1. Computer Networks Group Universität Paderborn Computer Networks Chapter 1: A brief KMS recap Holger Karl

2. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap2 Goals of this chapter  Briefly repeat the central aspects of communication networks that have been already treated in KMS

3. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap3 Overview  Communication basics  Duplexing, multiplexing, medium access  Routing, transport  Service vs. protocol  Reference models

4. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap4 Communication basics  Information: Human interpretation  Data: Formalized representation  Signal: Representation of data by characteristic changes of a physical variable  These changes can travel over distance  Example: current, voltage, tension, …  Can be generated by sender, interpreted by receiver  Immaterial signals in physical media enable data communication between remote senders and receivers  Signals represent bits Conventions for representation Information Data Conventions for representation Abstract world Signals Physical world

5. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap5 Signals propagate in medium, store data  Signals traveling in a medium take time to reach destination – delay d  Depends on distance and propagation speed in transmission medium  To represent one or several bits, a signal extending in time is needed – duration of transmission  Determined by rate r and data size  During time d, r*d bits are generated  Stored in the medium Message Sequence Charts (MSC) Start of transmission End of transmission Delay d Time Distance

6. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap6 Basic organization of communication  Duplexing: Given a single pair of communicating peers, duplexing describes rules when each peer is allowed to send to the other one  Using which resource  Mutiplexing: Given several pairs, multiplexing describes when which pair, using which resources, is allowed to communicate  Main resources: Time, frequency (+ some others)  Example combinations?

7. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap7 Multiplexing & shared resources  Multiplexing can be viewed as a means to regulate the access to a resource that is shared by multiple users  The switching element/its outgoing line  With the switching element as the controller  Are there other examples of “shared resources”?  Classroom, with “air” as physical medium  A shared copper wire, as opposed to direct connection  Characteristic: a broadcast medium! Virtually shared, but exclusively controlled! Shared!

8. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap8 Handling many devices: Introducing multiple hops

9. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap9 How to realize multiple hops: Switching  In absence of direct connection between communicating peers, some sort of switching becomes necessary  Option 1: Circuit switching  Request a (physical) connection  Turn knobs, switches, etc.  Use this connection as before – peers are now directly connected http://www.wdrcobg.com/switchboard.html

10. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap10 How to realize multiple hops: Switching  Option 2: Packet switching  Instead of building and releasing an end-to-end connection for each communication’s entire length, only  Use connections from one hop to another hop  Communicate well identified parts of a communication – packets – between these hop neighbors  Packets need to have addresses to find destination

11. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap11 Forwarding and next hop selection  Recall: A switching element/a router forwards a packet onto the next hop towards its destination  How does a router know which of its neighbors is the best possible one towards a given destination?  What is a “good” neighbor, anyway? A Z ? ? ? ? U V W X Y P M

12. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap12 Provide data for next hop selection  Construct routing tables  For each switching element separately  Separate entry for each destination  Contains information about the (conjectured) shortest distance to a given destination via each neighbor MPZ U234 V323 X432 Y443 Destination Neighbor Routing table of W

13. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap13 Handling errors  Transmission errors  Signals are mutilated, not correctly converted to (intended) bits  Local issue  Packets are missing  Local or end-to-end issue  Overload problems  Flow control: Fast sender overruns slow receiver  Congestion control: Receiver would be fast enough, but sender injects more packets into network than network is able to handle  Where and how to handle these errors?

14. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap14 Structuring communication systems  To handle complexity, partition into well defined subsystems  Called layers  Subsystem/layer offers a well defined functionality, a service  A promise on what will happen after certain interactions with the service access point take place  Services need distributed implementations  Parts at sender, parts at receiver, and possibly parts in the network  These various implementation parts interact with each other via protocols  Rules on interaction, how to achieve the service’s promise

15. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap15 Typical examples of services  Datagram service  Unit of data are messages  Correct, but not necessarily complete or in order  Connection-less  Usually insecure/not dependable, not confirmed  Reliable byte stream  Byte stream  Correct, complete, in order, confirmed  Sometimes, but not always secure/dependable  Connection-oriented  Almost all possible combinations are conceivable!

16. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap16 Analogy: Nested layers as nested translations  Layers rely on services of lower layers  Intermediate representation of data changes  Vertical vs. horizontal communication  Vertical: always real  Horizontal: may be real or virtual Horizontal communication Horizontal (real!) communication Vertical comm.

17. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap17 Protocols and FSMs  Finite state machines describe & implement actual behavioral rules of a protocol  Have to communicate with their remote peer  Cannot do so directly, have to use service of the underlying communication layer  Via service primitives, which can also provide arriving data to the protocol  E.g., indications from lower layer are events to higher layer protocol engine Layer n Use service primitives Layer n+1

18. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap18 Protocols and messages  When using lower-layer services to communicate with the remote peer, administrative data is usually included in those messages  Typical example  Protocol receivers data from higher layer  Adds own administrative data  Passes the extended message down to the lower layer  Receiver will receive original message plus administrative data  Encapsulating  Header or trailer Layer n Layer n+1 Packet arriving at Layer n+1’s SAP Extended packet passed to layer n Delivered by layer n

19. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap19 ISO/OSI 7-layer reference model (complete network) End-to-end Chained, local layers

20. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap20 TCP/IP protocol stack Nothing stated by TCP/IP model

21. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap21 TCP/IP – Suite of protocols  Over time, a suite of protocols has evolved around the core TCP/IP protocols So-called “hourglass model”: Thin waist of the protocol stack at IP, above the technological layers

22. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap22 ISO/OSI versus TCP/IP  ISO/OSI: Very useful model, non-existing protocols  TCP/IP: Non-existing model, very useful protocols  Hence: Use a simplified ISO/OSI model, but treat the TCP/IP protocol stack in the context of this model  With suitable add-ons especially for the lower layers

23. WS 05/06, v 1.0Communication Networks - Ch. 1 - A brief KMS recap23 Conclusion  These are the core functionalities a communication network has to provide  We shall investigate the main protocols of the abbreviated reference model in the remainder of this class