Debrecen, 2012. Train IT - Networks 1 Train IT – Computer Networks University of Debrecen

Debrecen, 2012.Train IT - Networks

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Train IT – Computer Networks

University of Debrecen


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I. – Basics of Computer Networks


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Computer Networks

Computer Networks :• Two or more computers linked together with some software

and hardware tools for an information transmission related purpose.

Objectives:• Sharing resources.• Increasing reliability.• Increasing speed.• Human communication.


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Classification of Computer Networks Based on their Sizes

Name

Multicomputer

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area Network (WAN)

Range

< 1m

1 km

10 km

100 km <


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Components of LANs

• Computers• Network cards• Peripherals• Network transmission lines• Network devices


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Functions of WANs

• Covering large geographical area

• Providing (real-time) communication between users

• Nonstop access to remote resources that can be connected to local services

• Providing e-mail, internet, file transfer, and e-commerce services


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Accessibility of Networks

• Public Network: A network that is accessible for everyone (eg. Internet).

• Private Network: A network that is accessible only for the owner organization (expensive for large networks).

• Virtual Private Network(VPN): A private network that provides private type access and data transfer via public infrastructure (eg. Internet).


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Computer Network Node

Node:• Device with own network address. It can communicate

independently (eg. computer, printer, router).• In a communication a node can act either as a transmitter (sender,

source) or as a receiver (destination).

Categories of network devices and tools:• End user node: computer, printer, scanner, and any other devices

that provide services directly to the user • Network linking/connecting tools: devices that enable

communication between end user nodes by connecting them to each other


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Signal, Signal Coding, Modulation

Signal: Physical quantities, depending on place and time, and carrying information. Information carrier on the communication chanel, it could be analog or digital.

Signal Coding: Mapping the (digital) information onto the (digital) carrier signal (eg. voltage levels, changing of voltage levels).

Modulation: Mapping onto analog carrier signal. The process of creating the (modulated) signal to be transmitted through the chanel from the modulating signal coming from the source and the analog carrier signal. The inverse process is the demodulation. A modem performs modulation and demodulation, as well.


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Transmission Media, Chanel, Collision

Transmission media:• Device or material on which the transmission of information

(signal) is performed. (Eg. twisted-pair cable, coaxial cable, fiber-optic cable, or air).

Transmission chanel:• Data path, frequency band for transmitting signals. Usually,

in a transmission media multiple chanels (data path) are formed.

Collision:• A collision occurs when two (or more) nodes transmit

information at the same time on a common transmission chanel.


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Transmission Speed

Transmission speed (network speed, bandwidth, bit rate):• Amount of information transmitted during a time unit.

Measure of unit: bit/sec, b/s, bps.• The throughput measured in applications is always lower

than the physical bandwidth.

• Larger units:1 Kbps = 1000 bps1 Mbps = 1000 Kbps1 Gbps = 1000 Mbps


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Connections of Data Transmission

Peer-to-peer (Point-To-Point) connection:• The propagation of information performed between two

points (a transmitter and a receiver) is called a peer-to-peer connection.

Multipoint connection, broadcasting:• A transmitter provided information to multiple receivers is

called a multiple nodes connection.Broadcasting is a multiple nodes connection, where all receivers get the information inside a given range (eg. radio broadcasting).


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Directions of Information Transmission

One way (simplex) connection:• The transmission of information allowed only one way is

called a one way (simplex) connection (eg. radio broadcasting).

Alternate way (half duplex) connection:• The transmission allowed both directions, but only one

direction at a time is called a half duplex connection (eg. CB radio).

Two way (full duplex) connection:• The traffic allowed in both directions simultaneously is

called a full duplex connection (eg. telephone).


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Basics of Addressing

Unicast (Unique) address:

• An identifier, assigned to a network interface of a node.

Broadcast address:

• An address, identifying all nodes (and interfaces of nodes) in a so called broadcast domain.


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Computer Network Protocol

Protocol:• The formal description of all rules and conventions which

determines the communication of the network devices (nodes) (set of communication rules).


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Server-Client Architecture

Server: A network node (and software) which provides services for other nodes. The service of a server is ensured by a server-software (eg. a web-server).

Client: A network node (and software) which has some kind of network service demand. For recourse to the service the client uses a client-software (eg. web browser).

The communication between the server and the client is described by a high level protocol (eg. http) .


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Layered Network Architecture


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Layers (Levels), Protocols, Interfaces

Layer 5

Layer 3

Layer 4

Layer 2

Layer 1

Layer 5

Layer 3

Layer 4

Layer 2

Layer 1

Layer 5 protocol

Layer 4 protocol

Layer 3 protocol

Layer 2 protocol

Layer 1 protocol

Transmission medium

Layer 4/5 interface

Layer 3/4 interface

Layer 2/3 interface

Layer 1/2 interface

Machine 2Machine 1


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Layered Network Architecture - Concepts

Layer N protocol:• A protocol which describes the specifications of layer (level) N.

Peers: • Entities which located on the same level of the two

communication endpoints (nodes). In some logical way the peers communicate each other by the help of the corresponding layer protocol.

Layer N/N+1 interface:• Connection of boundary surface of layers N and N+1.

Service of Layer N:• Set of actions (services) which are provided to layer N+1 by

layer N (through the interface).


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Scheme of Network Communication

4

Layer 5 protocol

Layer 4 protocol

Layer 3protocol

Layer 2 pr

DestinationLayer Source

3

2

1

5 M

H3 H4 M2

H4 M

H2 H3 H4 M2 T2

H3 H4 M1

H2 H3 H4 M1 T2

M

H3 H4 M2

H4 M

H2 H3 H4 M2 T2

H3 H4 M1

H2 H3 H4 M1 T2


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Network Communication - Concepts

Encapsulation:• Packaging the information arrived from a higher level with a

header of a specific protocol (it is similar when a surface mail letter is put in an envelope and the envelope is addressed).

Protocol Data Unit (PDU):• Entity (contains header and data) handled by the considered

protocol. (It is frequently mentioned as packet.)


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Encapsulation - Example

• Pictures and text are transformed into data.• Data are packaged into segments. • The data segment is encapsulated into a packet containing the

addresses (IP addreses) of the source and destination nodes.• The packet is encapsulated into a frame containing the physical

address (Ethernet address) of the next device connected directly.• The frame is transformed into series of bits (ones and zeros)

which can be transferred through the transmission media.


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OSI Reference Model

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

1

Presentation Layer

Application Layer

2

3

4

5

6

7

Layer Name of PDU

Packet

APDU

PPDU

SPDU

Frame

Bit

TPDU, Segment


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Mapping of TCP/IP - OSI Model

Physical Layer

Transport Layer

Session Layer

1

Presentation Layer

Application Layer

3

4

5

6

7

TCP/IP LayerOSI Layers

Host-to-Network Layer

Network Layer

Transport Layer

Application Layer

Data Link Layer

Network Layer

2

Not present in theTCP/IP model


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Hybrid Reference Model

Physical Layer

Transport Layer

1

Application Layer

3

4

5

Data Link Layer

Network Layer

2


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Network Interconnection Devices


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Network Interconnection Devices - Basics

Collision domain; Bandwidth domain:• Part of a network , where collisions can be detected (a

common communication channel that is shared by multiple nodes).

• In a collision domain only one information transmission can be performed at a time.

Broadcast domain:• Part of a network, where information transmitted with a

broadcast destination address can be detected.


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Subnetworks – based on the functionality of network interconnection devices – can be connected in different OSI Layers:

Repeater

Bridge

Router

Gateway

Physical Layer

Data Link Layer

Network Layer

Above Transport Layer

Connector itemOSI layer


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Repeater:• Amplifies and repeats the signals sent on transmission media.• Does not separate the connected subnetworks.• Repeater with multiple ports is called a HUB.

Bridge:• Working in Data Link Layer it performs selective connection.• The interconnected subnets form separate collision domains.• Usually transmits the broadcast messages towards all

interconnected subnets.



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Switch:• A multiple port device with bridge functionality between any

two ports.

Router:• Working in Network Layer it performs selective connection,

routing, and traffic control.• The interconnected subnets form separate collision domains

and separate broadcast domains.• It is a node with own IP address.• It is also called a gateway in the Network Layer (default

gateway).



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Topologies


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Physical topology:• Investigates the placement of nodes and their connection

possibilities. (Cable topologies, Physical topologies).

Logical topology:• Investigates the logical sequence and order of nodes.

Topologies


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Star (extended star)

Ring

Host (workstation or server)

Center

Physical Topologies


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Repeater

Bus

Bus

Physical Topologies


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Tree

Physical Topologies


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II. – Physical Layer


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• Electricity is the free flow of electrons.• Materials highly restraining the flow of electrons are called insulators. Materials

slightly restraining the flow of electrons (free of resistance) are called conductors.Semiconductors are able to control accurately the conducted electricity. The resistance is denoted by R, the measure of unit is ohm, (Ω)

• The current is the quantity of electric charge through a circuit in a second. It is denoted by I, the measure of unit is ampere (A)

• The voltage is an electronic force or pressure which arises at separation of electrons and protons. It is denoted by U, the measure of unit is volt (V)

• Ohm’s Law: U=I*R

Electricity - Basics


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• The amplitude of a signal is decreasing during its way in a transmission media.

• The length of a transmission media is determined such a way that the signal should be interpreted securely by the receiver.

• If large distance has to be covered, the signal has to be restored by the help of amplifiers (repeaters).

• The attenuation depends on frequency, thus the amplifiers have to compensate this with frequency dependent amplification.

• The quantity of attenuation and amplification is expressed in decibels (dB) on a logarithmic scale.

Attenuation


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Optical fiber

3/8” coaxial cable

Twisted pair

1 kHz 1 MHz 1 GHz 1 THz 1000 THz

Frequency

Att

enu

atio

n (

dB

/km

)

0,3

1

3

10

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Attenuation of Guided Media


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Four pairs are typically grouped in a plastic sheath.

The grouped pairs can be shielded (Shielded Twisted Pair, STP) or unshielded

(UTP).

Twisted Pair


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• The cheapest and most commonly used transmission media.

• Two insulated copper conductors are twisted according to a regular

pattern.

• Usually several pairs are grouped (UTP has 4 pairs) and protected with

a plastic sheath.

• Number of twists decreases crosstalk between pairs and ensures noise

protection.

• The length of twists might be different in pairs in order to decrease

crosstalk.

• The diameter of conductor is 0.4 - 0.9 mm .

• It is the cheapest media, easy to use but the data transmission speed and

the distance to be covered are highly limitated.

Twisted Pair– Physical Characteristics


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Transmission characteristics• The attenuation of twisted pairs is highly dependent on frequency.• It is sensitive to interference and noise. For example easily takes the

50Hz energy from the parallel AC network.• A shield can be used against disturbance (STP, FTP).• Crosstalk between neighbour pairs can be decreased by variable twist

length.• With a point-point analog signals a frequency bandwidth of cca.

100KHz can be obtained (transmission of multiple voice channels).• For short distance over 100 Mbps speed can be achieved.

Twisted Pair– Transmission Characteristics


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Category 5. UTP cable and connector for 100 MHz transmission.

For limited distance (100 meters) it provides 100 Mbps speed transmission.

Cabling of new buildings usually is done with this cable.

New standards: Cat5e – enables simultaneous use of the 4 pairs,

Cat6: ~250MHz; Cat7 STP: ~600MHz.)

STP: Twisted pairs are shielded separately.

FTP: Twisted pairs has a common shield cover (foil) . (It is cheaper than

STP, and better than UTP.)

Types of Twisted Pairs


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Twisted Pair with RJ-45 Connector

Pair 2 Pair 3

Pair 3 Pair 1 Pair 4 Pair 2 Pair 1 Pair 4


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Straight RJ-45 female (PC, Router):Tx+ Tx- Rv+ Rv-1 2 3 4 5 6 7 8

Cross RJ-45 female (Switch, Hub):Rv+ Rv- Tx+ Tx-1 2 3 4 5 6 7 8

• RJ-45 females with same assignment (eg. pc-pc, hub-hub) are connected with a crossover cable (568A – 568B).

• The different RJ-45 females (eg. pc-hub, pc-switch, router-switch) are connected with a straight-through cable (568A – 568A or 568B – 568B).

• Certain devices can detect the assignment of RJ-45 female on the other side, and are able to handle it automatically (auto sense).

Assignment of Contacts of RJ-45 Connectors


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Physical Characteristics

Cross section of a coaxial cable

Inner conductor

Outer cover

Insulation

Outer conductor

• Diameter of the cable: 5 - 25 mm.• Due to the concentric structure, it is less sensitive for interference and crosstalk, than the twisted

pair.• It can be used for larger distance and in multipoint application supports more stations, than the

twisted pair.

Coaxial Cable– Physical Characteristics


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Applications

• Transmission of television broadcasting.

• Large distant telephone transmission.

• Connection of computers

• Local networks.

Transmission characteristics

• In case of analog transmission, amplifiers are required in every several

km. It can be used up to 400 MHz.

• In case of digital transmission amplifiers are required in every km.

Coaxial Cable


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Optical Fiber

Core

Cladding

Buffer

Stiffener (Aramid string)

Protecting cover (usually PVC)


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Optical fiber

Cladding

CoreProtecting cover

Angle of incidence

Angle ofreflection

A light ray incident below the critical angle

is absorbed in thecladding

Optical Fiber– Physical Characteristics


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• A floppy optical fiber with 2 - 125 μm of diameter is able to transmit light ray.• Optical fiber is made of glass or plastic.• The cladding is also made of glass or plastic, but it has different optical

characteristics than the core.• The outer plastic cover protects against impurities, wearing, and other outer

effects.

Optical Fiber– Physical Characteristics


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Applications (benefits):

• Larger capacityHigh transmission speed can be achieved (2 Gbps in10x km).

• Smaller size and weight• Smaller attenuation

The attenuation is smaller, and it is constant at a wide frequency

range.• Electromagnetic isolation

Not sensitive for outer electromagnetic effects, there is no crosstalk. Does not emit energy, thus it can not be intercepted. It is difficult to tap an optical fiber.

• Larger repeating distanceSmaller the number of repeaters is, smaller the possibilities of errors and the costs are.Technology keeps developing. Eg. 3.5 Gbps transmission speed over 318 km without any amplifier (AT&T, 1990).

Optical fiber- Advantages


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Applications• Big city trunks• Long-haul country trunks• Telephone center trunks• Subscriber loops• Local area networks


• Works in 1014 - 1015 Hz (infrared) domain.

• 3 versions are used:• multi mode• single mode• multi mode graded index

Possible light sources:• LED

• Laser diode.

Optical Fiber


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Multi mode fiber

Light rays coming with different angles from light source are reflecting in

different angles at the boundary of the two materials, thus they travels different

distances during different time. So light impulses will be distorted. Consequently,

the data transmission speed is decreasing.

Single mode fiber

Decreasing the diameter of core, only rays paralell with fiber axis will pass through

the fiber. Light impulses will not be distorted, higher data transmission speed can

be achieved.

Multi mode graded index fiber

The refractive index of core material is increasing when the radial distance from

the fiber axis become larger. Due to this property the light rays will be focused.

The characteristics of this type can be placed between the two other types.

Types of Optical Fibers


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Types of Optical Fibers

Single mode

Multi mode graded index fiber

Multi mode

Wavelength and diameter of fiber 3 – 10 μm

Optical detectorOptical source


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Propagation and detection of electromagnetic signals are performed by antennas.

The two ways of broadcasting:• Directed• Omnidirectional (not directed)

In directed case the antenna radiates a focused electromagnetic ray. The receiver antenna should be positioned very precisely.

The omnidirectional radiation can be received with multiple antennas.

Signals with higher frequency can be focused better.

Three frequency ranges can be used for wireless transmission:• 2 - 40 GHz (microwave transmission) (directed)• 30 MHz - 1 GHz (radio frequency) (omnidirectional)• 3 1011 - 2 1014 Hz (infrared)

Wireless Transmission


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WAN Technologies


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WAN Cabling Technologies

• Serial connections: provide reliable, long-distance communications • ISDN: For dial-based on demand services or spare connections• DSL: For achieving a T1/E1 (1,544 - 2,048 Mbit/s) speed via telephone line • Cable modem connection: Cable providers use the coaxial TV-cables. The

coaxial cables are appropriate for achieving high-speed connections. These speeds can even be higher, than the speed of the DSL-accesses

• These technologies require specific transmission media and connectors.


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Technical Structure of ADSL

Subscriber side Network side

CPEFilter

ADSL NT

Computer POTS or ISDN Phone

ATM Backbone

DSLAM

Telephone center

Data network

Phone network PTSN


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WAN Technologies

WAN connectionSerial or smart-serial ports of router can be used.Connection based on clock signal:

Data Communication Equipment (DCE, provides clock signal eg. modem, CSU/DSU).

Data Terminal Equipment (DTE, eg. router)

ISDN concepts, equipementsTE1 Terminal Equipment 1

ISDN compatible device, connecting to NT1 or NT2.TE2 Non ISDN compatible device, connecting to TA.TA Terminal Adapter

Connects a non ISDN device to an ISDN network.NT1 Connects a 4-wire ISDN to a 2-wire ISDN (2-wire connects to CO).NT2 Connects different „subscriber devices” to NT1 (switching and connectivity)


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III. – Data Link Layer LAN Data Link Layer Solutions


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IEEE802

Logical Link Control

Medium Access Control

Physical

802.3 802.4 802.5

802.2

Network Layer

Transmission media

Physical Layer

Data link Layer ISOreference

model

802.2 = Logical Link Control Protocol802.3 = CSMA/CD802.4 = Token bus802.5 = Token ring

Media access protocols

The IEEE 802 protocol family

IEEE LAN Standards


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Ethernet (CSMA/CD)


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10BASE-T: transmission speed 10 Mbit/s, type of transmission is digital, T

refers to using a twisted pair). Maximum cable length is 100m.

10BASE-2: transmission speed 10 Mbit/s, type of transmission is digital, no. 2

refers to the maximum 200 of segment size. A 10BASE2 segment contains up

to 30 station. 10BASE-5: transmission speed 10 Mbit/s, type of transmission is digital, no. 5

refers to the 500 meters maximum distance of signals transmitted to.5-4-3-2-1 rule:

• Five segments of transmission media (max. 2500 meter).• Four repeater or hub.• Three segments for connecting stations.• Two connector segment without any station.• One large collision domain.

Ethernet – Physical Technologies


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Destination address

Preamble

Source address

Start of Frame

Pad (if necessary)

Data

CRC

Length/Type

7 bytes: 7 x ‘10101010’ (Synchronization)

1 byte: ‘10101011’

6 bytes: 1-3 bytes ID of manufacturer,4-6 bytes offset

6 bytes: 1-3 oktet ID of manufacturer,4-6 offset

2 bytes

0 - 1500 bytes

4 bytes

Direction of transmission

IEEE 802.3 / Ethernet frame format

0 - 46 bytes: minimum frame length is 64 bytes

Ethernet Frame Format


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Parameters of functionality

Transmission speed 10 Mbps (Manchester kódolás)

Slot time 512 bit-time

Gap time 9,6 sMax. no. of transmission frames 16

Jam size 32 bits

Maximum frame lenght 1518 byte

Minimum frame length 512 bits (64 bytes)

Destination address can be• Exact address of a station• All bits are ‘1’ : broadcast, message is sent for all stations.

Source address must not be a multiple address!

Ethernet


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Waiting for frame to be transmitted.Formatting the frame.

Is the chanel busy?

Waiting the gap time.Starting transmission

Is there a collision?

Complete transmission..

Indicating of reaching the maximum numbers of trials

Sending JAM signals.Increasing the number of trials.

Max. number of trialsis reached (16)?

Computing the delay and waiting a random time period.

IN

I

I

N

N

Functionality of MAC sublayer: transmitting frames

Ethernet Frame Transmission (CSMA/CD)


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Determining the time of repeated transmission of a frame:

The slot time or round-trip delay is twice the time it takes for the first bit of frame to travel the length of the maximum distance between two nodes. During this time tne nodes detect the collisions almost surely. (Cable delay: ~5μs/1000m.)

Slot time = 2 * (cable delay + repeaters delay)+ reserve timeSlot time = 51.2 μs (2 * (2.5 km + delay of 4 repeaters), transmission time of 512 bits)

Waiting time is the multiplication of slot time with a random integer, which depends on the number of transmission trials:

1 collision 1 Waiting randomly 0 or 1 slot time 2 collisions Waiting randomly 0, 1, 2 or 3 slot times3 collisions Waiting randomly 0, 1, 2 …7 slot times.

10 collisions Waiting randomly 0 – (210-1) slot times11 collisions - ” -. - “ -15 collisions - “ -

After 16 collisions the interface card does not try any more time. It indicates that the transmission failed.

Ethernet Frame Transmission


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Is there any incoming signal?

Indicating the busy state of chanel.Bit synchronization, waiting for the

frame delimiter.Reading the frame.

CRC and frame lengt are OK?

Forwarding the frame to a higherprotocol layer for processing

Discarding the frame.

NI

N

Functionality of MAC sublayer: receiving frames

Dest. addr = own addr.or a broadcast addr.?

I

I

N

Receiving an Ethernet Frame


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Goal of developement:• For 10 Base T Ethernet(IEEE 802.3) 10 times transmission speed,• Preserve the cabling system,• Keep the MAC method and the frame format.

Major part of 10 Base T networks were connetcted to repeater with cables less than 100m.

So the distance between two stations is at most 200m. In case of 100 Mbps transmission

speed the outermost stations can detect collision, too during the time of 512 bits data

transmission.

Thus, shortening the maximum lenghts the method of CSMA/CD MAC can be retained.

The standard:100BASE-TX can reach 100 Mbit/s transmission speed in half-duplex mode, and 200 Mbit/s in full-duplex mode.100BASE-FX separate transmit (Tx) and receive (Rx) lines together ensure 200 Mbit/s transmission speed.

Fast Ethernet (802.3u)


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100 Base X (100BaseTX, 100BaseFX)

It was designed for divverent media (X):• Category 5 unshielded (UTP) cable, • Category 5 shielded (STP) cable,• Optical fiber

The 4B5B (4B/5B) binary coding developed for FDDI networks was adapted for 100 Base X.Every 4 bits of data (nibble) is coded on 5 bits. Only those 5-bit symbols are used, where exist at most two adjacent ‘0’ bits.The guaranteed 2 bit signal transmit ensures a good bit synchronizing.

The 100BASE-TX version performs first 4B/5B coding, then performs shuffling and multi-level transmit, (MLT-3) coding.

Fast Ethernet (802.3u)


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Data symbols Control symbols

0000 111100001 010010010 101000011 101010100 010100101 010110110 011100111 011111000 100101001 100111010 101101011 101111100 110101101 110111110 111001111 11101

IDLE 11111J 11000K 10001T 01101R 00111S 11001QUIET 00000HALT 00100

4-bitdata group

5-bitsymbol

4B5B codes

4B/5B binary coding


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• 1000BASE-TX: – for Cat5e UTP cable(802.3ab). • A Cat5e cables can reliably reach up to 125 Mbit/s transmission speed.• For Gigabit bandwidth all of 4 wires is used .• Hybrid cirtuits are required, which enable duplex transmission on a single pair. Thus the

bandwidth increased up to 250 Mbit/s.• The required 1000 Mbit/s speed can be achieved by applying the four pairs.

• 1000BASE-SX: 850 nm laser or LED sources on a multi mode optical fiber. It is not so expensive and suitable for short distances.

• 1000BASE-LX: 1310 nm laser sources on a single or multi mode optical fiber. Using laser on a single mode optical fiber the signals can be transferred even a distance of 5000 m.

• Separate optical fibers are set for transmitting (transmit, Tx) and for receiving (receive, Rx). The connection originally has duplex characteristics.

Gigabit Ethernet (802.3ab, 802.3z)


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• At Ethernet transmisson, each transmission has to be at least the length of slot time.

• The slot times can be calculated by the largest regular network architecture and the longest allowed cable parts.

• For a 10 and 100 Mbit/s speed Ethernet the slot time is 512 bit ie. 64 byte• For a 1000 Mbit/s speed Ethernet the slot time is 4096 bit, ie.512 byte

• In case of collision detection a32 bit congestion signal has to be used• The minimum gap between two non-colliding frames is called Frame gap

• On a10 Mbit/s speed Ethernet network after sending a frame each node has to be wait at least the time of 96 bits (9.6 microsecundum)

• In case of collision, each node – waiting the time of frame gap– leaves the cable idle

Ethernet - Timing


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Ethernet Switching


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• A collision domain occurs when multiple computers are connected to the single, shared transmisson media (line).

• Devices in second layer divide the collision domains. These devices control the transmission of frames by Mac-addresses assigned to the Ethernet devices. Second layer devices are the bridges and switches.

• The devices in second and third layers do not transmit collisions. The collision domains are divided smaller domains by the third level devices, as well.

Ethernet - Segmentation


76

• A switch is essentially a fast speed bridge (in 2. layer) with multiple ports.• Each port forms a separate collision domain. (Eg. A 24-port switch forms 24

separate collision domains.).• Switches for each port stores the Ethernet addresses (Mac addresses) of the

accessible devices from that port in a table (switching table).• Switches upload and maintain their switching tables dynamically (based on the

source addresses of incomingframes).• The switching table is stored in a content-addressable memory (CAM). • A CAM memory works reverse way compared to a common memory: if you enter a

data (Ethernet address), the output will be the corresponding memory address. By the help of CAM, switches are able to find the port corresponding to a given MAC address without using any search algorythm.

Ethernet – Switches


77

The switch looks for the destination address of the incoming Ethernet frames in its switching table:

• If the destination address is a broadcast address (48 times ‘1’ bit value), then the frame will be transmitted on each port (except the incoming port).

• If the destination address can not be found in the switching table, then the frame will be transmitted on each port (except the incoming port).

• If the destination address can be found in the switching table, then the frame will be transmitted on the corresponding port (assuming, that this port is not identical with the incoming port of the frame).

Ethernet Switching


78

Switching methods:• Store-and-forward: Transmission of the frame will start

after the whole frame has been arrived. The switch recomputes the frame checksum (FCS). If the frame is corrupted, it is discarded.

• Cut-through switching: After arriving the destination address (6 bytes), the transmission of frame starts immediately on the outgoing port.

• Fragment free switching: After arriving a minimum frame length (64 bytes), the transmission of frame starts on the outgoing port. (Collided frames will not be transmitted.)

Ethernet Switching


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IV. – Network Layer


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The IP Network Protocol

IP (Internet Protocol) RFC 791• The network layer protocol of TCP/IP reference model.• Widely used, it is the basic element of Internet.• Most important characteristics:

– Structure of IP header.» Consists of 32-bit words.» Length: Minimum 5, maximum 15 words.

– IP addressing, address classes.– Fragment supporting.– Datagram services towards Transport Layer.– Ethernet frame type value: 0x0800.


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Structure of Internet Protocol

Whole lengthVersion IHLType of service

Fragment offsetIdentifier MF

DF

Header checksumUpper Layer

protocol TTL

Optional field(s)

Sender (Source) IP address

Receiver (Destination) IP address


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IP Addresses

• Identifies the node in Network Layer.• Dotted decimal notation eg. 157.45.190.57• Managing identifiers – InterNIC, IANA.• For organisations not unique addresses but address domains

(network identifiers) are assigned.• The first part on an IP address identifies the network, the

second part identifies the node (inside the network).• The IP routing based on the network identifiers.• How many bits should be in network IDs?

– If too small, the large domains will be unused.– Il too large, only small subnetworks can be handled.


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Classes of IP Addresses

Class C 1 Host #Network #

Bit# 1 21 8

Class A 0 Host #Network #

Bit# 1 7 24

0

1

Class B 0 Host #Network #

Bit# 1 14 16

1

1

1

1


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The Rule of the first byte

110

Leading bit(s)

C

Class

192 - 223

Value of 1st byte

0 A0 - 127

10 B128 - 191


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Network mask

Network mask (netmask):• A 32 bit mask, which contains bits with values of 1 in place

of network and subnetwork identifiers, and bits with values of 0 in place of host identifiers.

By the help of netmask the boundary of network-machine can be modified, which boundary originally was determined (static way) by the classes.

Prefix length:• The number of value 1 in the netmask (number of network

identifier bits in the netmask).


86

Default Network Masks

Network masks belonging to each classes:• Class A :

Network mask: 255.0.0.0 Prefix lenght: 8.• Class B :

Network mask : 255.255.0.0 Prefix lenght: 16.• Class C:

Network mask : 255.255.255.0 Prefix lenght: 24.


87

Special IP addresses

11111111.11111111.11111111.11111111 Broadcast on actual network

ID of the specific networkNetwork 00000000….00000000

01111111 Anything Loopback

00000000.00000000.00000000.00000000 Non specificated host

Broadcast on the specific networkNetwork 11111111….11111111


88

Data Link and Network Addressing Problems


89

In Network and Data Link Layers two independent address classes (IP addresses and Ethernet addresses) are considered.

• For encapsulation of Data Link Layer (forming an Ethernet frame) the physical address (MAC address) belonging to the IP address has to be determined.

• In certain cases (eg. Network boot, central IP address assignment) it could be necessary to determine the IP address by the help of Ethernet address.

Problems of Dual Address Classes


90

Network Address -> Physical Address (ARP)

ARP (Address Resolution Protocol) RFC 826• Each node records physical addresses belonging to the

network addresses in a table (ARP table).• How get a new data (pair of addresses) into the table?

1. Broadcasted ARP question: Who knows the physical address of the network address X?

2. Each node of of subnet receives and processes the frame of the question by a broadcast message.

3. If a node ‘identifies itself’ with network address X, sends an answer to the ARP question with own physical address.


91

Structure of ARP Frame

Type of protocol

Action code

Type of hardware

Lenght of net. add.

Physical address of destination

Physical address of sender

IP address of sender

Length of ph. add.

IP address of destination

IP address of destination

Words 1 - 2: General ARP header.

Words 3 - 6 : IPv4/Ethernet-specific data segment.

Ethernet frame type value: 0x0806

Physical address of sender

Physical address of destination


92

Physical Address -> Network Address (RARP)

RARP (Reverse Address Resolution Protocol) RFC 903• It is needed only in special cases (eg. network boot).• The RARP server(s) records the network addresses belongig

to physical addresses in a table (RARP table).• The table is maintained by the system administrator.• The assignment of physical address – network address is

static.• In case of more RARP servers, the same network address has

to be assigned to a physical address on each RARP server (the assignment must not depend on servers).


93

Physical Address -> Network Address (RARP)

RARP (Reverse Address Resolution Protocol) RFC 903

Scheme of functioning:1. RARP question: Who knows the network address of the physical

address X?

2. Each node of of subnet receives the frame of the question by a broadcast message.

3. The RARP servers process the question: If the physical address X is found in their tables, send an answer for RARP question with the network address found in tables.


94

Physical Address -> Network Address (BOOTP)

BOOTP (BOOTstrap Protocol) RFC 951• The RARP works within only one broadcast domain.• The BOOTP is an IP/UDP-based protocol, where the client

and the server could be in different broadcast domain.• Phases of boot based on BOOTP:

– Determine IP address.– Download Boot file (not investigated).

• Working scheme is the same as RARP.• BOOTP agent – boot support through a router.


95

Physical Address -> Network Address (DHCP)

DHCP (Dynamic Host Configuration Protocol) RFC 1531• Allows assignment of IP address domain.• In case of more DHCP servers, the handled address domains

should not overlep (in default).• Packet structure similar to BOOTP.• Clients get the IP address for a (renewable) time period.


96

Physical Address -> Network Address (DHCP)

DHCP (Dynamic Host Configuration Protocol) scheme of functioning :1. DHCP question: Who can give me an IP address? (DHCPDISCOVER)

2. Each node of of subnet receives the frame of the question by a broadcast message (DHCP relay agent).

3. A DHCP servers process the question: If there is a free IP address in the handled address domain, then send an answer to DHCP question with that IP address. (DHCPOFFER)

4. The client chooses one from the received DHCP answers, and sends a feedback of its choice to the corresponding DHCP server. (DHCPREQUEST)

5. The DHCP server „books” the choice of address (the address became occupied), and confirms client on booking. (DHCPACK/DHCPNAK)

DHCPDECLINE: The assigned IP address by the server is invalid (in use).

DHCPRELEASE: The client does not need for an IP address any more.


97

IP Subnets


98

IP Subnets

Why is it necessary to create subnets?• The logical functionality of the institute can be a reason.• On an IP network more than one broadcast domains (usually

with the same size) have to be created.

How can we create a subnet?• Some of the higher position bits of the host ID of the IP

address will be used identifying the subnet.• The new network-node boundary is denoted with the

network mask (longer prefix is used).


99

Subnets - Example

Example:• IP address of network: 197.45.112.0• Default subnet mask: 255.255.255.0• 3 bits are used identifying the subnet.• Network mask: 255.255.255.224• 8 subnets can be created.


100

Subnets - Example

Addresses of the subnets:

No. Network ID Host Addresses

1. 197.45.112.0 197.45.112.1-30

2. 197.45.112.32 197.45.112.33-62

3. 197.45.112.64 197.45.112.65-94

4. 197.45.112.96 197.45.112.97-126

5. 197.45.112.128 197.45.112.129-158

6. 197.45.112.160 197.45.112.161-190

7. 197.45.112.192 197.45.112.193-222

8. 197.45.112.224 197.45.112.225-254


101

Subnet Routing

197.45.112.64

197.45.112.96

197.45.112.32

197.45.112.35Destination IP address:

E2

E1E0

Routing table: Destination Netmask Int. Next hop Metric197.45.112.32 255.255.255.224 E0 0.0.0.0 0197.45.112.64 255.255.255.224 E1 0.0.0.0 0197.45.112.96 255.255.255.224 E2 0.0.0.0 0


102

Subnet Routing

197.45.112.64

197.45.112.96

197.45.112.32

197.45.112.35Destination IP address : &197.45.112.32

255.255.255.224Network mask:


103

IP Network Problems in ‘90s


104

The growth of Internet

90 January90 April90 July90 October91 January91 April91 July91 October92 January

92715251727206323382622308635564526

Routing table sizes of IP backbones


105

Depletion of Internet Addresses

Assignment status of classful addresses in 1992 (RFC 1466):

Total Assigned Assigned (%)

Class A 126 49 38%

Class B 16383 7354 45%

Class C 2097151 44014 2%

Short Time solution: CIDR


106


107

Problems of Classful IP Addressing- CIDR

The solution: CIDR (Classless Inter-Domain Routing) RFC 1519.• Assigning continuous class C addresses (instead of class B).• By the help of a network mask with a variable length the

network-machine boundary can be shifted with arbitrary number of bits (supernetting) or right (subnetting).

• Forming address domain zones based on locations.• Joined routing informations by the help of network masks.• Giving the mask representation of network addresses is

mandatory: <Network IP address, Network mask>


108

IP Address Domains of Continents

The class C IP addresses are assigned based on the continents (sizes of routing tables can be reduced significantly) RFC 1366,1466:

202.0.0.0 - 203.255.255.255Asia, Australia

Address domainContinent

194.0.0.0 - 195.255.255.255Europe

198.0.0.0 - 199.255.255.255North America

200.0.0.0 - 201.255.255.255Middle & South America


109

Long Time solution: NAT


110

NAT

• The major part of nodes take part „only” as clients in communication.

• It is sufficient to provide for clients possibilities of access to services for clients.

Application dependent solutions: Proxy, ALG.

Non application dependent solutions : NAT, PAT

RFC 1631. 2633, 2766, 3022.


111

NAT Basics

Address realm: Part of a network, where the uniqueness of IP addresses has to be ensured.

Public/Global/External Network: Address realm with address domain managed by IANA.

Private/Local Network: Address realm with local, organizational addressing.

10.0.0.0/8 172.16.0.0/12 192.168.0.0/16


112

NAT, Address Realms – Principle of Function

Global address realm

Local (private)address realm

Private IP addresses:• 192.168.0.0/16• 172.16.0.0/12• 10.0.0.0/8

Local (private)address realm

Addressinginformations


113

Server

NAT, Address Realms – Sending Packets

Global address realmLocal (private)address realm NAT Box

Sender

Data

ClientSender

Data

Clientprivate adr

Clientglob. adr

NAT table


114

Server

NAT, Address Realms – Sending Packets

Global address realmLocal (private)address realm NAT Box

Cél

AnswerClient

Destin.

Answer

Clientprivate adr

Clientglob. adr

NAT table

• IP address• IP addr + Port no.


115

NAT Resources

NAT solutions require resources (processor).•Searching address transformation tables.•Address exchange (port number exchange).•Recompute checksums.

There is no need to perform the computations for the whole PDU:

– „Substracting” the old addresses from the old checksums.– „Adding” new addresses to the obtained result.


116

IP - Routing


117

Routing Basics

Routing:• Make decisions on the directions of transmitting packets (IP

datagrams).

Routing table:• A table containing necessary informations for routing.

Typical (most important) fields:

Destination net Netmask Outgoing int. Next hop Metric


118

Routing Basics

Routed protocol:• A general data transmission protocol belonging to Network

Layer, which can be controlled by the router (eg. IP).

Routing protocol:• A protocol describing the propagation of necessary

informations (exchange between routers) for building up routing table(s) (eg. RIP, OSPF, BGP).


119

Routing Basics

Autonomous System (AS):• The routing administration unit of a network, where a

common routing strategy (routing protocol) works.

Metric:• Measuring method of the quality of a path resulted by a

given routing process. Basically there are two categories (can be transformed into each other):– Metric based on distance (cost).– Metric based on goodness.


120

Basic Operations of Routers – Fitting IP Addresses

1./ The rows of the routing table is ordered descending by prefix length. N=1.

• This ensures, that in case of more than one matching lines the one with the longest prefix will be the result.

2./ If the line N does not exist in the table, then there is no matching line.

• The router discards the packet. The algorythm is terminated.

3./ A Bitwise AND operation is performed on every bit of the destination address of the packet and the network mask of line N.

4./ If the result of this AND operation is identical with the address of destination network in line N, then the address matches line N.

• The router transmits the packet towards the direction in line N. End of algorythm.

5./ N=N+1, continue with Step 2.


121

Classification of Routing Configurations

Minimal routing:• Fully isolated (without router) network configuration

(eg. giving IP address and network mask).Static routing:

• The routing table is maintained by the system administrator (eg. giving default gateway).

Dynamic routing:• The routing tables are maintained by some routing protocol.

– Interior routing protocols (IGP - eg. RIP, OSPF).» The main principle is to determine the „best path” with a method of distance

vector or link state.– Exterior routing protocols(EGP - eg. EGP, BGP).

» The aim is not necessarily to determine the best path (policy based routing - BGP).


122

Distance Vector Routing


123

Distance Vector Routing

Basic principle of operation:• The routers keep records for all accessible destinations

(machine or network) how can a given destination be reached on the best path (direction and distance – distance vector).

• The routers exchange these information in a certain time period.

• Concerning the new information, the routers check whether there is any necessary modification regarding the previously known best path.


124

Distance Vector – Routing Table Problems

Problem of too small initial value: • If the optimal path is„corrupted” a path with larger cost

(longer) can not replace it.• Solution: Larger cost from the direction of optimal path

overrides the (lower) cost.

Problem of counting to infinity:• In certain cases the procedure responds very slow to

changing of topology.


125

Count to Infinity- Example

Consider the routing towards D.

Starting routing entries (optimal paths to D):• A: B,2• B: D,1• C: B,2

1

1

1

10

1

C

B

DA


126

Count to Infinity- Example

Consider changing of routing tables in case of corruption of link B-D:

A B,2 C,3 C,4 C,5 … C,10 C,11 C,11

B --- C,3 C,4 C,5 … C,10 C,11 C,11

C B,2 A,3 A,4 A,5 … A,10 D,10 D,10

1

1

10

1

C

B

DA


127

Routing Information Protocol - RFC 1058

Characteristics of Routing Information Protocol (RIP):• Distance vector based IGP protocol.• Old, but continuously developed and enhanced.• Metric: Number of hops (16=infinite distance).• It can be used in cases of optimal paths with maximum

length of 15 routers.• Propagate routing informations in every 30 seconds.• „Triggered update” for reducing the time of count to

infinity.• RIP V2 (RFC 1723) CIDR compatible.


128

Enhanced Interior Gateway Routing Protocol (EIGRP)

• It is the own distance vector based routing protocol of Cisco.• Multiple goals, flexible, scaleable.• Metric: compound (computed from 5 variables, can be

weighted):– bandwidth– delay– load– reliability– MTU


129

Enhanced Interior Gateway Routing Protocol (EIGRP)

Most important characteristics:• CIDR compatible.• The metric based on „Bandwith” in default.• Mechanism of exploring neighbours (avoid broadcast).• Handling count to infinity:

– Split Horizon, Holddown Timer, Triggered update.– Recording potential substitute paths.

• Propagating updates (not the whole table).• Integrated routing (can be used for multiple directed

protocol).


130

Link State Routing


131

Link State Routing

Scheme of functioning of Link State Routing:

1./ Exploring neighbours

2./ Measuring the costs (length) of the link to the neighbours.

3./ Creating packets containing the measurement results.

4./ Propagate the packets to all routers of the network unit.

5./ All routers know the topology of the network and can compute (eg. with Dijkstra algorythm) the optimal paths (spanning tree) to the other routers.


132

Link State Routing– Processes (IS-IS)


133

Open Shortest Path First - RFC 1131

Characteristics of Open Shortest Path First (OSPF):• Link State Routng IGP protocol.• New, recommended as default protocol since 90er years.• Using smaller network unit (area) than AS.• (Non disjunct) classification of routers:

– Routers working inside the area.– Routers working on boundaries of areas.– Routers working on backbones.– Routers working on AS boundary.

• Able to perform equal cost, multiple paths routing.• Using the „Service type” field of IP header.• Recent version: OSPF V2 (RFC 2328).


134

OSPF Areas

The base of the decision process (Dijkstra algorythm) is the area.

The areas form a „star pattern” with a special area – the backbone – in the center connecting the areas.

Tasks of routers on area boundaries are complex:• Separate decision process for all areas.• Summarizing informations get from areas.• Propagating the summarized informations to other areas.

Inter area routing:• Routing in source area as far as the boundary router.• Routing on backbone as far as the boundary router of the

destination area.• Routing in destination area to destination network.


135

Administration of OSPF Data

Most important items of an OSPF router table:• Type of destination (network, area boundary router, AS

boundary router).• Id. of destination (IP address).• Type of service.• Giving the path/paths towards destination:

– Type of path (intra-area, inter-area, AS-external).– Cost of path.– Next router (IP address, interface of access).


136

Transport Layer


137

Transport Layer Protocols

UDP - User Datagram Protocol RFC 768• Connection free, non-reliable Transport Layer protocol.

TCP - Transmisson Control Protocol RFC 793• Connection based, Összeköttetés alapú, reliable Transport

Layer protocol.


138

UDP

• The UDP (User Datagram Protocol) is the connection free transport protocol of the TCP/IP protocol set.

• Transmission of datagrams without any guarantee (without confirmation).

• Failure management is to higher level (applications) protocols.• The UDP protocol is suitable applications which do not need to

concatenate sequences of segments. Eg. TFTP, SNMP, DHCP, DNS .


139

Structure of UDP header

Destination port numberSource port number

ChecksumLength (byte)


140

PORT Numbers- Protocols (RFC 1700)

echo 7/tcp echo

echo 7/udp echo

ftp-data 20/tcp # File Transfer [Default D]

ftp-data 20/udp # File Transfer [Default D]

ftp 21/tcp # File Transfer [Control]

telnet 23/tcp telnet

telnet 23/udp telnet

smtp 25/tcp mail # Simple Mail Trans

smtp 25/udp mail # Simple Mail Trans

http 80/tcp # World Wide Web HTTP

http 80/udp # World Wide Web HTTP


141

TCP

• The TCP (Transmission Control Protocol) is the connection based transfer protocol of the TCP/IP protocol set. It provides a reliable (receipted) bit stream for applications.

• Before starting data transmission, the two nodes build up a TCP connection (TCP session) by the help of the so called three-way handshake.

• Information to be submitted is divided into segments (64KB).• Before transmission each segment gets a number. The sender keeps

record on each transmitted TCP segments, and requires a receipt.• The destination node reverts the original message from the segments, and

receipts the segment(s).• If a segment is missing, the TCP protocol ensures re-transmission of the

missing segment.


142

Structure of TCP Header

Source port number

No. of sequence (SEQ No.)

No. of acknowledgement(ACK No.)

Filling

DataOffset

URG pointer

Destination port number

ACK

PSH

RST

SYN

FIN

URG

Window sizeBusy

Checksum

Options


143

TCP – Three-Way Handshake

TCP Client TCP Server

0. CLOSED LISTEN

1. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED

2. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

3. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED

4. ESTABLISHED <-- <SEQ=301><ACK=102><CTL=ACK><DATA> <-- ESTABLISHED


144

Application Layer


145

Application Layer Protocol

The highest level protocol which ensures the communication required by the users via applications typically with client-server architecture).

The most common protocols:• Name-IP address assignment – dns• Web, www – http• File transfer – ftp, sftp• Remote login – telnet, ssh• E-mail– smtp; pop3, imap4


146

HTTP Protocol

• The HTTP (HyperText Transfer Protocol) is the protocol of web, which is the most quickly spreading and most widely used part of internet.

• It provides easy information access• We can navigate on World Wide Web by the help of hyperlinks.

(A hyperlink is an object on a web page pointing to an other web page.)

• The URL address (Uniform Resource Locator) necessary for this is contained by the web page, usually embedded in the HTML code.


147

HTTP Protocol

• The HTTP protocol is based on a client-server architecture. • The web browser is a client application. A web browser requires components

with client and server functionality, as well.• At first the web browser examines the protocol, from which it determines

whether an other program should be called or launched.• It determines the IP address of web server via DNS.• It builds up a session with web server via TCP.• The data sent to HTTP server give the name of the folder (and the HTML file)

containing the web page. • The server responds the request by sending to client the text, voices, clips, and

graphics defined on the HTML page.• The client-side browser composes files, displays the web page, and closes the

session.


148

Traffic Filtering – Access Control Lists

• Without filtering the whole traffic is enabled.• With traffic filtering you can control the traffic on a specified interface of

the firewall (router): what kind of traffic is enabled or disabled.• The control is described with a list (ACL):

– Condition1 specification / enable– Condition2 specification / disable– …

• In the control list conditions can be stated for IP addresses, protocols (TCP/UDP), and port numbers (80=http, 25=smtp).

• If a packet does not match any list items, the packet is discarded (implicit deny).

• The traffic filtering works independently of routing.


149

Wireless Network Technologies


150

Wireless Network Technologies

Local network technologies (WLAN, Wi-Fi):• Wireless extension of an organizational LAN.• Free frequencies for use (2.4 GHz, 5 GHz).

– Spreading as the light.– 2.4 GHz is near to resonance frequency of water!

• Ensures mobility on the organizational data communication network.

Provides long distance connection (GPRS, EDGE, UMTS).• Enables global network access.• Extension of mobile phone technology to data transmission.• Data (transmission) charges.


151

Bases of Wireless Communication

• Wave: A kind of changing, which results point-to-point (cyclic) energy transfer.

• Amplitude: Distance between the zero and the maximum signal height.

• Frequency (F): Number of cycles in one second.• Time of period (T): The time of one cycle T = 1/F.• Wave length ( ):l Distance between two identical signal height

values. C = Fl = 300 000 km/s (light speed)


152

WLAN Devices

Access Point, AP: Provides the access to a wired network (Internet) for clients (mobile devices).

Bridge: A device for connecting two wired LAN. The connection can be done in Data Link Layer.

Client adapter: Network card for wireless networks.

RF devices: antennas, cables, connectors.

Wi-Fi Certification!


153

Antennas

Antenna: A device developed for transmitting and receiving RF signals.The strength of signal transmission of an antenna can be controlled or uncontrolled.

Antenna gain: Changing the strength of signal (power) compared to isotropic antenna (theoretical point source of waves). The gain measures how efficiently the antenna focuses the radiated RF energy. (measure of unit: dBi)


154

Omnidirectional Antennas

Isotropic antenna: A theoretical point source of waves (0 dBi).

Dipol antenna: Rod shaped circle source of waves (2.2 dBi).


155

Directional Antennas

Yagi antenna:

An antenna equipped with RF Mirrors and elements providing direction for waves (12 dBi).


156

Directional Antennas

Yagi antenna:

An antenna equipped with RF Mirrors and elements providing direction for waves (12 dBi).

Parabolic antenna: An antenna equipped with a parabolic mirror and a head positioned in the point of focus (22 dBi).


157

WLAN Modes

Infrastructure: The mobile devices are connected to the (wired) organizational network via a radio access point (AP).

The mobile devices do not perform direct radio communication.

Ad-hoc: The mobile devices are connected directly each other via their own radio interface. In case of large numbers of machines it is not efficient.


158

WLAN Logical Architectures

BSS (Basic Service Set): Network environment working within a coverage of a radio interface (AP). Identifying the network environment a text identifier (SSID) is used.

IBSS (Independent BSS): Several independent BSS. It is used typically in Ad-hoc networks.

DS (Distributed Systam): Several BSS connected via radio or wired infrastructure.

ESS (Extended Service Set): Several BSS (with DS connection). The transit between BSSs is possible without any loss of network connections (Roaming).


159

Scheme of WLAN Transmission

Client AP

1. --> <Request to Send> -->

2. <-- <Clear to Send> <--

3. --> <DATA> -->

4. <-- <ACK> <--

The measured throughput is significantly less, than the theoretical bandwidth!


160

802.11b

• 13 (overlapping) channels (EU) with 5 MHz bandwidth on 2.4 GHz.

• Four different data transmission speeds with four different coding and modulating technologies:• 11 Mbps (least coverage)• 5.5 Mbps• 2 Mbps• 1 Mbps (largest coverage)


161

802.11g and 802.11a

802.11g:• New coding and modulating technology on 2.4 GHz (PBCC,

OFDM).• 54 Mbps data transmission speed.• Retain frequency (2.4 GHz) provides a backward

compatibility for 802.11b systems.

802.11a:• Technology working on 5 GHz (light-like propagation).• 54 Mbps data transmission speed.• Requires a separate RF unit (5 GHz).


162

Wireless Security

SSID: Not a protective factor (it is only an identifier, can be intercepted).

WEP (Wired Equivalent Privacy): RC4 encryption, tha standard specifies a 64-bit key, or in some manufacturer implementation a 128-bit key. Nowdays it is not safe, using free softwares for analyzing the trafic the key (even the 128-bit key) can be „obtained”.

MAC address filtering: Based on the MAC addresses of clients the connection can be enabled or disabled. Not specified in 802.11.

WPA (Wi-Fi Protected Access): A system, (partly) containing the (second generation) security elements of 802.11i (Extensible Authentication Protocol, EAP; Temporary Key Integrity Protocol, TKIP).


163

XP – The Most Important Network Commands


164

Writing and Renewing the IP Configuration

• ipconfig – Writes the IP address, netmask, and the default gateway (for all network cards).

• ipconfig /all – Writes the whole configuration (MAC address, address of DNS server, expiration dates).

• ipconfig /release – „Releases” the IP address (previously assigned by DHCP).

• ipconfig /renew – Renew (asking again) the IP address assigned by DHCP. Before this one submitting an ipconfig /release command is recommended.


165

Testing the Accessibility of a Node

• ping machine – Tests the accessibility of the given (by name or IP address) machine with 4 packets.

• ping –t machine – Tests the accessibility of the given machine with continuous packet sending (until pressing Ctrl+C).

• ping –a IP address – Beside testing it writes the DNS name belongigng to the IP address (Fully Qualified Domain Name).


166

Testing the Accessibility of a Node

• ping –l size machine – Tests the accessibility of the machine with packets containing bytes given with parameter of size.

• ping –i ttl machine – Adjust the TTL field of IP packets to ttl during testing. (Investigates, wheter the machine can be accessed through routers max. number of ttl.)


167

Exploring a Path to a Destination

• tracert machine – Writes the sequence of routers (hops) on a path leading to the machine (given with name or IP address). The DNS names of routers are displayed in the list (DNS names are backtracked from IP addresses).

• tracert –d machine – The IP addresses of routers are displayed in the list (faster, because DNS names are not backtracked from IP addresses).


168

Displaying and Modifying the Routing Table

• route print – Displays the routing table: destination network, netmask, gateway, connection (interface), metric.

• netstat -nr – Identical with the route print command.• route ADD destination MASK mask gateway METRIC metric – Inserts a new row into the routing table for the given destination network with the specified parameters (mask, gateway, metric). Despite of the occasionally displayed „Network database can not be open” error message the command can be executed normally (can be verified with the route print command).

• route DELETE destination MASK maszk – Deletes the given row (specified with the destination, mask parameters) of the routing table. Despite of the occasionally displayed „Network database can not be open” error message the command can be executed normally (can be verified with the route print command).


169

Displaying and Modifying the ARP Table

• arp –a – Displays the ARP table (a table, containing the IP address – MAC address assignment).

• arp –s IP address Eth. address – Assigns the given IP address with the Eth. Address inserting a new entry in the ARP table (creates a static ARP entry).

• arp –d IP address – Deletes the row belonging to the given IP address from the ARP table.


170

Writing NetBIOS Network Configuration

• nbtstat -n – Writes NetBIOS names of local machine.• nbtstat –a machine – Lists the NetBIOS nametable of

remote machine. Displays the Mac address of the given machine, as well.

• nbtstat –A IP address – Lists the NetBIOS nametable of remote machine with IP address. Displays the Mac address of the given IP address.

• nbtstat -r – Lists the names resolved by broadcasting or WINS.

• nbtstat -R – Deletes the list of resolved names by broadcasting.

Bibliography

Andrew S. Tanenbaum, David J. Wetherall: Computer Networks, 5th edition, 978-0132126953, Prentice Hall, 2010.

Fred Halsall: Computer Networking and the Internet, 5th edition, 978-0321263582, Addison Wesley, 2005.

William Stallings: Data and Computer Communications, 8th edition, 978-0132433105, Prentice Hall, 2006.

RFC Documents: http://www.rfc-editor.org


171