Packet Switching
9 important questions on Packet Switching
What is main problem in this chapter?
Directly connected networks have limitations
- number of hosts
- geographical area
Requirements
- interconnect links
- interconnect networks
- scalability
Switching and forwarding
Bridges and LAN switches (layer 2)
Internetworking (layer 3)
Routing (layer 3)
Switch & Router Design
What is Datagram Switching?
Each packet forwarded independently
- analogy: postal system
Each switch maintains forwarding table
- the hard problem is constructing this table
Forwarding table
- list with addresses (A....Z), take path X to get there. Keep going through the path and tables till destination
What is Virtual Circuit Switching?
- explicit connection setup (and tear-down) phase
Subsequent packets follow same circuit
- analogy: phone call
Each switch maintains VC table
- Higher grades + faster learning
- Never study anything twice
- 100% sure, 100% understanding
For my 1st exam I got a C. Then, I started looking for ways to learn better and I came across Study Smart. Since then I have scored a A+, A, A- and another A. I highly recommend it to everyone who wants to hear it. I am so happy with it!!
I was struggling to finish all my first-year subjects for 3 years. Then I discovered Study Smart, which helped me to finish all of them within 3 months.
Because of StudySmart, things are going well in many ways. Before, I always had study stress and no time for anything. Now I feel calm and confident (because of the program). I used to score a B, now I usually score an A or A+.
Hey Chris, I really want to thank you very much for developing the platform, I usually got around C for my exams, now I scored an A+, an A and a A- !!
Since using Study Smart, my average has increased significantly. I even completed a statistics course with a A+ while I'm bad at statistics. I took on extra courses this year, because of Study Smart. Thanks so much!
Ever since I started studying with StudySmart I passed all my exams !!! No more re-sits!! At first, I was happy with a pass and now I only get good grades. Many thanks!
I aced all the courses I studied with Study Smart. For law, I first scored a D, and now an A! I’m 100% sure that I’ll pass all the courses I study with Study Smart.
I always thought I studied well. My grades were fine. I could never have imagined that applying these study skills would have such a huge and direct impact on my grades, my speed and the pleasure I have in learning.
For the first time, I finally feel totally confident about the way I learn. I have the perfect overview and make great progress in terms of speed and the grades I get. Thanks, Chris!
I immediately started to enjoy learning again, and my grades soared. All of a sudden, everything went better. I highly recommend Study Smart to all students..
Being able to create my materials online made the process of studying so much smoother and quicker. I no longer have to spend a huge amount of my time on creating my summaries.
At first, my notes were a mess. Now I use Study Smart to take my notes in a perfectly structured way. It helps me to save a lot of time and do other things I enjoy.
Students that study with Study Smart get better grades. The system applies all the learning methods in such a way that both well-performing students and students who struggle perform much better.
Hi Chris, I would like to thank you very much. Last year I scored C on a course, and this year I got an A+ thanks to your method! Thank you very much!
The largest effect is that my child is more enthusiastic about school! Every parent wants their children to be happy at school, Study Smart made it happen.
In high school, I failed several exams, and that has really changed now. I actually needed this tool very much back then. Since I started studying with StudySmart, I haven't had any D's. My average score is now an A and those are results I would have never had in the past
With my old method, I passed only 3 out of 8 courses. Since I started taking my notes digitally in Study Smart, I have passed all my courses on the first try. For me, StudySmart takes away the stress of wondering if I will pass or not.
Thanks to StudySmart, I passed all my exams, and with better grades than before! On top of that, I have mastered a very good study method now, which I am confident will help me earn my degree.
What is Source Routing?
- no forwarding table in switches
- packet gives rotating list of ports for switches to use. Switch uses port and places it back of the list
Variable length path info
- where? In header? Variable length
Scaling problems!
Examples
- IP (option), Token Ring (bridges), SDN (controller)
What are Bridges and LAN switches
Connect two or more LANs with a bridge
- bridge operates in promiscuous mode (accept all frames)
- accept and forward strategy
- using only frame header (does not use/change layer 3 packet header)
Ethernet Switch = Bridge on Steroids
Extended Lan: a collection of interconnected LANs
Collissions only happen in 1 ethernet. Instead of the full extended Ethernet.
What are learning Bridges
Learn table entries based on source ddress
- initially table is empty
Table is an optimization; need not be complete
- timout to protect against invalid information
Always forward broadcast frames
Problem: Loops
- loops are causing "broadcast storms"
- loops disturb learning process
Explain the Spanning Tree Algorithm
Bridges run a distributed Spanning Tree Protocol (STP)
- select which bridges actively forward
- Developed by Radia Perlman
- now part of IEEE 802.1 specification (802.1D)
Overview
- Each bridge has unique ID
- Select bridge with smaller ID as root
- Select bridge on each LAN closest to root as designated bridge
- Each bridge forwards frames to/from each LAN for which it is the designated bridge
How Broadcast & Multicast for bridges
- forward frame on each active port
Multicast:
- broadcast all multicast frames
Better: learn when no group members downstream
- accomplished by having each member of group G send a frame with multicast address G in source address field
- (not used in practice)
What are the limitations of bridges?
- use of hardware addresses does not scale
- spanning tree algorithm does not scale
- broadcast does not scale
- solution: VLAN (infra)
How to accommodate hetergeneity?
- different frame formats
- different address formats
- different transmission speeds and throughputs
Transparency is an advantage, but can be dangerous
- frames can be delayed, dropped, reordered
- result: an extended LAN does not behave as an single Ethernet
The question on the page originate from the summary of the following study material:
- A unique study and practice tool
- Never study anything twice again
- Get the grades you hope for
- 100% sure, 100% understanding
