Network Fundamentals:
Chapter 1.3: Network core
The network core is a mesh of interconnected routers.
Packet switching: hosts break application-layer messages into packets. Forward packets from one
router to the next, across links on path from source to destination. Each packet is transmitted at full
link capacity.
Routing: determines the source destination route taken by packets (routing algorithms).
Forwarding: ove pakets fo oute’s iput to appopiate oute output.
Packet switching vs. Circuit switching:
Circuit-switch network: data travels via a physical connection e.g. telephone networks
Packet-switch network: not physical, data travels in packets e.g. internet
Circuit-switch networks are more appropriate for a fixed or constant data arrival rate, while packet-
switch networks are more appropriate for applications with varying rates of transmission.
Internet structure: network of networks:
Chapter 1.4: Delay, loss, throughput in networks
Loss and delay can occur when the packet arrival rate to link (temporarily) exceeds the output link
capacity, causing the packets to queue in router butters. There are four sources:
Processing delay: Ous at the oute. Refes to the tie euied to eaie the paket’s
header and determine where to direct the packet. Includes other factors such as time
needed to check for bit-level errors in packet. Delay is fixed.
Queueing delay: Occurs at the queue when packet is waiting to be transmitted onto the link.
Length of queuing delay is dependant on congestion level of router, number of arriving
packets waiting for transmission onto the link, and amount of incoming traffic. Delay is
variable.
Transmission delay: Refers to the amount of time amount of time required to transmit of a
paket’s its ito the lik. Formula: L/R (L: packet length (bits) / R: link bandwidth bits/sec)
Transmission delay is the amount of time required for the router to transmit a packet: it is a
futio of the paket’s legth ad tasissio ate of the lik. Dela is fied.
Propagation delay: Refers to the time required for a bit that has entered the link to
propagate from beginning of the link to the router. The bit propagates at propagation speed,
dependant on the physical medium of the link (e.g. fibre optics, twisted-pair copper wire,
etc. )
Formula: d/s (d: length of physical link / s: propagation speed (~2x108 m/sec)
The propagation delay is the time required for a bit to propagate from one router to the
next: it is a function of the distance between two routers. Delay is fixed.
Total delay (end-to-end) = Processing + Queueing + Transmission + Propagation delays
---dnodal = dproc + dqueue + dtrans + dprop---
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Document Summary

The network core is a mesh of interconnected routers. Packet switching: hosts break application-layer messages into packets. Forward packets from one router to the next, across links on path from source to destination. Each packet is transmitted at full link capacity. Routing: determines the source destination route taken by packets (routing algorithms). Forwarding: (cid:373)ove pa(cid:272)kets f(cid:396)o(cid:373) (cid:396)oute(cid:396)"s i(cid:374)put to app(cid:396)op(cid:396)iate (cid:396)oute(cid:396) output. Circuit-switch network: data travels via a physical connection e. g. telephone networks. Packet-switch network: not physical, data travels in packets e. g. internet. Circuit-switch networks are more appropriate for a fixed or constant data arrival rate, while packet- switch networks are more appropriate for applications with varying rates of transmission. Loss and delay can occur when the packet arrival rate to link (temporarily) exceeds the output link capacity, causing the packets to queue in router butters. There are four sources: processing delay: o(cid:272)(cid:272)u(cid:396)s at the (cid:396)oute(cid:396).

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