Computer Network Design Review - Polito · Pag. 3 Computer Network Design – Review Andrea Bianco...

Pag. 1

Computer Network Design – Review

Computer Network Design - 1 Andrea Bianco & Paolo Giaccone – TNG group - Politecnico di Torino

Review of fundamentals

networking concepts

Andrea Bianco Paolo Giaccone

Telecommunication Networks Group

[email protected]

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


Copyright

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Way, Stanford, California 94305, USA.


Review

• What is a (modern) telecommunication and computer network?

• Topologies

• Multiplexing/multiple access

• Switching techniques

• User (traffic) characterization

• Properties of telephone and computer networks

• Protocol architectures for computer networks

– Physical layer (multiplexing)

– Data link layer (multiple access)

– Network layer (routing, congestion)

– Transport Layer (congestion)

– Application Layer (client server, P2P, CDN, Data Center)

Pag. 2



Networks

• Focus on modern telecommunication and computer networks – Telephone and Internet

• One possible network definition (service oriented view) – Infrastructure that provides services to applications:

• Web, VoIP, email, games, e-commerce, social nets,

• phone calls, fax, …

– Internet • provides programming interface to apps program to “connect” to

Internet

• provides service options


Networks

• Another network definition:

– A set of nodes and channels that offer a connection

among two or more points to make telecommunication

possible among users (i.e. move infos (data/flows)

among nodes)

– Represented as a topology (graph)

• Different levels of detail

– Key issue in networks is resource sharing

• Node is the point where

– Multiplexing /multiple access (link sharing) and switching

(node sharing) occurs


Type of channels

• Point-to-point channel

• Only two nodes connected to the channel

• The channel is used by both nodes (often in

the same fashion)

• Sharing the channel among flows is called

multiplexing

A B

Pag. 3



Type of channels

• Broadcast channel

– Many Tx and many Rx

• Sometimes one (many) Tx and many (one) Rx

• Single communication channel shared by all nodes

– This room!

• The information sent by one node is received by all

other nodes (with some delay)

• Sharing the channel among flows is called multiple

access


Channel properties

• Many quality indices

– Attenuation, robusteness to mechanical stress, ease of

installation, robusteness to interference, cost, etc,

• Mainly interested in

– bit rate [bit/s]

• Also named bandwidth, throughput, with slightly different

meaning

– delay [s]

• propagation delay

• depends on the channel length

– Bandwidth x delay [bit]

• channel ‘’size’’

• how much we can ‘’push’’ on the channel


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)

• Edges may be:

– direct (directed segments (arrow) – unidirectional channels)

– undirect (non directed segments – bidirectional channels)

• Abstraction of real networks

– Several levels of abstraction are possible

Pag. 4



Many topologies

• Full mesh, mesh, tree, bus, star, …

A

E B

C D

A

E B

C D

A

E B

C D

A

E B

C D

A

E B

C D A E B C D

A

E B

C D


Physical and logical topology

• It is important to distinguish between the

physical and the logical topology

– Logical topology: logical interconnection among

nodes via logical channels

– Physical topology: takes into account transmission

media constraints


Physical topology

• E.g., satellite network

A B

C D

Pag. 5



Logical topology

• Private network over satellite network

A B

C D


Logical topology

• Router interconnection for an ISP

Frame Relay

or ATM network

Router

Router Router

Router

Node

Node

Node

Node

Virtual

Circuits


Logical topology

• Overlay among peers in a P2P network

Pag. 6



Logical topology

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

ISP B

ISP A

ISP C

IXP

IXP

regional net


Logical topology

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

IXP


Tier-1 ISP: e.g., Sprint

Pag. 7



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. 8



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


Channel, sharing

Multiplexing and multiple access

Pag. 9



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


Channel sharing techniques

• Frequency (FDM - FDMA)

• Time (TDM - TDMA)

• Code (CDM - CDMA)

t

f

channel


Frequency division

• Each flow is transmitted using different frequency

bands

• Need for band guard

f

t

Pag. 10



Time division (TDM – TDMA)

• Each flow exploits different time intervals (slots)

• Define frame over which slot allocations are repeated

• Need for time guard

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

c

t

f


Code division

(CDM – CDMA)

f

t

Pag. 11



Code division

• Example

– Code word used by 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


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

• We are interested in receiving data from user 1

• Over the channel, transmitted signals sum up

(need to equalize power)

– Tx of 1+2+3: +3 +1 +1 -1

– Tx of 2+3 (noise): +2 0 +2 0


Code multiplexing

• Reception = correlation with code words

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

– Everything we receive • Transmissions of 1+2+3: +3 +1 +1 -1

• Correlation with +1 +1 - 1 -1 = 4

– The noise • Transmissions of 2+3: +2 0 +2 0

• Correlation with +1 +1 -1 -1 = 0

Pag. 12



Multiplexing or multiple access

• Time, frequency, code (and space: multiple

parallel 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 (by allowing more users) if using

pseudo-orthogonal codes but degrading the

signal to noise ratio at the receiver (increase the

bit error rate)


FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

125 ms


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

Pag. 13



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


Switching techniques

Circuit and packet switching



• Circuit switching

– Telephone networks

• Packet switching

– Two flavours (datagram and virtual circuit service)

– Internet, computer networks

Pag. 14



Circuit switching

• Three phases

– Opening

– Data transfer

– Closing

• Network transparently

forwards user

generated data

– No operation on user

data

• Fixed bit rate 64kbit/s

U1 N1 N2 U2

t t t t


Space vs time switching

125 ms

125 ms

U1 N1 N2 U2

U1 N1 N2 U2



• Circuit switching – Resources allocated uniquely to a circuit

• Physical channel, time-slot in TDM frame

• Resources are shared among connections but are statically allocated to data

– Connection oriented • Need to open (and close) the circuit prior (after) data transmission

• Store state information on each circuit (stateful approach)

– Address (unique for each user in the network) used only when opening the circuit, not carried in data

– Data unit identified by position

– Routing (choice of the best route) performed only when opening the circuit

• Done through routing table lookup

– Data forwarding • Through forwarding table look-up (one entry for each active circuit)

• Static (always the same scheduling, unless circuits are closed or opened)

Pag. 15



Packet switching

U1 N1 N2 U2

t t t t

Transmission time

Propagation time

Processing

time


Data transfer over virtual circuits

Forwarding table

DTE1 DTE2 1 2 3

4 5

1

2

3 4

5

1 2

3 1 2 3 4 5

32 612

1 32 5 612

216

314

In Label Out Label


Grouping virtual circuits

• A virtual circuit is logically identified by a

label

– The label may change value over each link

• Relabeling

• Label = often a pair of identifiers (VCI-VPI in

ATM)

– Virtual channel (VC): identifies a single

connection

– Virtual path (VP): identifies a group of virtual

channels

Pag. 16



Grouping virtual circuits

• The grouping allows flow aggregation

– Eases network management

– Increases scalability

• Possible use

– LAN inter-connection to crete a VPN (Virtual

Private Network)

– Multimedia flows (video, audio, data)


VPI 1

VPI 6

VCI 1

VCI 2

VCI 3

VCI 4

VCI 5

Virtual circuits and paths (ATM)


VP SWITCHING

VCI 21

VCI 22

VCI 23

VCI 24

VCI 25

VCI 24

VCI 23

VCI 24

VCI 25

VCI 24

VCI 21

VCI 22

VPI 1

VPI 2

VPI 3 VPI 8

VPI 4

VPI 5

ATM: VP switching

Pag. 17



VCI 25 VCI 25

VCI 21 VCI 21

VCI 23

VCI 24

VCI 23

VCI 24

VC SWITCHING

VPI 4

VPI 5

VPI 2

ATM: VC switching


Virtual circuits

• Switched virtual circuit (SVC) – Established on-demand, through signaling, in real-time

– Three phases

• Virtual circuit opening

• Data transfer

• Virtual circuit closing

– Users (and network) exchange signaling packets (over dedicated VCI/VPI) to establish a virtual circuit; then, data transfer can occur

• Permanent virtual circuit (PVC) – Established through agreement among user and network provider

• Off-line, through management procedures

– Define a semi-static network

• Logical topology

– Users can immediately exchange data, with no delay



• Packet switching, with datagram service – Shared resources

• Ideally the full network is available to a single user

• Resources are “continuously” shared with all other users at the data level

– Connectionless

• Free to send data when available, no need to check network or user availability

• Stateless approach

– Each packet must carry the destination (and source) address

– Data unit identified through source and destination addresses (unique for each pair of users in the network)

– Routing and forwarding performed independently over each packet

• Through routing table look-up

Pag. 18




• Packet switching, with virtual circuit service – Shared resources

• Resources are shared with all virtual circuits sharing the same link

– Connection oriented • Need to open (and close) the virtual circuit prior (after) data transmission

– Permanent virtual circuits available

• Store state information on each virtual circuit (stateful approach)

– Address (unique for each user in the network) used only when opening the virtual circuit, not carried in data

– Data unit identified through a label (unique for each existing virtual circuit on each link in the network)

• Label is unique on each link, but has a local scope, i.e. the value assumed is different on each link for simplicity

– Routing (choice of the best route) performed only when opening the virtual circuit

• Done through routing table lookup

– Data forwarding • Through forwarding table look-up (one entry for each active virtual circuit)

• Re-labelling needed


Fundamentals of packet

switching • Data sent as packets

• Nodes operate in store&forward (almost always)

– Buffers

– Delay

• Many operations on data in the network (not in

circuit switching)

– Error detection, error recovery, flow control, routing,

forwarding, congestion control, packet inspection ……..

– Need to define a network architecture to organize

functionalities (see later)


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. 19



Delays

• Delays suffered by each packet from source to destination

node

• In each link

– Transmission (and reception) delay

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

– Propagation delay

• It is a function of the link length in meters

• In each switching node

– Processing time

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

procedures on the packet header

• Normally negligible with respect to transmission time

– Queuing delays

• Depend on the traffic generated by all users

• Highly variable


Traffic characterization

and QoS Provisioning


User traffic characterization

• Need to know user behaviour to design a

network

• Traffic sources

– CBR (Constant Bit Rate)

– VBR (Variable Bit Rate)

• Characterized by their rate [bit/s]

Pag. 20




• CBR (Constant Bit Rate) sources:

– Rate (bit/s)

• “Perfectly” known

– Call duration (s)

– Call generation process

• Only statistically known



• VBR sources: – Average rate (bit/s)

• Known?

• Over which period?

– Peak rate (bit/s) or

– Burstiness (Peak rate/ average rate) • Known (worst case)

– Burst duration • Known?

– Call duration (s)

– Call generation process • Only statistically known


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


Pag. 21



Integrated vs dedicated networks

• Telecommunications networks were traditionally defined to provide a specific service – one service one network paradigm

• Telephone network for the interactive human voice transportation service

• Internet for data exchange among computers

• TV or radio distribution for the TV or radio system

• Integrated networks – one network for any service

• narrowband ISDN o N-ISDN

• broadband ISDN o B-ISDN


Integrated vs dedicated networks

• Dedicated networks – Easier to optimize for the specific service

– “Optimal” engineering solutions for the specific requirements of the service

• Integrated networks advantages – No need to create an independent infrastructure for each

service

– Supporting different requirements implies sub/optimal choices

• Integrated networks trade flexibility and infrastructure cost reduction with perfomance and increased control complexity


Quality of service provided by

networks • Networks used as examples

– Fixed telephone network: POTS

– Internet

– B-ISDN

• Describe in an informal way the quality of

service provided by these networks

Pag. 22



POTS

• Characteristics – CBR source completely known (generated by the

network)

– Circuit switching • Constant, dedicated bit rate no congestion

• Minimum possible delay (only propagation): order of tens of ms (real time)

• Zero loss probability

• Requirements – Error probability smaller than few %

– Small or negligible blocking probability

• QoS largely independent of other users (apart from blocking probability)

• Network utilization can be really low, user satisfaction very high


Internet

• Characteristics – Source behavior unknown

– Packet switching with datagram service • Complete sharing of network resources

• Bit rate and delay unknown

• Possible congestion

• Loss probability may be significant

• Requirements

– Error probability negligible in wired networks • Zero blocking probability

• QoS largely dependent of other users

• Network utilization can be very high, user satisfaction can be very low


B-ISDN

• Intermediate situation – Source known (either deterministically or statistically)

– Packet switching with virtual circuit service • May introduce algorithms to control network resources sharing

• Bit rate and delay negotiable

• Loss probability negotiable

– Requirements • Blocking probability reasonably small

• Error probability negligible

• QoS dependent of other user behavior and of algorithms used to manage network resources

• Trade network utilization and user satisfaction

Pag. 23



Network design

• Design problem

– Given:

• Network topology (nodes, link speed)

• Traffic characterization

– Jointly obtain:

• Guarantee some sort of QoS for user connection

• High network utilization

• Without the objective of high network utilization, the

problem becomes “trivial”

– overprovisioning (power line or water distribution

networks)


Network design

• Several flavour

• Running at different time scale (with different level of

complexity)

• Mainly focus on network design and planning (resource

deployment) • On the basis of traffic estimates and cost constraints

• Exploits routing criteria and traffic engineering

• Other examples

– Network management (running a network)

• Measurements

• Fault management (protection and restoration)

• Includes re-design and re-planning

• Connection management

• Data unit transport


Design to obtain QoS

• Different time scale (with different level of complexity)

• Network design and planning (resource deployment) – On the basis of traffic estimates and cost constraints

– Exploits routing criteria and traffic engineering

• Network management (running a network) – Measurements

– Fault management (protection and restoration)

– Includes re-design and re-planning

• Connection management

• Data unit transport

Pag. 24



Our definition of QoS

• Assume that a network has been designed and is properly

managed

– Available resources are given

• Mainly study algorithms operating at the following time-

scale:

– Connection management

– Data unit transport

• Also named traffic control problem

• Must define what is meant by connection. Also named data classification problem.

• Two different traffic control principles: – Preventive control : mainly executed at network ingress, with fairly

tight traffic control to avoid congestion insurgence in the network

– Reactive control: react when congestion situation occur, to reduce or eliminate congestion negative effects


Traffic control:

essential elements • Connection oriented network

• User-network service interface – Traffic characterization

– QoS negotiation

• Resource allocation (bit rate and buffer)

• Algorithms for traffic control – CAC (Connection Admission Control) and routing

– Scheduling and buffer management (allocation, discard) in switching nodes

– Conformance verification (policing or UPC: Usage Parameter Control)

– Traffic shaping to adapt it to a given model

– Congestion control


Traffic control:

connection oriented network • The connection oriented paradigm permits to know

which are the network elements over which traffic

control algorithms must be executed (path known)

– Circuit switching

– Packet switching with virtual circuit service

• If high utilization is a major objective:

– Packet switching

• As such, the most suited switching technique to

obtain QOS is packet switching with virtual circuit

service

Pag. 25



Traffic control:

user-network service interface • The capability to control the network increases with the

knowledge of user traffic. Limiting factor is the complexity.

• Over the service interface

– Traffic characterization

– QoS parameters negotiation

• Can be defined on a call basis or on a contract basis

• POTS: implicit, on a contract basis

• Internet: not existing

• Frame relay: negotiable, normally on a contract basis

• B-ISDN: negotiable with traffic contract on both contract and

call basis

• Internet extended to support QoS: negotiable through a SLA

(Service Level Agreement) mainly on a contract basis


Traffic control:

resource allocation • Main resources:

– Bit rate over transmission links

– Buffer

• Resources can be allocated

– On a contract basis (booking)

– On a call basis

– Packet by packet

• Allocation

– Exclusive (dedicated resource)

– Shared


Algorithms: CAC and routing

• Routing – QoS based path selection to router a connection

• CAC – Determine whether to accept a connection or not, depending on

• The path chosen by the routing algorithm

• Traffic characterization

• QoS requests

• Network status

• Constraints – It is not acceptable to destroy or even reduce the quality of service

guaranteed to already accepted connections • Can be relinquished

– Connection must be refused to avoid network overload or congestion

• Preventive control (but can become reactive)

Pag. 26



Algorithms: scheduling and buffer management

• Scheduling – Choice of the data unit to be transmitted among data unit stored in

the switch

• Buffer management – Allocation (partial/total, exclusive/shared) of memories in the

switch

– Dropping policies

• Mandatory in an heterogeneous environment to support different QOS requests – FIFO (First In First Out) or FCFS (First Came First Served) policy

with drop-tail discard is optimal in a homogeneous environment

– Counter for less than 10 pieces at supermarket

• Preventive and reactive


Algorithms: policing e shaping

• Policing (traffic verification)

– Network control of user behavior to guarantee conformance to traffic

characterization

• Shaping (traffic conditioning)

– User/network adaptation of data traffic to make it conformant to a

given characterization

• Mandatory to control user honesty and to adapt traffic which

is difficult to generate as conformant a priori

• Where algorithms must be executed?

– Only at network edge, i.e., when user access network?

– Multiplexing point modify traffic shape

• Both at network access and internally to the network

• Mainly preventive, but they can become reactive if QoS

level may change over time


Algorithms: congestion control

• Congestion – Traffic excess over a given channel (link)

• Can occur due to – Short term traffic variability

– Allocation policies that share resources to increase network utilization

• Congestion effects: – Buffer occupancy increase

– Delay increase

– Data loss

• Needed to obtain high link utilization

• Must execute at network edge, within the network or….?

• Reactive

Pag. 27



Protocol architectures


Architectures and protocols

• Communication requires cooperation

• One abstract description of the communication paradigm between two or more users requires the definition of a reference model specifying a network architecture

• A network architecture defines

– the communication process

– the relation among objects used in

communication

– the functionalities to support the communication

• Layered architectures!


Layered architectures

• Layered architectures are used because of – simple design

– simple management

– simple standardization

– separation among functions

OSI

Application

Presentation

Session

Transport

Network

Data Link

Physical

DECNET

User

Netw. Appl.

Session

End to End

Routing

Data Link

Physical

ARPA

Application

Service

Internetwork

Network

SNA

Transaction

Service

Presentation

Service

Data

Flow

Trans.

Control

Manag.

Service

Virtual Route

Explicit Route

Transm. Group

Data Link

Physical

path

control

half

session

Pag. 28



router 1

router 2 router 3

host 1

host 2

host 3

host 4

subnet 1

subnet 2

subnet 4 subnet 3

packet

transfer

routing

error control

applications

Separation among functions:

Internet


Management plane

Control plane User plane

High layers

AAL

ATM

Physical

High layers

B - ISDN


Protocols

• Formal definition of the procedures adopted to

guarantee the communication between two or

more objects on the same hierarchical level to

execute a specific function

• Protocol definition:

– semantics

• set of commands and answers

– syntax

• structure of commands and answers

– timing

• temporal sequence of commands and answers

Pag. 29



Layer and entities

• Active elements in a subsystem

• Run the functions of the layer (exploiting service provided

by lower layers)

• Interact (cooperate) within the same layer

(N) - layer

System A System B

(N) - entity

transmission media


Services

• A service can be:

– connection-oriented (CO): a preliminary agreement

(connection) is established between the network and

the communication end-points, then the data is

transferred and finally the connection is released

– connectionless (CL): data is sent to the network

without any preliminary agreement and is treated

independently from each other


(N) - service

N+1 N

N+1 N (N) – service

provider

Black-box for the

(N+1) - entity

Services

Pag. 30



N N-1

N N-1

(N-1) - service

(N-1) – service

provider

Black-box for the

(N) - entity

Services


(N) - layer

interface

(N-1) - layer

(N-1) - SDU

(N-1) - PCI (N-1) - SDU

(N-1) - PDU

SAP

(N) - PDU

PDU creation


Application

Presentation

Session

Transport

Network

Data link

Physical

Receiver Transmitter

Application

Presentation

Session

Transport

Network

Data link

Physical

data

APCI ASDU

PPCI PSDU

SPCI SSDU

TPCI TSDU

NPCI NSDU

DLPCI DLSDU bit or symbols

Information transfer

Pag. 31



application

presentation

session

transport

network

data link

physical

transmission media

Application protocol

Presentation protocol

Session protocol

Transport protocol

Network protocol

Data link protocol

Physical layer protocol

application

presentation

session

transport

network

data link

physical

The seven OSI layers


Functions (OSI-like)

• Physical layer:

– allows to transfer binary digits exchanged among the

data link entities

– deals with bits or symbols

– defines transmission codes, connectors, voltage levels,

etc…

• Data link layer

– error detection and error correction

– flow control

– data unit (packet, cell, datagram) delimitation

– addressing


Functions

• Network layer

– routing

– congestion control (moved to layer 4 in the Internet)

– pricing

– addressing

• Transport layer

– error control

– sequence control

– flow control (end to end)

– congestion control (Internet)

Pag. 32



Functions (OSI)

• Session layer

– provides synchronization points to recover for

interruption of the transport layer

• Presentation layer

– Deals with data representation

• Application layer

– provides the application processes with the

means to access the OSI environment

• Often merged in a single layer in the Internet

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Computer Network Design Review - Polito · Pag. 3 Computer Network Design – Review Andrea Bianco...

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