Review of Topology and Access Techniques / Switching Concepts

1. Review of Topology and Access Techniques / Switching Concepts BSAD 141 Dave Novak Sources: Network+ Guide to Networks, Dean 2013

2. Overview • Three “base” wired topologies • Bus, star, ring • Two wireless topologies • Ad-hoc, infrastructure • Three basic access techniques • CSMA/CD, CSMA/CA, and token passing • Packet switching versus circuit switching

3. Architecture • Basic level – any topology will support any architecture • Star topology may support client – server (CS) or peer-to-peer (P2P) architecture • Likewise, P2P architecture may be implemented using star, bus, or ring topology

4. Architecture / Topology • Topology –physicalor logical design or layout of the network • What is the difference between a physical star and logical star topology? • Architecture –describes the functionality and “administrative” structure of the network • How devices interact • Whether there are servers and exactly what these devices do

5. Access Technique • The access technique describes how the devices using a particular networking technology coordinate the use of, or share, the communication medium • How is CSMA and how does it work?

6. Access Technique • What about • CSMA/CD? • CSMA/CA?

7. Access Technique • Token Passing?

8. Packet Concept • Computer networks do not transfer data as a string or stream of continuous bits • Bits or data are divided into small generic blocks called packets • Dividing data into small packets helps determine which blocks have errors and which do not • Packets promote “equity”

9. Packets –vs- frames • What is the difference between a packet and a frame?

10. Switching • Determines how connections are established between different nodes on the network • Focus on 2 types of switching • 1) • 2)

11. Switching • Many / most LAN technologies use baseband communication • Why would this generally be the case?

12. Switching • To enable many devices to share the medium, transmission data are broken up into packets

13. Circuit switching • A connection-based telecommunications transmission technology where 2 nodes establish a dedicated connection / communication channel for the duration of the communication session • Requires that the sending node contact receiving node and establish a connection (what is a potential drawback to this type of connection?)

14. Circuit switching • Concept is a dedicated path that provides non-stop transfer • Bandwidth on circuit is dedicated and remains available until connection is terminated • Not shared • While nodes connected, all data follow same path • Data arrive in order

15. Circuit switching • Misconception that circuit switching only used for connecting voice circuits (either analog or digital)

16. Advantages of Circuit Switching • Dedicated connection between 2 hosts – full use of communication channel for duration of communication session • Highly reliable (with respect to path, variation in delay, and constant bit rate) • Requires little packet overhead during transmission because path / communication circuit is dedicated and known • Makes optimal use of available bandwidth (ratio of overhead to data is very small) during communication

17. Disadvantages of Circuit Switching • Inflexible with respect to many internet-based applications (data are bursty) • Involves a lot of communication overhead and time with respect to ESTABLISHING communication channel • Can waste a lot of bandwidth if data are not continually being sent • In event of failure, data are not packetized and do not seek an alternate path – path may be lost

18. Packet switching • A digital networking communication method where data are broken into small packets • Delivers variable rate data streams • During transmission packets are buffered and queued which leads to variable delay and throughput • No communication is needed with receiving node ahead of time - asynchronous

19. Packet switching • Two modes • 1) Connectionless (datagram) • 2) Connection-based (virtual circuit) • Implication is that data-oriented, digital, packet-switched networks can take advantage of both connectionless and connection-based transfer

20. Packet switching • 1) Connectionless (datagram concept) • Asynchronous communication – no coordination • Each packet much include complete routing information because each packet can be routed individually • Out-of-order delivery, different paths

21. Packet switching • 2) Connection-based (virtual circuit concept) • Protocols establish a connection between sender and receiver • A connection is established via software rather than physically • Connection set up and torn down as needed • Packets include connection identifier • Packets are typically delivered in order

22. Advantages of Packet switching • Bandwidth from end-to-end is optimized • Unused circuits are torn down and reallocated • Supports variable data transfer rates • In event of failure, packets can reroute • Designed to support “bursty” data transfer

23. Disadvantages of Packet switching • Variable delay • Packets can be lost or corrupt • Sophisticated protocols are needed to provide some level of reliability during transfer • Not good for certain types of applications that require constant bit rates, limited variation in delay, or no data loss (e.g. real-time video)

24. Distance limitation and LANs • Distance is a fundamental consideration in the design of any network – particularly LAN technologies • Why?

25. LAN extension • Why can’t the length of an Ethernet network continually be increased adding new segments using hubs or repeaters?

26. Ethernet and Delay • CSMA/CD (and CSMA/CA for that matter) designed to function with limited latency • Scheme fails if length is too long or too many segments are connected • Serious performance problems if you string a bunch of Ethernet segments together with repeaters

27. Summary • Three “base” wired topologies • Bus, star, ring • Two wireless topologies • Ad-hoc, infrastructure • Three basic access techniques • CSMA/CD, CSMA/CA, and token passing • Packet switching versus circuit switching