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Communication Systems

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2008

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Communication SystemsCourse Information – General

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:

Tuesday, 2:00 – 4:00pm, Friday, 9:00 – 11:00am in lecture room 01-009 in building 101 at the former airfield

Time & Place for practical exercises: first date: 25th of April (this Friday) basement of computing

department (H.-Herder-Str. 10, institute quarter – see any map)

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 SystemsCourse Information – Course Layout

Lecture earns 6 credit points (ECTS) Area of specialization: “communication and data bases (6)” Typical 3 + 1 course

three quarters are lectures (here in this room)

one quarter is a practical course, variable dates – see lecture plan (and explanation on Friday)!

theoretical exercises sheets are handed out every Friday (if public holiday, the lecture, practical before or after)

Bachelor course: lecture in the area of specialization

Master/Diploma: entry/mid level course

Different exams for bachelor and master/diploma students most probably oral exam (directly after the end of term)

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Communication SystemsCourse Information – Job Offer

Easy money: looking for a student managing the lecture recording Requirements

being here for every lecture in time

preparing the device

after the lecture – recoding of the data stream and putting it to the e-lectures server, annotation

Get paid 20h/month for May, June, July For application – check the homepage!

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Communication SystemsSyllabus and Scope of course

Communication systems lecture is on the broad topic of communication

data and voice communication in circuit switched and packet orientated networks

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)

broad overview on different kind of networks

partly in depth discussion of some concepts

Different kind of networks for different purposes

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Communication SystemsSyllabus and Scope of course

Communication models among networked devices Focuses on network layer and application layer

for underlying hardware, LAN, W-LAN, WAN technologies and

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 :-)) not all topics are handled in the lecture!

practical exercises will introduce new topics too and deepen the insight into topics presented in lecture

the theoretical exercises could not be answered completely out of the lecture, you should consult other sources too

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Communication SystemsSyllabus and Scope of course

This lecture extends the Systems II lecture of the Bachelor program, will held in Winter in the near future

focus on traditional IP communication with a little bit security background

central topic will be voice communication and its challenges in different network structures

Sources of information lecture slides and exercise sheets of past lectures

literature hints

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 SystemsCourse Outline first part

Might be modified a little throughout the course, number of lectures in brackets for orientation (~20 lectures + 6 practical courses)

Introduction, broad-brush picture of computer networking [2]

Internet Protocol; current implementation and restrictions [1]

the new IP standard IPv6 [1]

network layer security: IPsec (v2)

Routing protocols, like RIP, OSPF, BGP, ... [2]

Helper protocols and applications (ARP, ICMP, DHCP) for address assigning and packet routing [1]

TCP as a connection orientated transport layer protocol with congestion and flow control [1]

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Communication SystemsCourse Outline second part

Digital Telephony Networks and Voice over IP [6+] GSM, UMTS

data connections GPRS, EDGE, HSDPA, ...

voice digitization and quality of service

SIP and H.323 and its challenges in complexer setups

More wireless / mobile communication technologies [2] WLAN

Security in communication networks [1+]

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Communication SystemsLiterature on IP part

Books (certain number of copies available at the faculty library) Kurose & Ross, Computer Networking (best at the moment)

Douglas E. Comer, Computer Networks and Internets

Andrew S. Tanenbaum, Computer Networks

Patterson & Davie, Computer Networks, A Systems Approach

R. Stevens, TCP/IP Illustrated Vol. 1

Other useful texts ... are given during the lectures or on the web page

RFC – request for comment documents on Internet standards

ACM and other journals articles on selected topics ...

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IntroductionWhat are communication networks?

Telephony (mostly voice) networks exists for more than 100 years 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

digital mobile telephony networks of the second generation (2G) extremely changed the style of human communication in the last decade

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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IntroductionWhat are communication networks?

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

IP connections use traditional telephone lines (modem connections over POTS, ISDN data connections, ...)

Mobile Telephone systems of third generation (3G, UMTS) provide broader bandwidth for data centric applications

Voice over IP replaces parts of traditional telephony networks (beginning at the core system and from the end user devices)

UMTS moves towards IP in network subsystem (NSS)

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IntroductionWhat is the Internet?

Start with introduction to TCP/IP and Internet Distinction: TCP/IP is merely the set of protocols usable for

network communication Internet is

using TCP/IP

mostly public network interconnecting millions of computing devices spread over the whole world

Most of them traditional desktop PCs and workstations of any kind, servers for web pages and mail, ...

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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IntroductionWhat is the Internet?

End systems connected together by communication links Many types of links

copper wire of different type, like twisted pair, telephone line, coaxial cable (e.g. broadband TV via Hybrid Fiber Coax (HFC) networks)

Fiber optics (most of wide area connections up to connections of continents)

Radio spectrum for air transmission

Link transmission rate is bandwidth

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IntroductionWhat is the Internet?

Hosts seldom connected by direct links but over intermediate switching devices called routers

Routes get chunks of information and forward it to one of its other links The term for chunks of information is packet Way of packet through the net – path or route Seldom dedicated paths, so we speak of packet switching networks

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IntroductionWhat is the Internet?

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

Network connecting the universities and colleges in south-west 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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IntroductionService Description

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 Connection orientated, reliable service, guaranteeing the user

delivering of data in order and entirety (hopefully)

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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IntroductionWhat is a protocol?

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 ISDN D channel or Q.931 call setup and info signaling

Signaling System 7 (SS7, core network (CN) signaling)

DTAP for signaling between mobile stations (MS – end user device in GSM) and base stations (BS)

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IntroductionWhat is a protocol?

Telephony networks aim at interoperability through definition of interfaces

centralized standardization bodies, nationally and internationally, like ITU (International Telecommunications Union)

interoperability is a much stronger issue in the Internet community

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

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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IntroductionWhat is a protocol?

Human analogy: Ask for the time If polite: You do not ask directly someone you do not know

So “protocol” of exchanging information between A and B on time of the day may be as follow:

A: Hi

B: Hi

A: May I ask the time?

B: Yes, it is 5 p.m.

B could answer “I do not know”, “I dont understand you”, ..., so protocol should have ability to handle unsuccessful cases

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IntroductionWhat is a protocol?

In this analogy 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:

H: TCP connection request to W

W: TCP connection reply

H: GET http://www.ks.uni-freiburg.de

W: deliver <file>

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IntroductionNetwork protocols

Occur when two or more Internet entities communicate Often more than one protocol must be run, e.g.

Protocols running on hardware to encode data to physical states on wire

Controlling the flow of bits between two network adaptors

Routing protocols to determine path of packets from source to destination

Congestion control protocols

Protocol for retrieving webpages from a webserver

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IntroductionProtocol - Definition

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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IntroductionEdges of networks

After rough overview more detailed description of parts defining internets

Hosts called end system, because sitting at the edge of internet End users directly interface to them 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

A client program requests a service from a server program client/server model is most prevalent structure of internet

applications Most applications, like mail, ssh, web, ... work that way Intermediate infrastructure serves as black box Of course not all applications and servers work that way

Peer-to-peer networks, like bittorrent, edonkey, gnutella, ...

Servers may receive services from other servers as clients, e.g. DNS

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CommunicationDifferent services ...

May be used by networking applications When applications use connection-orientated-service they exchange

control packets before sending data Procedure is called handshaking This service provides reliable data transfer, flow and congestion control Reliability ensures proper order of packets and no errors (achieved

through acknowledgment and retransmission of packets)

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CommunicationDifferent services ...

Flow control avoids overwhelming of each side with data packets Congestion control helps preventing of grid-locking the internet 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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CommunicationNetwork Core (taxonomy of networks)

Network core inside of the network not visible to the end user (application)

Sample pictures of (IP based) network cores given some slides before Main distinction of network types Important concepts of network taxonomy Two fundamental approaches in network cores:

Circuit switching

Packet switching

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Network CoreCircuit Switching (CS)

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 CoreCircuit Switching (CS)

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

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-over-IP services (in contrast to traditional Telco services)

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Network CorePacket Switching (PS)

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

Packet must be received completely before it could sent out an outgoing line

It is queued into outbound packet queue to handle busy links

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Network CorePacket Switching

Packets therefore suffer from Transmission delays – if packet consists of L bits and the outgoing

link handles R bps delay is L/R seconds

Switching delays (routing decisions are to be made)

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 CorePacket Switching

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 Cheaper, easier to implement (less complex)

More efficient, no waste of bandwidth

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CommunicationComparison

Efficiency of the use of a 10 Mbps link shared by some users Suppose users generate data at 1 Mbps in 10 percent of there

online time (idle reading webpages, analyzing data, ...)

Circuit switching would reserve 1 Mbit per user, so at max 10 Users may share the link

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

Thus probability that less than 10 users share the link is 0.9996 (no delay or packet discarding occurs)

Packet switching allows much more users sharing one link!

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Communication SystemsPacket switching networks

How to determine the size of packets? What about message switching?

Remember: “Proposal” of protocol to obtain a webpage: H: TCP connection request to W

W: TCP connection reply

H: GET http://www.ks.uni-freiburg.de/index.php

W: deliver <file>

Every step could be one message (= one single packet) sent over the network

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Communication SystemsPacket switching networks

Or: larger messages could be segmented (split into packets of a defined maximum size)

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Communication SystemsMessage switching

Depicts network of four links and two end systems (sender and receiver)

Store and forward sending In upper part of picture message is kept intact

Complete message has to be received before sent out again

Four subsequent steps (sender to switch S1, S1 to S2, ...)

1.25 Mbyte of message in 10 Mbits network – message needs 1 second to travel over one link (1.25 Mbyte = 10 Mbit)

Result: 4 seconds from sender to receiver

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Communication SystemsSegmented message switching

In lower part of picture message is segmented Segmented message could pipelined – packets could travel in

parallel

If first packet is sent out on second link, second packet can use the first link same time etc.

Same four subsequent steps

0.25 Mbyte of message in 10 Mbits network – segmented message needs 0.2 seconds to travel over one link

9 steps needed – see next picture

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Communication SystemsSegmented message

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Communication SystemsSegmented message switching

Message has to be disassembled and reassembled after all packets received

We get: 9 * 0.2 seconds = 1.8 seconds Better than half the time of unsegmented message

Time for

Disassembling and reassembling Header overhead (five headers instead of one) 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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CommunicationEnd / literature

Next lecture: Tuesday the 29th of April, here in this seminar room Next exercise: Friday 25th at computing department (seminar room -114) Literature

any of the given textbooks – introductory chapters

network introduction from the view of telephony people: Kaaranen and others, “UMTS Networks”, Wiley 2005, first chapters

homepages of belwue, DFN, GEANT(2)

on packet and circuit switching: Kurose ...