COMPUTER NETWOKRS Network fundamentals -...

Pag. 1

COMPUTER NETWOKRS – Network fundamentals

COMPUTER NETWORKS – Network fundamentals - 1 Copyright Gruppo Reti – Politecnico di Torino

Network fundamentals

Gruppo Reti TLC

[email protected]

http://www.telematica.polito.it/

COMPUTER NETWORKS – Network fundamentals - 2

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Copyright Gruppo Reti – Politecnico di Torino


Telecommunication chronology?

• “Tele” means distant

– Smoke signals?

– Drums in the jungle?

– Mirrors?

– Flags?

• Focus on moder telecommunication

networks

– Telephone network

– Internet


Pag. 2



Telephone network: chronology

• 1837: Morse code

• 1876: Bell phone patent

• 1891: Strowger selector patent

• 1894: First electromechanical telephone

exchange

• 1895: Marconi radio experiments

• 1923: first automatic (electromechanical)

long-distance service (Baviera - Siemens)



Telephone network: chronology

• 1938: relè

• 1960: telephone exchange electronically

controlled

• 1964: first exchange fully electronic controlled

(Succasunna - USA)

• 1975: first exchange fully electronic (Chicago)

• 1980: common channel signalling, ISDN,

analog mobile networks

• 1990: Intelligent network (signalling), digital

mobile networks Copyright Gruppo Reti – Politecnico di Torino


Data networks: chronology

• 1969: ARPANET (primo RFC: Host software)

• 1971: ARPANET: 15 nodes e 23 host. E-mail

• 1973: First FTP version

• 1973: First TCP version

• 1976: Ethernet and X.25

• 1980: OSI

• 1982: Token ring (IBM)

• 1982: TCP and IP are defined as the

reference protocols (since 1/1/83) Copyright Gruppo Reti – Politecnico di Torino

Pag. 3




• 1984: DNS

• 1985: FDDI

• 1990: Tim Berners-Lee (CERN) presents a

document (nobody cares) on Hypertext

Information Management. The project name

must be chosen: “Information Mine”,

“Information Mesh” ,“World Wide Web”

• 1993: Web traffic is1% of total Internet traffic




• 1997: Internet2, new network for

experimental research

• 1999: peer-to-peer applications to exchange

file file (MP3, video) directly among users (no

server intereaction): Napster, Gnutella,

Kazaa

• Since then most of the novelties in the

application side (Social networks, Streaming,

On-line gaming, P2P TV)

• Today: you know better than us Copyright Gruppo Reti – Politecnico di Torino


TLC networks:

services and functions

Gruppo Reti TLC

[email protected]


Pag. 4



Some definitions

• Taken from the CCITT “Blue Book”

(Reccomendation I.112)

– CCITT: The International Telegraph and Telephone

Consultative Committee of Telecommunication Union

(ITU).

– Since 1994 CCITT become ITU-T


ITU-T

• Communication: information transfer according to

pre-established rules.

– Agreement

• Telecommunication: trasmission and reception of

signale representing signs, text, images, sounds,

info of any nature, via cables, radio, optical or

electromagnetics media


Example

• Telephones are user terminals connected to a

network providing telecommunication services

TLC

NETWORK

Pag. 5



ITU-T: definitions

• Telecommunication service: offered from a public

or private provider to his customers to satisfy a

specific telecommunication need

• Functions in a telecommunication network:

operations internally executed in a network to

provide services to users


Functions: an example

• By raising the handset the user signals to the

network that he is willing to start the call

procedure




• The user waits for the free tone from the

central office


TU - TU

Pag. 6




• The user dials the number to signal to the

network the user (more precisely the user

terminal) he/she is willing to be connected to


00390118935649

(not my phone number!)


Functions: user signalling

• Control information transfer between the user

and the network

• SIGNALLING.



ITU-T definition

• Signalling: exchange of control information

associated with the setup and release of a

telephone call on a telecommunications

circuit.

• Example of this control information:

– digits dialed by the caller

– the caller's billing number

– call-related information

• Actions order is important

• Time is important Copyright Gruppo Reti – Politecnico di Torino

Pag. 7



Functions: switching

• The network identifies resources needed to

connect the two users

• In the case of telephone networks, a circuit is

established. Circuit switching.


ITU-T definition

• Switching: the process of interconnecting

functional units, transmission channel or

telecommunication circuits for the time

needed to transfer signals



Functions: network signalling

• To execut the switching function, i.e., to build

a circuit

– Nodes need to exchange control information

(e.g. callee phone number)

– Information exchange within the network is

needed

– Network signalling


Pag. 8




• In the early, roaring years of telephony

– signalling was voice based

– switching was manual






– switching was hand based





– switching was hand based

Pag. 9



Functions: transmission

• The two users can (happily?) exchange

information


Hi, how are

You doing?

I was fine


ITU-T definition

• Transmission: signal transfer from one point to

one or several points



• When the call is ended, the circuit is

released (to free network resources)

Pag. 10



Functions: management

• Connecting new users

• Adding channels or nodes

• Upgrade or substitute devices (to follow technological evolution)

• Reconfiguration after fault

• Performance monitoring

• Device control


Functions in a TLC network

• Signalling (implies routing)

• Switching (implies routing)

• Transmission

• Management

• And many others ….



Network Topologies

Gruppo Reti TLC

[email protected]


Pag. 11



Network

• Definition:

– A set of nodes and channels that offer a

connectionamong two or more points to make

telecommunication possible

• Node is the point where switching occurs

• Channel is any communication media

– The channel may be

• Monodirectional (one way)

• Bidirectional



Type of channels

• Point-to-point channel

• Only two nodes connected to channel end

points

• The channel is used by both nodes in the

same fashion


A B


Type of channels

• Multipoint channel

• Several nodes connectes to a single chanel

• One master and several slaves


Slave

Master

Pag. 12



Type of channels

• Broadcast channel

• Single communication channel shared by all

nodes

– This room!

• The information sent by one node is received

by all other users

• Data should contain the destination address

– YOU pay attention!



Topologies in TLC networks

• The network topology is defined by the

relative position of nodes and channels

• A network topology is a graph G=(V,A)

– V = set of vertices (represented as circles -

nodes)

– A = set of edges (represented as segments -

channels)



Topologies in TLC networks

• Edges may be:

– direct (directed segments (arrow) – unidirectional

channels)

– Undirect (non directed segmetns – bidirectional

channels)

• Define:

– N= |V|

– C= |A|

Pag. 13



Fully meshed topology

• C = N(N-1)/2

• Advantage: highly fault tolerant (many paths available among pair of nodes)

• Disadvantage: too many channels needed

• Many alternative paths, but only one shortest path (one hop)

• There is an obvious, minimunm distance, routing choice

• Used only when N is small (e.g., core nodes in a national telephone network)

A

E B

C D


Tree topology

• C = N -1

• Disadvantage: fault vulnerable (only one path exist for any node pair)

• Advantage: smallest possible number of channels for a connected topololgy

• Used to reduce cost

• No routing alternatives

A

E B

C D


Active star topology

• C = N (star centre is not a node, used only to enable communication)

• Disadvantage: vulnerability to faults on the star centre

• Advantage: low number of channels

• Used to keep cost under control

• Each node has a unique routing

• All routing complexity in the star centre

• Used – in LANs (switch)

– in satellite networks (satellite)

– in cellulare mobile networks (base station)

A

E B

C D

Pag. 14



Passive star topology

• C = 1 (even if N wires)

• The star node passively propagates signals on any wire

• Broadcast channel

• Disadvantage: vulnerable to star center fault

• Advantage: small number of channels

• Low cost

• Each node has a unique routing • Used

– in LANs (hub)

– in distribution networks (reflector)

A

E B

C D


Meshed topology

• N-1 < C < N(N-1)/2

• Not a regular topology

• Advantage: fault tolerance and

number of channels can be traded

off

• Complex routing: many alternative

(good) paths

• Most commonly used (Internet,

telephone)

A

E B

C D


Ring topology

• Can be unidirectional or biderectional

Pag. 15



Ring topology

• C=N/2 for the unidirectional ring

• C=N for te bidirectional ring

• Used in LAN and MAN but also to build meshed topologies (SDH)

• Two alternative paths for each node pair


Ring topology

• In case of faults, the bidirectional ring

ensures network survivability (with reduced

capacity)

• The bidirectional ring is the simplest topology

that provides an alternative path in case of

faults



Bus topology

• C=N-1 for the active bus (peculiar tree)

• C=1 for the passive bus (broadcast channel)

• Only one path for any pair of nodes

• Used in LANs

A

E B

C D

Pag. 16



Physical and logical topology

• It is important to distinguish between the

physical and logical topology

– Logical topology: logical interconnection among

nodes via channels

– Physical topology: takes into account transmission

media constraints


Physical topology

A B

C D


Logical topology

A B

C D

Pag. 17



Topologies and performance

• The amount of traffic that can be succesfully

transferred (throughput) in a network is

– for a given channel available capacity

– inversely proportional to the average distance

among node pairs

– weighted by the amount of traffic exchanged

between the two node

• For uniform traffic and regular topologies the

average distance on the topology establish the

throughput



• Comparison among topologies, with the same

number of nodes (4) and (almost) the same

number of channels

• Uniform traffic

– Every node pair exchange x bit/s. Total generated

traffic is 12x.

• Every unidirectional channel has capacity B bit/s.

• Compute: average distance, network capacity

(maximum throughput), maximum channel

load,maximum node load



• Capacity: 3x2B=6B

• Average distance: 20/12=1.66

• Consider only traffic from left to right (simmetry)

– maximum channel load is 4x. Thus, x <= B/4

• Node 3 (or 2) must handle 7B/4 of traffic unit

• Uniform traffic, non regular topology, unbalanced

channel load, unbalanced node load


1 2 3 4 3x 2x

2x

x

x

x

Pag. 18



Topologies and performance • Capacity: 3x2B=6B

• Average distance: 1.5

• Considering the traffic from node 4.

– Maximum load on (all) channel is 3x. Thus x <=B/3

– The same holds for the other direction

• Node 4 must handle 3B of traffic unit

• Uniform traffic, non regular topology, balanced channel load,

unbalanced node load

x

x

x

x

x

x x

x

x 1 2

4

3


Topologies and performance • Capacity: 4x2B=8B

• Average distance: 1.33

• For clockwise traffic the maximum channel load is 2x. Thus x <=

B/2.

– The same holds for counter clock wise traffic

• Each node must handle 2B unit of traffic

• Uniform traffic, regular topology, balanced channel load, balanced

node load 3x/2

x/2

3x/2

x/2 3x/2

x/2

3x/2

x/2


Topologies: average distance

Manhattan

E[d2] =O(N)

E[d1]> E[d2] >E[d3]

Ring

E[d1]=O(N) Shuffle

E[d3]=O(logN)

Pag. 19



TLC networks: services

Gruppo Reti TLC

[email protected]



ITU-T: definition

• Telecommunication service: offered from a public

or private provider to his customers to satisfy a

specific telecommunication need


Integrated vs dedicated networks

• TLC networks can be:

– Dedicated (to a specific service)

• Telephone

• TV distribution

• Internet (in the early days)

– Integrated (several services)

• Narrowband ISDN o N-ISDN

• Broadband ISDN o B-ISDN

• Internet (today)


Pag. 20



TLC service taxonomy

• Bearer service

– Permits transmission of information between network

interfaces. These services give the subscriber the

capacity required to transmit appropriate signals between

certain access points, i.e. user network interfaces.

– Example: Point to point direct circuit

• Teleservices

– provide the full capacity for communications by means of

terminaland network functions and possibly functions

provided by dedicated centers.

– Examples: phone calls, telefax

– Classified in base services or supplementary services



Teleservices

• Base service

– provides minimal functionalities needed by the

service itself

– Examples:

• POTS (phone calls)

• Television



Teleservices

• Supplementary service

– Provides to the user additional functionalities

with respect to the base service

– Can be offered only together with a base service

– Modifies or extends a base service

• Examples

– Telephone networks: call waiting alert, toll free

number, automatic redial, answsering machine

– Television: Video-on-Demand (VOD)


Pag. 21



Teleservices: information flow

• Information flows can be:

– Symmetric bidirectional

• Phone calls

– Asymmetric bidirectional

• ADSL

– Monodirectional

• TV distribution

• Information flow can be:

– Point to point

• Phone calls

– Point to multipoint

• Multimedia streaming

– Multipoint to multipoint

• Teleconference


Teleservices taxonomy

• Should be taken with some care like any

taxonomy

• Interactive

– conversational

– messaging

– searching/retrieval

• Diffusive

– With or withouth presentation control from the

user



Conversational interactive

teleservices • End to end data transfer in real time

• Examples (video, voice)

– Phone calls

– Videoconference

– Videosurveillance

• Examples (data)

– File trasnfer

– Computer interconnection

– Real time control

– Multiplayer games

Pag. 22



Messaging interactive teleservices

• End to end communication exploiting

intermediate devices for information storage

• Examples (video, voice)

– Answering machine

– Image transfer

• Examples (data)

– e-mail

– SMS


Searching/retrieval

interactive teleservices • Permits user to search/retrieve information

stored in public data base

• Examples:

– Web

– Teleteaching

– Telesoftware

– Teleshopping



Diffusive teleservices without

presentation control

• The information flow is distributed from a source

to a set of receivers (authorization may be

required)

• Users cannot control information flow (starting

time, presentation order, etc)

• Examples

– TV

– pay TV

– radio

Pag. 23



Diffusive teleservices without

presentation control • The information flow is distributed from a

source to a set of receivers (authorization may be required)

• Information is organized in sequences that are cyclically repeated

• Users may select starting time and/or presentation ordering

• Examples

– televideo

– near VoD



How to provide teleservices?

• Two (Internet derived) models

– client-server

– peer-to-peer

• Can be distinguished looking at how user

application software is behaving


Client-server model

• Two clearly different roles

– Client • start interaction with the server (“talks first”), asking for a

service (es. web page, e-mail transfer)

– Server • provdes the requested service to the client( sned the web

page, receives and stores e-mails)

• Idle while waiting for clients

• Most of the time (not always) the information is stored in the server

• Most of Internet applications follow the client server model

Pag. 24



Client-server models

• Servers

– are activated when the device hosting them is

switched

– Wait for client requestsa

– Always available

• Clients

– Activated when the user requires a service

– After the service has been provided they are de-

activated



request

reply

Client-server model


Peer-to-peer model

• Introduced more recently in the Internte

• Phone calls, fax follow a similar model

• Mainly used for interaction among large groups

of users

• All applications are peers

• Information distributed and shared among all

peers

Pag. 25



Transmission techniques

in networks

Gruppo Reti TLC

[email protected]



Types of transmission

• Analog transmission

– Information is transferred via a signal

• continuous

• limited

• assuming infinite possible values

• Digitral transmission

– Information is transferred via a signal

• Not continuous

• limited

• assuming a finite set of possible values



t

Analog transmission

Pag. 26



t

Digital transmission


From analog to digital

t t

Sampling process

t

Quantization process

Analog signal

t

Digital signal 1010

1001

1000

0111

0110

0101

0100

0011

0010

0001

0000

1010

1001

1000

0111

0110

0101

0100

0011

0010

0001

0000

0011 – 0011 – 0100 – 0100 – 0100 – 0100 – 0010 - 0010


From digital to analog

• If the analog signal is bandwidth limited and

the signal bandwidth is B

– The sampling process is lossless (from the

samples the original signal can be exactly

rebuild) if samling at a frequency 2B

• The quantization process is lossy

– Quantization error

• By selecting a proper number of levels, the

quantization error can be controlled a priori


Pag. 27



Type of tranmissions

• Analog

– Information, assuming values in a continuous set, is

represented as a continuos variation of an electrical/optical

parameter

• Digital

– Information is represented as an electrical/optical parameter

assuming a set of finite values (discrete)

– However, signals are physically continuos!

– So, where is the difference?

– In the receiver!

• The receiver makes a decision process and tries to rebuild the discrete

information

• Clock synchronization is critical


Parallel vs serial tranmission

• Parallel

– Information is transferred in parallel over a

communication bus (in a computer 8 bit=1byte in parallel)

or on a serial line or through several parallel wires.

Clock (synchronization) signals are sent together with

data to align receiver clock to transmitter clock

• Serial

– Information is sent bit by bit on the channel.

Synchronization mechanism are adopted between the

transmitter and the receiver

– More common in networks



1

0

0

0

1

1

1

1

0

0

0

1

1

0

1

1

Bit 1

Bit 8

Parallel

Parallel transmission

Pag. 28



1 0 1 0 0 1 1 1 0 0 1 1 1 0 0 1

Serial transmission


Serial transmission

• Asynchronous

– Every information byte (or packet) is transmitted

separately. The reception clock must be

synchronized to the transmitter clock on any data to

compensate for clock skew

• Synnchronous

– Data to be sent are structured in frames. Transmitter

and receiver clock are continuously synchronized to

ensure that bit detection is correct also at high bit

rate


Asynchronous transmission

• S: Start Bit

• P: Parity Bit

• Stop Bits: 1, 1.5, 2

LSB P MSB STOP

BITS S LINE

IDLE

FROM 5 TO 8 BIT

1 CHARACTER

Pag. 29



Synchronous trasmission

• Synchronization overhead is reduced

LSB MSB

CHARATER N

MSB LSB

CHARACTER N-1 CHARACTER N+1

CLOCK


Reception

• Continuous reception on point-to-point

channels for synchronous transmissione

– Easier to keep clock synchronized

• Burst mode reception

– Ove broadcast or multi-point channels

synchronous transmission is not possible (many

transmitters schare che channel)

– Receiver must adapt is clock to different

transmitters for any data send over the channel


Transfer modes in networks

Gruppo Reti TLC

[email protected]


Pag. 30



Sharing channel resources

• Sharing of channel resources among data flows comes in two different flavours – Multiplexing

• All flows access the channel from a single point

• Single transmitter scenario

• Centralized problem

• A radio access from an antenna (base station in a cellular network, access point in a WI-FI network, satellite transmission), an output link in a switch or a router

– Multiple-access • Flows access the channel from different access points

• Many transmitters are active

• Distributed problems

• Local area networks (if not switched), mobile phones in a cellulare network, PC accessing via a Wi-FI hot-spot


Sharing channel resources

• Sharing of channel resources among data blonging to several flows comes in two different flavours – Multiplexing

• All flows access the channel from a single point

• Single transmitter scenario

• Centralized problem

• A radio access from an antenna (base station in a cellular network, access point in a WI-FI network, satellite transmission), an output link in a switch or a router

– Multiple-access • Flows access the channel from different access points

• Many transmitters are active

• Distributed problems

• Local area networks (if not switched), mobile phones in a cellulare network, PC accessing via a Wi-FI hot-spot

– Multiple access is a more complex task


Channel sharing techniques

• Frequency (FDM - FDMA)

• Time (TDM - TDMA)

• Code (CDM - CDMA)

• Space

t

f

channel

Pag. 31


COMPUTER NETWORKS – Network fundamentals - 91 Copyright Gruppo Reti –

Politecnico di Torino

Frequency division

• Each flow is transmitted using different frequency bands

• Need for band guard

• Filters needed to receive the proper channel

f

t


Time division (TDM – TDMA)

• Each flow exploits different time intervals (slots)

• Define frame over which slot allocations are repeated

• Each frame last 125 us

• Need for time guard

• Need to know

the proper time slots

for reception

f

t


Code division

(CDM – CDMA) • Each flow exploits a different code (waveform

with higher frequency than the bit tx rate)

• Need for orthogonal codes

• Code is not a

new dimension c

t

f

Pag. 32




Code division

(CDM – CDMA) • Code is not a subset of frequencies and/or a

a set of timeslots

f

t


CDMA • Data flows separated using orthogonal codes

• Neither time nor frequency separation

• Better noise protection

• Transmission: scalar product of data signal and of the transmitter code signal

• Reception: scalar product of data signal and of the transmitter code signal (must be known)


Code division

• Example

– Code word used bu user i: +1 +1 -1 -1

– Coded sequence = information bit x code word

– Information bit: -1 -1 1 1 -1

– Coded sequence: -1-1+1+1 -1-1+1+1 +1+1-1-1 +1+1-1-1 -1-1+1+1

Pag. 33



Code multiplexing

• Example:

– Code word for user 1: +1 +1 -1 -1

– Code word for user 2: +1 +1 +1 +1

– Code word for user 3: +1 -1 +1 -1

– Code word for user 4: +1 -1 -1 +1

• Over the channel, transmitted signals sum up

(need to equalize power)

– Transmissions of 1+2+3: +3 +1 +1 -1

– Transmissions of 2+3: +2 0 +2 0


Code multiplexing

• Un esempio (segue):

– Reception = correlation with code words

– Reception of user 1 = scalar product of the received sequence with the code word +1 +1 -1 -1

– Transmissions of 1+2+3: +3 +1 +1 -1

– Correlation with +1 +1 - 1 -1 = 4

– Transmissions of 2+3: +2 0 +2 0

– Correlation with +1 +1 -1 -1 = 0


Space multiplexing

• Networks exploit also space multiplexing

• First idea is to use multiple parallel wires

• Routing techniques may also try to exploit space

multiplexing to increase network capacity

– Cell in wireless access are an example of space

reuse

Pag. 34




Multiplexing or multiple access

• Time, frequency, code and space (multiple wires)

are all equivalente alternatives

– Given a channel capacity we can choose one among

the above techniques depending on technological

constraints

• Code division permits to “increase” channel

capacity (allowing more users) if using pseudo-

orthogonal codes but degrading the signal to

noise ratio at the receiver (increase the bit error

rate)


Statistical multiplexing

• Multiplexing can be

– deterministic, fixed in time, on the basis of

requirements determined at connection setup

– statistical, variable in time, to adapt to

instantaneous traffic requirements


Statistical Multiplexing

• Sequence of A & B packets does not have

fixed pattern, bandwidth shared on demand

• Dynamic TDM scheme

A

B

C 100 Mb/s Ethernet

1.5 Mb/s

D E

statistical multiplexing

queue of packets waiting for output

link

Pag. 35



Switching techniques

• Resource allocation process for the time

needed to transfer information on the

newtork

• Two main technique

– Circuit switching

• Useful for continuous, regular flows

• Voice on a telephone network

– Packet (and cell) switching

• Useful for bursty, unpredictable flows

• Data transmission on Internret



Circuit switching

• Resources are allocated on demand (through

signalling) to each user call to create a circuit

• A circuit is (or is equivalent to) a physical link

directly connecting the two user terminals


Circuit switching

• The circuit is uniquely allocated to the two users

for the whole call duration

• No other user can exploit the circuit until the call

is over

• Resources are made available at the end of the

call, when requested by one of the two users

through a signalling procedure

Pag. 36



Circuit switching

• Opening

• Data transfer

• Closing

U1 N1 N2 U2

t t t t


Circuit switching

• Example: telephone network


1. Call request 2. Incoming call

3. Call accepted 4. Call accepetd

5. Data transmission 6.Data reception


Circuit switching node architecture

Input

interface

Output

interface

Inerconnection

network

Switching

matrix

Command

system Signalling Signalling

command

Pag. 37



Space vs time switching

125 ms

125 ms

U1 N1 N2 U2

U1 N1 N2 U2


Circuit switching

• Advantages:

– Fixed guaranteed bit rate

– Fixed transfer delay

– Circuit transparency (format, speed, protocol)

• Changing the user terminals is enouigh to change

application

– Negligible delays in crossing nodes



Circuit switching

• Disadvantages:

– Resources allocated to a single call

• Efficient only for non bursty (regular) flows

– Time needed to open the circuit

– No conversion of format, speed, protocols

performed by the network (e.g. no error control is

possible)

– Rate (price) time based


Pag. 38



Packet switching

• No resource allocation

• Well suited for bursty (non regular) sources

• Similar to the mailing system


P.T.

P.T.

ADDRESS


Packet switching

• Data to be sent are organized in packets, (PDU)

including user data and control information

• PDU = protocol data unit

• PCI = protocol control information (control)

• SDU = service data unit (user data)

PCI SDU


Packet switching

• Data unit is the ISO definition.

• Other names :

– Packet

– Cella (fixed size data unit)

– Datagram

– Segment

– Message

– Frame


Pag. 39



Packet switching

• Data unit are delivered to the network

• Each node

– Stores the packet while receiving it

– Process the packet to define the output channel

over which to send it (routing)

– Stores the packet in the queue for transmission

• Store and forward behaviour

– implies delays:processing, tx, rx AND queueing



Store and forward

• Dominant operating mode:

– Need to read the header to execute routing

– Routing (and other processing) requires time

– Some form of header error protection is needed

– Different links may have different speed



Packet switching node architecture

• Buffering

– At outputs

– At inputs

– Mixed

Input

interface

Processing

and switching

Buffer Output

queues

Output

interface

Pag. 40



Packet switching

• It may be necesary to partition user data in

pieces due to packet size limitations

– Every packet has its own header

• Fixed vs variable packet size?


PCI SDU


Packet switching

U1 N1 N2 U2

t t t t

Transmission time

Propagation time

Processing

time


Packet size

• Packet size P

– Measured in bit

• Packet size in time TTX

– Transmission time measured in s

– Different on every link

– TTX = P/VTX where VTX is the link bit rate

• Packet size in meter M on a given link

– M = Speed of light x TTX


Pag. 41



Delays

• Delays suffered by each packet from source to destination

node

• In each link

– Transission (and reception) delay

• It is a fiunciton of packet size in bit and of the link bit rate

– Propagation delay

• It is a function of link length in meters

• In each switching node

– Processing time

• Function of the processing speed and of the complexity of the executed

proceures on packet header

• Normallu negligible with respect to transimssion time

– Queuing delays

• Depend on the traffic generated by all users

• Highly variable Copyright Gruppo Reti – Politecnico di Torino


Packet switching

• Packet lenght depends on

– Pipelining

• Small packets increase the tranmsission

pipeline, parallel transmission on different

links of packets belonging to the same flow

– node latency (tx/rx delay) reduction

• Latency can always be reduced increasing

the link bit rate



Pipelined transmission

• HP: negligible propagation and processing

delay


A B C D

80 kb/s 80 kb/s 80 kb/s

10 kB

1 kB

1s + 1s + 1s = 3s

{0.1s + … + 0.1s} + 0.1s + 0.1s = 1.2s

Pag. 42



Pipelined transmission

• Increasing by 10 the link bit rate


A B C D

800 kb/s 800 kb/s 800 kb/s

10 kB

1 kB

0.1s + 0.1s + 0.1s = 0.,3s

{0.01s + … + 0.01s} + 0.01s + 0.01s = 0.12s


Packet switching


– Pipelining

– Packetization delay

• Small packets reduces the packetization

delay

– Time needed to create the packet

– Critical for delay sensitive voice applications

over packet switching networks

– Cannot be reduced by increasing bit rate Copyright Gruppo Reti – Politecnico di Torino


Packet switching


– Pipelining


– Ratio between header and payload

• Large packets permit to reduce the

percentage of control information (header)

• Suppose

– PCI of size p bit

– SDU of size s bit


p s

p

+

Pag. 43



Packet switching


– Pipelining


– Ratio between header and payload

– Error probability

• Small packets reduce the bit error probability

– Packet of n bit

– Bit error probabiltiy p with idependent bit error

– Probability of correct reception (1-p)n



Packet switching

• Advantages with respect to circuit switching

– Efficient resource utilization also for bursty sources

– Possible to introduce error control in network nodes

– Possible to convert formats, bit rate, protocol

– Accounting based on the amount of transmitted data

• Disadvantages with respect to circuit switching

– Difficult to obtain bit rate guarantees

– Each packet is processed in each node

– Highly variable delays



Packet switching

• Two different techniques/services to transfer

packets

– datagram

– virtual circuits

• The initial description referred to the

datagram service


Pag. 44



Datagram service

• No end to end agreement is needed to

provide the data transfer service among the

two users and the newtork

• Connectionless approach

• Each packet contains the source and

destination address

• Each packet is routed independently

– Packets with the same addresses may follow

different paths unique

– No flow concept in the network Copyright Gruppo Reti – Politecnico di Torino


Virtual circuit service

• Communication is organized in three phases

– Connection opening (signalling)

– Data transfer

– Connection closing (signalling)

• Connection oriented approach

• The two users and the network must agree

before the data transfer can take place

• Packets with the same source and

destination addresses follow the same path




• It is packet switching

– There is no static allocation of network resources

to the connection

• It is not equivalent to circuit switching

– Although time to set up the connection may be

needed


Pag. 45




• Advantages with respect to datagram

– Sequence is guaranteed

– Delays are more controlled (reduced variability)

– Routing (which is the best path?) executed only

when opening the connection

– Addressing (see later)

• The network becomes aware of the notion of

flow of packets



Addressing

• In the datagram service in each packet any

pair of users must be uniquely identified

network wide

– Global identifiers

• In the virtual circuit service it is only needed

to distinguish among each virtual circuit

– Local (on each path/link) identifiers (labels) are

enough

– Labels can be reused on different paths



TE1

TE2

1 2 3 4

5

1

2 3 4

5

1 2

3 1 2 3 4 5

32 612

216

IN label OUT label

1 32 5 612

TE3 512

Virtual circuits: identifiers

Pag. 46



Virtual circuits identifiers

• Logically one connection is identified one label

– Labels are changed on each link for practical issues

• Label sometimes are in pairs

– LCG-LCN in X.25/ISDN, VCI-VPI in ATM

• Virtual circuit: associated with a single connection

• Group: associated with a set of virtual circuits

• Groups permit flow aggregation

– Ease network management

– Examples: LAN connection, multimedia flows, etc,

sharing the same end to end path



VPI 1

VPI 6

VCI 1

VCI 2

VCI 3

VCI 4

VCI 5

ATM terminology for grouping

VSc


Permanent vs switched VCs

• SVC (Switched virtual circuit)

– Created on demand, upon user request, via

signalling procedures

• PVC (Permanent virtual circuit)

– Created through management procedure, not in

real time

– Define a semi-static network configuration


Pag. 47



Signalling techniques

Gruppo Reti TLC

[email protected]



ITU-T

• Signalling: exchange of control information

associated with the setup and release of a

telephone call on a telecommunications

circuit.



Signalling techniques

• Classification:

– User signalling: exchange of control information

between the user and network access node

– Network signalling: exchange of control

information among network nodes

• Type of signalling

– Associated to the channel:

• In band

• Out of band

– Common channel


Pag. 48



Associated signalling

• There is a one-to-one correspondance

between:

– Controlling channel (signalling info)

– Contolled channel (user info)

• Used in the early days of telephone netwoks


1

2

k

userdata

1

2

k

signalling

association


Signalling associated with the

channel • In band

– Controlling and controlled channels are the

same

– Used in different times


1

2 k

signalling


Signalling associated with the

channel • Out of band.

– Separate controlling and controlled channels


1

2 k

1

2 k

User data

signalling

Pag. 49



Common channel signalling

• A single signalling channel control many user

information channels

• Singalling channel is packet switched

• ITU-T SS (Signalling System) n.7


1

2

k

user data

signalling

1 k


Common channel signalling

• Leads to the definiton of a signalling network

– Logically (and sometimes physically) separated from

the circuit switching data networksegnalazione

• Among signalling information

– Addresses

– Addressing scheme


Management techniques

Gruppo Reti TLC

[email protected]


Pag. 50



Network management

• Network Management organized according

to several functions:

– Configuration Management

– Performance Management

– Fault Management

– Security Management

– Accounting Management



Quality of service

and traffic characterization

Gruppo Reti TLC

[email protected]



Quality of service (QoS)

• In network design, traffic characteristics must

be known to provide a given service

• The amount of available resources and the

adopted switching technique (which establish

resource allocation) define QoS provided to

the users

• Network analysis and design exploit

mathematical models that permit the QoS

estimation given the available resources and

traffic behaviour Copyright Gruppo Reti – Politecnico di Torino

Pag. 51



Quality of service

• Network analysis

– Given

• Service requests

• Available resources

– Define

• Quality of service

• Network design

– Given

• Service requests

• Quality of service

– Define

• Needed network resources



Quality of service

• Need to rely on mathematical models to:

– Characterize service requests

– Describe interaction among activities and

resources

– Compute quality of service



Traffic characterization

• Analog sources: voice, video

– Defined through their spectral characterization

(required bandwidth, correlation, ...)

• Digital (or digitalized) sources: data, voice,

video

– Defined through average bit rate and burstiness


Pag. 52




• Digital sources

– Constant Bit Rate - CBR

• Voice (64 kb/s)

• Videoconference (n x 64 kb/s)

• Uncompressed video (180Mb/s)

– Variable Bit Rate - VBR

• Compressed voice (tens of kb/s)

• MPEG video (Mb/s)

• file transfer (from kb/s to Mb/s to Gb/s)




• CBR sources

– Bit rate (bit/s)

• Packet size

– Call duration (s)

– Call generation process




• VBR sources

– Peak rate (bit/s)

– Average rate (bit/s)

• Over which time window?

– Burstiness is defined as peak rate/average rate

– Call duration (s)

– Call generation process


Pag. 53




• How to control traffic shape


L

V1

V2

source network

token

bucket


QoS parameters

• Different applications have different

requirements

• QoS indices:

– Delay (average, percentile, wors case)

– Bit rate

– Error probability

– Loss probability

– Call blocking probability



QoS indices: example

• Phone calls

– CBR source

– Worst case delay of hundredths of milliseconds

• Real time communication

– Bit rate of 64 kbit/s

– Error probability of few %

– Negligible call blocking probability


Pag. 54




• E-mail

– VBR source

– No requirements on delay (minutes is fine)

– No requirements on bit rate

– Negligible error probability

– Negligible blocking probability




• Video on demand

– VBR source

– Maximun delay of few seconds

– Bit rate of Mbit/s

– Small error probability

– Negligible call blocking probability



105

104

103

102

101

1 hour

1 min

telemetry

101 102 103 104 105 106 107 108

hi-fi audio video

videoconference low speed data

fax

voice video

telephony

Co

nn

ecti

on

du

rati

on

[s]

1

Speed

[bit/s] 1 kbit/s 1 Mbit/s

high

speed

data

Service characteristics

Pag. 55



low

speed

data

alphanumeric

terminals

LAN alta velocità

immagini

connectionless

data

transmission

supercomputer

interconnection

voice audio

HDTV VIDEO

compressed

non compressed

Bu

rsti

ne

ss

Peak rate [bit/s]

103 104 105 106 107 108 109 1010

1000

100

10

1

LAN

graphical

terminal

Burstiness= Peak rate/ Average rate


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