Transcript 01_1_Introduction

Communication
Systems
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2008
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Communication Systems
Course Information – General
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Summer course: 04/22/2008 - 07/25/2008
Instructors: Prof. Dr. G. Schneider, Dirk von Suchodoletz, Rui
Zhou and Klaus Rechert
Time & Place of lecture:
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Tuesday, 2:00 – 4:00pm, Friday, 9:00 – 11:00am in lecture room 01009 in building 101 at the former airfield
Time & Place for practical exercises:
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first date: 25th of April (this Friday) basement of computing
department (H.-Herder-Str. 10, institute quarter – see any map)
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infrequent dates: combined exercise 10th and 13th of June and 8th
and 11th of July
Any further information / organizational stuff on Friday in the
exercises
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Communication Systems
Course Information – Course Layout
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Lecture earns 6 credit points (ECTS)
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Area of specialization: “communication and data bases (6)”
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Typical 3 + 1 course
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three quarters are lectures (here in this room)
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one quarter is a practical course, variable dates – see lecture plan
(and explanation on Friday)!
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theoretical exercises sheets are handed out every Friday (if
public holiday, the lecture, practical before or after)
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Bachelor course: lecture in the area of specialization
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Master/Diploma: entry/mid level course
Different exams for bachelor and master/diploma students
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most probably oral exam (directly after the end of term)
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Communication Systems
Course Information – Job Offer
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Easy money: looking for a student managing the lecture recording
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Requirements
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being here for every lecture in time
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preparing the device
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after the lecture – recoding of the data stream and putting it to the electures server, annotation
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Get paid 20h/month for May, June, July
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For application – check the homepage!
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Communication Systems
Syllabus and Scope of course
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Communication systems lecture is on the broad topic of
communication
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data and voice communication in circuit switched and packet
orientated networks
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that means telephony networks and the Internet
Introduction to terminology, concepts and approaches of different
communication systems
Presentation of a wide variety of protocols and concepts (with
detailed introduction to some of them)
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broad overview on different kind of networks
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partly in depth discussion of some concepts
Different kind of networks for different purposes
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Communication Systems
Syllabus and Scope of course
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Communication models among networked devices
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Focuses on network layer and application layer
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for underlying hardware, LAN, W-LAN, WAN technologies and
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Internet programming (sockets, services, ...) you will find other
lectures and seminars
Detailed information at the homepage (lecture plan)!!
Please note (for your personal advance in this topic or for the
written exam – put it as you like :-))
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not all topics are handled in the lecture!
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practical exercises will introduce new topics too and deepen the
insight into topics presented in lecture
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the theoretical exercises could not be answered completely out of
the lecture, you should consult other sources too
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Communication Systems
Syllabus and Scope of course
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This lecture extends the Systems II lecture of the Bachelor
program, will held in Winter in the near future
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focus on traditional IP communication with a little bit security
background
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central topic will be voice communication and its challenges in
different network structures
Sources of information
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lecture slides and exercise sheets of past lectures
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literature hints
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sources of similar lectures found at other universities
Lecture will recorded an available then from the computer science
departments e-lectures server
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Communication Systems
Course Outline first part
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Might be modified a little throughout the course, number of lectures
in brackets for orientation (~20 lectures + 6 practical courses)
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Introduction, broad-brush picture of computer networking [2]
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Internet Protocol; current implementation and restrictions [1]
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the new IP standard IPv6 [1]
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network layer security: IPsec (v2)
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Routing protocols, like RIP, OSPF, BGP, ... [2]
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Helper protocols and applications (ARP, ICMP, DHCP) for address
assigning and packet routing [1]
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TCP as a connection orientated transport layer protocol with
congestion and flow control [1]
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Communication Systems
Course Outline second part
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Digital Telephony Networks and Voice over IP [6+]
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GSM, UMTS
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data connections GPRS, EDGE, HSDPA, ...
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voice digitization and quality of service
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SIP and H.323 and its challenges in complexer setups
More wireless / mobile communication technologies [2]
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WLAN
Security in communication networks [1+]
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Communication Systems
Literature on IP part
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Books (certain number of copies available at the faculty library)
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Kurose & Ross, Computer Networking (best at the moment)
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Douglas E. Comer, Computer Networks and Internets
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Andrew S. Tanenbaum, Computer Networks
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Patterson & Davie, Computer Networks, A Systems Approach
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R. Stevens, TCP/IP Illustrated Vol. 1
Other useful texts
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... are given during the lectures or on the web page
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RFC – request for comment documents on Internet standards
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ACM and other journals articles on selected topics ...
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Introduction
What are communication networks?
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Telephony (mostly voice) networks exists for more than 100 years
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the technology of end users systems (TE for terminal equipment in
Telco lingo) has not changed much – try your grandmothers old dial
phone to your “analogous” telephone line or ISDN a/b connector
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digital mobile telephony networks of the second generation (2G)
extremely changed the style of human communication in the last
decade
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but rapid changes are under the way – reason for the outline of this
lecture
Different kinds of mobile networks allow “ubiquious
communication”
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Introduction
What are communication networks?
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Internet is complex, both in terms of hardware and software
components, so difficult to provide “one-sentence-description”
The public computer network most of you using throughout the day
Other way – networking infrastructure that provides services to
distributed applications
We experience a merging of networks
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IP connections use traditional telephone lines (modem connections
over POTS, ISDN data connections, ...)
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Mobile Telephone systems of third generation (3G, UMTS) provide
broader bandwidth for data centric applications
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Voice over IP replaces parts of traditional telephony networks
(beginning at the core system and from the end user devices)
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UMTS moves towards IP in network subsystem (NSS)
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Introduction
What is the Internet?
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Start with introduction to TCP/IP and Internet
Distinction: TCP/IP is merely the set of protocols usable for
network communication
Internet is
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using TCP/IP
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mostly public network interconnecting millions of computing devices
spread over the whole world
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Most of them traditional desktop PCs and workstations of any kind,
servers for web pages and mail, ...
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Nontraditional Internet end systems: PDAs, mobile computers, TV
set top boxes, cell phones, fridges, ...
In network terminology they are end systems or hosts
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Introduction
What is the Internet?
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End systems connected together by communication links
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Many types of links
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copper wire of different type, like twisted pair, telephone line, coaxial
cable (e.g. broadband TV via Hybrid Fiber Coax (HFC) networks)
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Fiber optics (most of wide area connections up to connections of
continents)
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Radio spectrum for air transmission
Link transmission rate is bandwidth
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Introduction
What is the Internet?
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Hosts seldom connected by direct links but over intermediate switching
devices called routers
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Routes get chunks of information and forward it to one of its other links
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The term for chunks of information is packet
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Way of packet through the net – path or route
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Seldom dedicated paths, so we speak of packet switching networks
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Introduction
What is the Internet?
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End users, organizations or enterprises connected to the Internet
through Internet Service Providers (ISPs) of different levels
End user providers mostly telecommunication firms like German
Telekom, ARCOR, ...
Companies often use regional ISPs
Universities have their own ISPs, like BelWue in south-west of germany,
the DFN (broadband and Gigabit infrastructure) and GEANT(2) on the
European level (next slides)
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BelWue
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Network connecting
the universities and
colleges in southwest of Germany
Blue is 10GBit/s,
black 1Gbit/s
Ethernet
Green leased line
2.4Gbit/s, red
622Mbit/s
Purple 622Mbit/s
backup links
See Freiburg in the
left down corner
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DFN (B-Win)
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DFN (G-Win,
successor)
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GEANT2
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Introduction
Service Description
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Internet allows distributed applications running on different end systems
to exchange data
Services include: remote login, mail, web services, databases, instant
messaging, audio and video streams, ...
Internet provides two types of services
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Connection orientated, reliable service, guaranteeing the user
delivering of data in order and entirety (hopefully)
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Connectionless, unreliable service, which does not make any
guarantees about eventual delivery
But no services which makes promises on how long delivery takes
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Introduction
What is a protocol?
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Hosts, routers and other networking pieces run protocols
controlling the sending and receiving of packets
IP (Internet Protocol) and TCP (Transmission Control Protocol)
most important protocols used in Internet communication
Protocols and Internet standards are often discussed in RFCs
(Request for Comment)
In telephony networks protocols defined to, like
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ISDN D channel or Q.931 call setup and info signaling
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Signaling System 7 (SS7, core network (CN) signaling)
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DTAP for signaling between mobile stations (MS – end user device
in GSM) and base stations (BS)
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Introduction
What is a protocol?
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Telephony networks aim at interoperability through definition of
interfaces
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centralized standardization bodies, nationally and internationally, like
ITU (International Telecommunications Union)
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interoperability is a much stronger issue in the Internet community
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Telcos hope to cut off competitors using incompatible protocols –
e.g. the ISDN used for telecommunication within the university is not
compatible to the ISDN used in public networks
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One reason, that pace of technology is much faster in the Internet
domain than in telephony networks, example is the data rate in G2
mobile telephony networks (much to slow for most modern
networked applications)
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Introduction
What is a protocol?
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Human analogy: Ask for the time
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If polite: You do not ask directly someone you do not know
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So “protocol” of exchanging information between A and B on time of
the day may be as follow:
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A: Hi
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B: Hi
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A: May I ask the time?
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B: Yes, it is 5 p.m.
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B could answer “I do not know”, “I dont understand you”, ..., so
protocol should have ability to handle unsuccessful cases
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Introduction
What is a protocol?
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In this analogy
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If not using a know protocol, you do not get an answer
Protocols in networking operate the same way: Host H asks the
webserver W for a specific page:
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H: TCP connection request to W
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W: TCP connection reply
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H: GET http://www.ks.uni-freiburg.de
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W: deliver <file>
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Introduction
Network protocols
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Occur when two or more Internet entities communicate
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Often more than one protocol must be run, e.g.
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Protocols running on hardware to encode data to physical states on
wire
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Controlling the flow of bits between two network adaptors
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Routing protocols to determine path of packets from source to
destination
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Congestion control protocols
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Protocol for retrieving webpages from a webserver
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Introduction
Protocol - Definition
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A protocol defines the format and the order of messages
exchanged between two or more communicating entities, als well
as the actions taken on the transmission and/or receipt of a
message or other event. (Kurose&Ross)
The layering of different protocols is one of the most important
parts in understanding the Internet
This layering will be defined later in so called protocol stack
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Introduction
Edges of networks
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After rough overview more detailed description of parts defining
internets
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Hosts called end system, because sitting at the edge of internet
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End users directly interface to them
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Every end user device able to run TCP/IP could be connected to
the net and is an end system in internet terminology
Hosts run end user applications, often divided into two categories:
clients and servers
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Introduction
Client-Server-Model
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A client program requests a service from a server program
client/server model is most prevalent structure of internet
applications
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Most applications, like mail, ssh, web, ... work that way
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Intermediate infrastructure serves as black box
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Of course not all applications and servers work that way
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Peer-to-peer networks, like bittorrent, edonkey, gnutella, ...
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Servers may receive services from other servers as clients, e.g. DNS
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Communication
Different services ...
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May be used by networking applications
When applications use connection-orientated-service they exchange
control packets before sending data
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Procedure is called handshaking
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This service provides reliable data transfer, flow and congestion control
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Reliability ensures proper order of packets and no errors (achieved
through acknowledgment and retransmission of packets)
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Communication
Different services ...
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Flow control avoids overwhelming of each side with data packets
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Congestion control helps preventing of grid-locking the internet
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Congested routers discard packets which require informing the sender
and requires retransmission
TCP will be inherently used and introduced during this course
UDP is very simple, connectionless, with none of the services named
above
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Communication
Network Core (taxonomy of networks)
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Network core inside of the network not visible to the end user
(application)
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Sample pictures of (IP based) network cores given some slides before
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Main distinction of network types
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Important concepts of network taxonomy
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Two fundamental approaches in network cores:
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Circuit switching
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Packet switching
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Network Core
Circuit Switching (CS)
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Resources needed along a path, like bandwidth, buffers reserved for the
duration of communication
Telephone systems operate that way – a connection is called a circuit
Reservation procedure may require a lot of complexity (and therefore
delays) and may produce costs
Connection quality in terms of bandwidth, delay, error rate, ... will remain
the same during communication
Quality of Service (QoS) is a big issue in telephony networks: Voice
connections are heavily influenced through delays, packet loss and
changing bandwidth)
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Network Core
Circuit Switching (CS)
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Concept known from the traditional world of analogous telephony
systems
Guaranteed reserved constant bandwidth may use a given connection
much below real capacity
Hardware protocols designed mostly for telephony, like ISDN and ATM
use circuit switching
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ATM still forms the backbone (core network) of UMTS mobile phone
network
Costs usually calculated in terms of time usage and possible maximum
bandwidth of a link not in term of transferred volumes
Problems can be seen with designing and establishment of Voice-overIP services (in contrast to traditional Telco services)
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Network Core
Packet Switching (PS)
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Completely different concept
Source breaks long messages (e.g. FTP file) into smaller data
chunks called packets
Each packet travels through communication links and most
inevitably crosses packet switches called routers
Packet switches use store-and-forward mechanism
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Packet must be received completely before it could sent out an
outgoing line
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It is queued into outbound packet queue to handle busy links
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Network Core
Packet Switching
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Packets therefore suffer from
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Transmission delays – if packet consists of L bits and the outgoing
link handles R bps delay is L/R seconds
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Switching delays (routing decisions are to be made)
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Queuing delays (wait in outgoing buffer)
If queue is full – packets are discarded and packet loss occurs
Share of bandwidth in packet switching networks via statistical
multiplexing
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Network Core
Packet Switching
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Circuit switching uses frequency division multiplexing (FDM) or
time division m. (TDM) instead
Statistical multiplexing is much more flexible than FDM or TDM
(with fixed frequencies and time slots) and can utilize a given
bandwidth much better
Packet switching networks
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Cheaper, easier to implement (less complex)
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More efficient, no waste of bandwidth
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Communication
Comparison
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Efficiency of the use of a 10 Mbps link shared by some users
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Suppose users generate data at 1 Mbps in 10 percent of there online
time (idle reading webpages, analyzing data, ...)
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Circuit switching would reserve 1 Mbit per user, so at max 10 Users
may share the link
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For packet switching the probability of user activity is 10%, if there
are 35 users probability of 11 active users (less bandwidth for every
user than required) is 0.0004
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Thus probability that less than 10 users share the link is 0.9996 (no
delay or packet discarding occurs)
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Packet switching allows much more users sharing one link!
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Communication Systems
Packet switching networks
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How to determine the size of packets? What about message
switching?
Remember: “Proposal” of protocol to obtain a webpage:
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H: TCP connection request to W
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W: TCP connection reply
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H: GET http://www.ks.uni-freiburg.de/index.php
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W: deliver <file>
Every step could be one message (= one single packet) sent
over the network
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Communication Systems
Packet switching networks
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Or: larger messages could be segmented (split into packets of a
defined maximum size)
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Communication Systems
Message switching
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Depicts network of four links and two end systems (sender and
receiver)
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Store and forward sending
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In upper part of picture message is kept intact
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Complete message has to be received before sent out again
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Four subsequent steps (sender to switch S1, S1 to S2, ...)
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1.25 Mbyte of message in 10 Mbits network – message needs 1
second to travel over one link (1.25 Mbyte = 10 Mbit)
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Result: 4 seconds from sender to receiver
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Communication Systems
Segmented message switching
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In lower part of picture message is segmented
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Segmented message could pipelined – packets could travel in
parallel
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If first packet is sent out on second link, second packet can use
the first link same time etc.
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Same four subsequent steps
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0.25 Mbyte of message in 10 Mbits network – segmented message
needs 0.2 seconds to travel over one link
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9 steps needed – see next picture
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Communication Systems
Segmented message
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Communication Systems
Segmented message switching
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Message has to be disassembled and reassembled after all
packets received
We get: 9 * 0.2 seconds = 1.8 seconds
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Better than half the time of unsegmented message
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Time for
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Disassembling and reassembling
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Header overhead (five headers instead of one)
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Switching delay
... assumed zero in this example (higher in reality but mostly much
smaller then transfer delays!!)
End: Think of message vs. packet switching for interactive, real
time communication like Voice over IP
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Communication
End / literature
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Next lecture: Tuesday the 29th of April, here in this seminar room
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Next exercise: Friday 25th at computing department (seminar room -114)
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Literature
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any of the given textbooks – introductory chapters
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network introduction from the view of telephony people: Kaaranen
and others, “UMTS Networks”, Wiley 2005, first chapters
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homepages of belwue, DFN, GEANT(2)
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on packet and circuit switching: Kurose ...
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