Computer Networks
Vít PÁSZTO
17.3.2022
Computer Networks
• Set of technologies allowing transfer and exchange of
information among computers
• Allows users to communicate (following some rules) in
order to share and exchange data (resources/information)
• History goes back to the second half of 20th century (US
Air Force WAN for semi-automatic ground envi (radar))
• Internet (WWW), storage services, printers/faxes
(peripherial devices), email, instant communication
(messengers, VoIP and others)
Computer Networks
• History goes back to the second half of 20th century (US
Air Force WAN for semi-automatic ground envi (radar))
ARPANET
• The first „modern“ computer network (1969)
• U.S. Ministry of Defence (ARPA) – Larry Roberts
• Testing network for packet switching
• No central unit because of destruction of any part
• Four initial members:
– UCLA, SCRI (Stanford Central Research Institute), UCSB
(University of California Santa Barbara), University of Utah
• First packet sent on 29th Oct 1969
• Network Control Protocol (NCP)
ARPANET
4
13
29
40
113
0
20
40
60
80
100
120
1969 1970 1972 1973 1983
No. of nodes
nodes
ARPANET
• 1973 – Europe joined the network
– Norway and UK („emails“)
– 1976 – Queen Elizabeth used the network
• 1983 – Separation of ARPANET  MILNET  68 nodes
lost from ARPANET
• 1983 – NCP replaced by TCP/IP
• Establishments of LANs (NSFNET)  ARPANET as
backbone network
• Disconnected in 1990
Computer Network
• Set of technologies allowing transfer and exchange of
information among computers
• Allows users to communicate (following some rules) in
order to share and exchange data (resources/information)
• History goes back to the second half of 20th century
• ARPANET – first „modern“ computer network
• Internet (WWW), storage services, printers/faxes
(peripherial devices), email, instant communitation
(messangers, VoIP and others)
Computer Network
• Set of technologies allowing transfer and exchange of
information among computers
• Allows users to communicate (following some rules) in
order to share and exchange data (resources/information)
• History goes back to the second half of 20th century
• ARPANET – first „modern“ computer network
• Internet (WWW), storage services, printers/faxes
(peripherial devices), email, instant communitation
(messangers, VoIP and others)
Computer Network
• Set of technologies allowing transfer and exchange of
information among computers
• Allows users to communicate (following some rules) in
order to share and exchange data (resources/information)
• History goes back to the second half of 20th century
• ARPANET – first „modern“ computer network
• Internet (WWW), storage services, printers/faxes
(peripherial devices), email, instant communitation
(messangers, VoIP and others)
Computer Networks
• Computers
– Multi-users with limited communication
• Computer network
– Computers are connected and cooperating
– They keep their identity
• Distributed operating system
– Cluster of computers, which is compact unit
– Inner structure is transparent to the user
Computer Networks
• Sharing resources
– Data sets and files (programs, data)
– Technical resources (peripheries, capacities)
• Communication
– Between users – email, messaging, social
networks
– Between programs – distributed apps
• Higher reliability
– Back-ups
– Redundant infrastructures
• Costs savings
– Network of rather small computers is cheaper
then one super-computer
– Nowadays we have computer clusters and grids
Computer Networks
Computer Networks - classification
• Switching
• Node connection
• Signal type
• Geographical scale
• Property/ownership
Computer Networks – classification (Switching)
• Circuit switched network
– Oldest technology (telegraphy, telephony, ISDN)
– Connection established for two nodes
– Communication channel open as long as the „call“
lasts (bit stream)
– Connection via operator‘s exchange/switchboard
– The link remains reserved even if no communication
– Virtual circuit switched network
Computer Networks – classification (Switching)
• Circuit switched network
–cons & pros:
- reduced communication speed, comm. between 2 nodes,
set up
+ „low“ acquisition, running costs, reliability (fast, errorless)
Computer Networks – classification (Switching)
• Circuit switched network
Computer Networks – classification (Switching)
• Packet network
– Most of data is transmitted in packets
– Packet is formatted unit of data (~10B to kB long)
– Packet = control information and data (payload)
– Topology and active network nodes (routers,
switches, usually FIFO)
– Basis for the Internet communication
– Used in mobile-phone comm. (GSM vs. GPRS/XG)
Computer Networks – classification (Switching)
• Packet network
–Bandwidth is shared among users (the packet route
is not known „a priori“)
Computer Networks – classification (Switching)
• Packet network
– Most of data is transmitted in packets
– Packet is formatted unit of data (~10B to kB long)
– Packet = control information and data (payload)
– Bandwidth is shared among users (the packet
route is not known „a priori“)
– Topology and active network nodes (routers)
– Base for the Internet communication
– Used in mobile-phone comm. (GSM vs. GPRS/3G)
Computer Networks – classification (node conn.)
• Peer-to-peer (P2P)
– all nodes are equal (no central coordination)
– all computers can share their resources (memory,
peripheries, files etc…)
– common for data sharing/exchange
– + more users --> overall available speed increases
– - (i)legality – authors rights, attacks („infections“, spams,
viruses, Troys), terrorism
– example – BitTorrent, Napster (music), DC, Bitcoin…
Computer Networks – classification (node conn.)
• Client-server
– One (or more) computer (servers) is superior to
another one (or more) computer (clients)
– Server offers services to others „common“
computers (workstations)
– example – file, email, print, WWW server etc.
Computer Networks – classification (signal type)
• Analogue network
– Works with analogue (continuous) signal
– Amplitude and frequency
– Example:
• you speak to handset – change in air pressure – collected
by a handset -- amplified and then converted into current,
or voltage fluctuations:
Computer Networks – classification (signal type)
• Analogue
Computer Networks – classification (signal type)
• Digital network
– Works with digital (discrete) signal pulses (1/0)
– All techniques valid with analogue signal
(amplifying, filtering etc.) could be applied on digital
– Analogue signal is chopped to form digital one
– advantages - ?
Computer Networks – classification (signal type)
• Digital signal
– sampling - continuous-valued discrete-time signal
– quantization (replaces each sample value by an
approximation selected from a given discrete set)
– digitalization
Computer Networks – classification (signal type)
Feature Analog Characteristics Digital Characteristics
Signal Continuously variable, in both
amplitude and frequency
Discrete signal, represented as either
changes in voltage or changes in light levels
Traffic measurement Hz (for example, a telephone
channel is 4KHz)
Bits per second (for example, a T-1 line
carries 1.544Mbps, and an E-1 line
transports 2.048Mbps)
Bandwidth Low bandwidth (4KHz), which
means low data transmission
rates (up to 33.6Kbps) because of
limited channel bandwidth
High bandwidth that can support highspeed
data and emerging applications that
involve video and multimedia
Computer Networks – classification (signal type)
Feature Analog Characteristics Digital Characteristics
Network capacity Low; one conversation per
telephone channel
High; multiplexers enable multiple
conversations to share a communications
channel and hence to achieve greater
transmission efficiencies
Network
manageability
Poor; a lot of labor is needed
for network maintenance and
control because dumb analog
devices do not provide
management information
streams that allow the device to
be remotely managed
Good; smart devices produce alerts,
alarms, traffic statistics, and performance
measurements, and technicians at a
network control center (NCC) or network
operations center (NOC) can remotely
monitor and manage the various
network elements
Network capacity Low; one conversation per
telephone channel
High; multiplexers enable multiple
conversations to share a communications
channel and hence to achieve greater
transmission efficiencies
Computer Networks – classification (signal type)
Feature Analog Characteristics Digital Characteristics
Power requirement High because the signal contains a
wide range of frequencies and
amplitudes
Low because only two discrete signals—the
one and the zero—need to be transmitted
Security Poor; when you tap into an
analog circuit, you hear the voice
stream in its native form, and it is
difficult to detect an intrusion
Good; encryption can be used
Error rates High; 10
–5
bits (that is, 1 in
100,000 bits) is guaranteed to
have an error
Low; with twisted-pair, 10
–7
(that, is 1 in 10
million bits per second) will have an error,
with satellite, 10
–9
(that is, 1 in 1 billion per
second) will have an error, and with fiber,
10
–11
(that is only 1 in 10 trillion bits per
second) will have an error
Computer Networks – classification (geo scale)
• PAN
• LAN (WLAN)
• MAN
• WAN
• others – HAN (Home), SAN (Storage), CAN
(Campus), GAN (Global), RAN (radio) VPN,
Ambient network
Computer Networks – classification (geo scale)
• PAN – Personal area network
• Using devices such as mobile phone, PDA,
laptop/notebook, tablet, smart watches, printers,
video games consoles etc.
• Transfer via USB/FireWire, IrDA, Bluetooth
• Short distances (few meters – up to 10 meters)
Computer Networks – classification (geo scale)
• PAN – Personal area network
• Using devices such as mobile phone, PDA,
laptop/notebook, tablet, smart watches, printers,
video games consoles etc.
• Transfer via USB/FireWire, IrDA, Bluetooth
• Short distances (few meters – up to 10 meters)
Computer Networks – classification (geo scale)
• LAN – Local area network
• smaller scale (room/building/block of buildings)
• own cabling and equipment (e.g. optical fiber,
active network nodes,…)
• runs at 10 Mb/s to 10 Gb/s
• low error rate (wired networks)
• formerly for resources sharing
• example: MVSO + Tesco SW
Computer Networks – classification (geo scale)
• MAN – Metropolitan area network
• covering the whole city/town (or several blocks)
• operated by one institution (company or local
authority), but…
• … cabling and network equipment is rent/leased
• technically – more LANs wirelessly connected
• example: DQDB – up to 100 km (two nodes max
2km), up to 155 Mb/s
Computer Networks – classification (geo scale)
• WAN – Wide area network
• broad area – up to international scale
• leased cabling and infrastructure (optical fibres,
microwave channels, satellites)
• great variety of bandwidth (65 kb/s to 100 Gb/s)
• formerly for remote access and communication
among users
• WAN is being replaced by VPN
Computer Networks – classification (geo scale)
Computer Networks – classification (ownership)
• PDN – Public data network
– operated by a telecommunications administration,
or a recognized private operating agency
• PN – Private network
– uses private IP address space, „private“ packets
cannot be transmitted via public Internet
• VPN – Virtual private network
– connection to private computers via public network
– establishes access to intranet (security issues)
Computer Networks – classification (others)
• Ambient network
– combination of PAN, LAN/WLAN and mobile
networks
– concept of „not disconnected“ communitation
(joining networks) --- EU FP6
• Example:
Laptop → (Bluetooth) → Mobile → (GPRS) → mobile
signal provider → Internet
PDA → (bluetooth) → Laptop → (WLAN) → AP
Computer Networks – classification (others)
• Ambient network
– combination of PAN, LAN/WLAN and mobile
networks
• HAN – Home area network
– communication among home digital devices
• SAN – Storage area network
– connection to storage capacity of various devices,
which appear like locally attached to the operating
system
Computer Networks – topology
• Arrangement of various elements in computer
network (mainly cabling and active components)
• Basic types:
– Point-to-point
– Bus network
– Star network
– Ring network
– Tree network
– Mesh network
• one transmitting medium
• data are sent in one direction (async.)
• if a single node is damaged – all network
transmission ceased
• every station receives all network traffic
• equal transmission priority
• + easy to connect, less cabling, small net
• - if something breaks, all net. collapse
Computer Networks – topology (Bus network)
• with one central station
• used for WLAN
Computer Networks – topology (Bus network)
• every node (computer) is connected
to another two (to form a circle)
• data have to pass through every single
node between start/end node
• + easy-to-transmit, minimal delay
time, no packets collision, highest
bandwidth
• - if failure, then problem
Computer Networks – topology (Ring network)
• most used network topology
• computers are connected to the
central point (hub or switch)
• one link between central point and
computer
• + if failure of one computer, it works;
no packets collision, easy to expand
• - many cables, special hw (central p.)
Computer Networks – topology (Star network)
• combination of Bus and Star
• broad area networks
• in most cases B-tree types
• + scalable, point-to-point connection,
easier fault identification and isolation
• - many cables, hard to maintain, if
backbone fails, the entire network falls
Computer Networks – topology (Tree network)
• connection among nodes
• no central feature
• represented by a general graph
• shortest path bridging
• + scalable, lower sensitivity to failures,
decentralized
• - ???
Computer Networks – topology (Mesh network)
Computer Networks – classification
VIDEO
ISO/OSI model
Computer Networks – former heterogeneity
• Experimental networks (ARPANET, CYCLADES,
SNA from IBM, DNA from DEC)
• Need for mutual inter-connection
• Layer architecture (main ideas):
– individual layer for different abstraction degree
– each layer should provide exactly specified functions
– interface should minimize data-flow
– number of layers should be optimized
Computer Networks – ISO OSI model
• Reference model OSI (Open Systems Interconnection)
– Specified by ISO (International Organization for Standardization)
– Needed for standardization of internet communication
– ISO 7498 (y. 1984); ITU-T (X.200)
– Layer-scheme describing the communication
– Layers are independent and substitutable
– Model objective = standards development
– Implementation is not specified – but principles
– Main aim = decomposition of the communication
Computer Networks – ISO OSI
• Example:
– Communication between companies‘ management
– From UP to BOTTOM
– Individual layers have communication „interface“
Computer Networks – ISO OSI
• Example:
– Communication between companies‘ management
Computer Networks – ISO OSI
• Example (simplified):
– Communication between two entities (people)
Computer Networks – ISO OSI
• Layer provides services to a higher-level layer
• Layer does not „bother“ higher layer
• Data are sent in packets and in each layer is enriched
with new information (formatting, addresses etc.)
• If any layer is not active = it is null/transparent layer
• No layer can be skipped
• Communication:
– Vertically -> via interface
– Horizontally -> via protocols
Computer Networks – ISO OSI
ISO OSI – Communication
• Entities:
– are active objects of a layer
– set of entities = layer
– communicate with other system entities
– carry functions and offer services (and consume services
from preceding layer)
– function = providing specified service
– interact directly only with neighbouring layers‘ entities
– example (at lower layers) – hardware devices (I/O ports)
ISO OSI – Entities
• Protocol:
– set of rules for communication of entities in the same layer
• Service:
– for higher-level layer; using lower-level layer
– offered in Service Access Points (SAP) having their addresses
– interaction:
• request (request for lower-level layer service)
• confirm (no comment)
• indication (info for higher-level layer to cause some action)
• response (user reaction/action-end to indication)
ISO OSI – Protocols & Services
• Protocol:
– set of rules for communication of entities in the same layer
ISO OSI – Protocols & Services
• Services:
– Connection-oriented services
• connection – transmission – termination
• connection is to be identified, both sides are communicating
• example – telephony (classic, ISDN) or virtual connections (ATM)
– Connectionless services
• data are sent as a packets with end-point addresses (more than 1)
• multicast (group of end-points) or broadcast (anyone could get it)
• packets are independent to each other
• example – link layer (LAN), IP protocol
– Reliable (confirmation messages) or unreliable services
ISO OSI – Services
ISO OSI – Encapsulation
• ISO OSI vs. Internet (TCP/IP)
Computer Networks – Network architecture
Computer Networks
Vít PÁSZTO
Network Layer
• Guarantee physical tasks
– transforms bits into physical signal (and vice versa)
• Ensures electrical properties of a network:
– voltage, frequency, modulation, speed, timing,
synchronization, coding etc.
• And also mechanichal properties of connectors:
– shape, size and connectors
• Works with signal => via hardware
• All about physical elements in a network
Physical layer
• Activation, sustaining and deactivation physical
connection for bits transmission
• Half or full duplex
• Services:
– open/close physical connection
– bits arranging into serial current
– error messages for link layer
• example: RS232, V.35
Physical layer
• What is it about?
– modems
– standards for communication
– ISDN, DSL
– cabels
– transmission technologies (Bluetooth, IrDA etc.)
– connectors
– active nodes
Physical layer
•
Physical layer
• Modem (http://goo.gl/SriaSc) - V.21, V.22, V.22bis,
V.26bis, V.27ter, V.32, V.32bis, V.34, V.90, V.92, V.44
• Group of EIA standards – RS-232, RS-485 and RS-422
• ISDN (Integrated Services Digital Network)
• DSL (Digital Subscriber Line) / ADSL
•
Physical layer
ITU Kb/s
V.32 9,6
V.32bis 14,4
V.34 28, 8
V.34+ 33,6
V.90
56 (switchboard to
modem)
33,6 (modem to
switchboard)
Physical layer
Physical layer
Physical layer
• T1, E1 (multiplex digital data transmission)
• SONET/SDH (Synchronous Digital Hierarchy) – optical
transmission
• 10Base2 (thin Ethernet) and 10Base-T (twisted pair cable)
Signal Speed
E0 64 kbit/s
E1 2,048 Mbit/s
E2 8,448 Mbit/s
E3 34,368 Mbit/s
E4 139,264 Mbit/s
Physical layer
Physical layer
Physical layer
Physical layer
• Bluetooth
• IEEE 802.11
• FireWire
• IrDA
• USB
Physical layer
Physical layer
connectors
USB 1.0
1996
USB 2.0
2001
USB 2.0
Revised
USB 3.0
2011
USB 3.1
2014
USB 3.2
2017
USB4
2019
Data rate
1.5
Mbit/s
(Low
Speed)
480 Mbit/s
(High Speed)
5 Gbit/s
(SuperSp
eed)
10
Gbit/s
(SuperSp
eed+)
20
Gbit/s
(SuperSp
eed+)
40
Gbit/s
(SuperSp
eed+)
USB camera Lumix, * mini USB, * type B, * female type A, * type A
Physical layer
Pin Color
Signal name
Description
A B
Shell N/A Shield Metal housing
1 Red VBUS Power
2 White D−
USB 2.0 differential pair
3 Green D+
4 Black GND Ground for power return
5 Blue
StdA_SSRX
−
StdB_SSTX
− SuperSpeed receiver
differential pair
6 Yellow
StdA_SSRX
+
StdB_SSTX
+
7 N/A GND_DRAIN Ground for signal return
8 Purple StdA_SSTX−
StdB_SSR
X−
SuperSpeed transmitter
differential pair
Physical layer
USB 3.0 host, VGA connector, DisplayPort connector, USB 2.0 host.
Physical layer
• HDMI
• High-Definition Multi-media Interface
• uncopressed transmission (video and sound)
Physical layer
• Repeater:
– signal correction (amplification), one input – one output
– if different cables = transceiver
• Hub:
– signal is multiplied (multiport repeater)
– collision detection
– sending signal to all (no adressing)
– has been replaced by switches (but it is „faster“ than switch)
Physical layer - homework
• 3 slides presentation about one of the technologies here:
LoRa
Low-voltage differential signaling
Mobile Industry Processor Interface physical layer
Modulated ultrasound
Optical Transport Network (OTN)
SMB
SONET/SDH
SPI
T1 and other T-carrier links, and E1 and other E-carrier
links
Telephone network modems — V.92
TransferJet physical layer
USB physical layer
Varieties of 802.11 Wi-Fi physical layers
Visible light communication co-ordinated under IEEE
802.15.7
X10
1-Wire
ARINC 818 Avionics Digital Video Bus
Bluetooth physical layer
CAN bus (controller area network) physical layer
DSL
EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
Etherloop
Ethernet physical layer Including 10BASE-T, 10BASE2,
10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T,
1000BASE-T, 1000BASE-SX and other varieties
G.hn/G.9960 physical layer
GSM Um air interface physical layer
IEEE 802.15.4 physical layers
IEEE 1394 interface
IRDA physical layer
ISDN
ITU Recommendations: see ITU-T
I²C, I²S
Network Layer
Data-link layer
• Differences:
– Data link layer -> larger data-blocks transfer -> frames
– Physical layer -> bit transfer
– Data link layer -> bits interpretation (distinguish what is info
part – header – and what is the data itself payload)
– Physical layer -> does not recognize bits (just sending them)
Data-link layer
• Main task:
– packet preparation for transport
– access control to medium
• Creates data frames and receives data from network layer
• Creates connection/interface between programme processes and
physical devices (IN/OUT)
• When data goes through different types of network (LAN vs.
WAN) -> data frame is modified and adjusted to a specific network
(no change of content)
Data-link layer
• Access control to a medium using several methods
• Source node (network device connected with transport
medium) -> Network interface controller (NIC) -> using
router to pass in between networks (e.g. LAN to WAN)
• Router unpacks a data frame (gets network adresss) ->
makes routing decision -> pack a data into different data
frame -> sends it to WAN
Data-link layer
• Data-link layer adds a header and footer to a data:
– Info about devices
– Timing informaiton
– Transmission errors
– Which nodes will be communicating next time
Data-link layer – data frame
• Data are sent in 1/0 (bits)
• Destination device recognizes Start and End delimiter
(special sample of 1/0)
• Data frame structure is changing with a network type
• Typical fields (excluding a data/payload):
– Start and End
– addresses (MAC)
– data type in a frame
– control field
Data-link layer – data frame
Control count
Payload
Data-link layer – data frame
Data frame – header
• Start flag
• Physical address (MAC) of destination device
– Set by a manufacturer (1st three) + uniques address
• MAC is important within one LAN
• If data frame is transmitted out -> router -> reads IP a
generates MAC of the opposite device
• After receiving a data frame, node reads MAC
addresses and decides, if A) receives a data frame or B)
discard a data frame
Data frame – header
• If A), data frame is unpacked, modified (using
protocols) and passed to higher level layer (network
layer)
• If a data frame is not addressed to a receiving
node, it could be passed via broadcast to others
• This is typical for LANs -> MAC is in the form of
FF:FF:FF:FF:FF:FF
• Header also contains info about a data type
Data frame – footer
• Info about a data frame status:
– if a data frame arrived with no change or errors
• Control field = FCS (Frame Check Sequence)
• Source node makes a logical count on sending data
= CRC (Cyclic Redundancy Check), which is stored in
FCS
• When data arrives, CRC is executed and is
compared with FCS
• If a count does not match -> a frame is discarded
Data-link layer – Protocols
• Most typical – Ethernet, PPP (Point-to-Point), ATM
(Asynchronous Transfer Mode), HDLC etc.
• Every protocol is for specific network (e.g. Ethernet and
IEEE 802 LAN; PPP and HDLC for WAN)
• Umbrella organizations ISO, IEEE, ANSI, ITU-T
• Protocols working on sw as well as on hw (network
devices)
• More protocols = more methods of medium access
control
Access control to shared medium
• Deterministic method
– Every nodes has set an access time (on network)
– Access is controlled and ruled = during transmission, no
other node can transmit -> transmission link (network)
might not be used effectively
– e.g. Token Ring
Access control to shared medium
•Non-deterministic method (stochastic method)
– nodes are „competing/fighting“ of transmission to shared medium
(I have something to send, so I try)
– In order not to overload a link -> CSMA (Carrier sense multiple
access)
– node is listening whether there is transmission -> waits or sends
– but transmission collision/conflict in the same moment => collision
and discard all data frames
CSMA/CD and CSMA/CA
• CSMA addition of CD (Collision Detection) and CA
(Collision Avoidance)
• CSMA/CD:
– Ethernet, collision detection; device is waiting, then sends a
data frame. If collision, then resends
• CSMA/CA:
– WLAN, tries to avoid collision; node listens and if OK, sends an
information that the node is about to transmit => avoids
collision
– But there could be a collision of information message
Access control to non-shared medium
• There is no need to avoid collision (example PPP)
• Protocol decides to send a data in modes:
– simplex
– half-duplex
– full-duplex (PC-PC, PC–switch, switch-switch)
• In case of full-duplex and UTP cabel (100 Mbps
Ethernet) one is receiving, second is transmitting (both in
the same moment)
• Before transmission – „agreement“ of what mode will
be used
Ethernet
• Technology/protocol used on LANs
• Many types according to speed (10 Mbps - 10 Gbps)
• Ethernet data frame is similar in all cases
(differences are in the data placement system)
Standard 802.2 and 802.3, method CSMA/CD -> frame
has to contain source and destination address (MAC)
• On TCP/IP networks - protocol Ethernet II
Ethernet
• Using IEEE 802.3 there are 4 Ethernet variants:
– 10 Mbps – Ethernet 10Base-T
– 100 Mbps – fast Ethernet
– 1 Gbps – gigabite Ethernet
– 10 Gbps – 10 gigabite Ethernet
• Values stands for maximal theoretical bandwidth
• Transfer medium – optical or metallic fibres/cabels
• Many connectors for data transmission
Wireless transmission
• Radio and microwave frequency
• Used in open space (air transmission)
• Could be used by third-parties => security and
encryption
• Advantages & Disadvantages
• Not so fast (max. hundreds of Mbps)
• 4 basic types:
– 802.11 (WLAN) - 802.16 (WiMAX)
– 802.15 (WPAN) - GSM (using GPRS protocol)
Wireless transmission
• 802.11 (WLAN)
– Wi-Fi, local networks, CSMA/CA
– sends a frame -> waits for confirmation -> if does not
come, sends a frame again
– supports authenticity (verification of communicating
sides), association with access-point and transmission
security/encryption
• 802.15 (WPAN)
– Wireless PAN, typically Bluetooth
– Transmission range from 1 to 100 meters
Wireless transmission
• 802.11 (WLAN)
– Wi-Fi, local networks, CSMA/CA
– sends a frame -> waits for confirmation -> if does not
come, sends a frame again
– supports authenticity (verification of communicating
sides), association with access-point and transmission
security/encryption
• 802.15 (WPAN)
– Wireless PAN, typically Bluetooth
– Transmission range from 1 to 100 meters
Wireless transmission
• 802.16 (WiMAX)
• Worldwide Interoperability for Microwave Access
• Broadband wireless internet access
• Outdoor networks, supplement to Wi-Fi (which is more or less
indoor)
• Range up to 50 km
• GSM (Global System for Mobile Communication)
• Digital transmission (voice call, SMS, MMS, …)
• protocol GPRS
• Range up to hundreds km (depending on antennas type and
height, terrain, etc.)
Wireless transmission
• Structure:
– ad hoc networks (peer to peer)
– access point (client-server type networks)
Wireless transmission – security issues
• Protection from „eavesdropping “ or unauthorized
use
• For user – there‘s no need to be connected via wirebased
devices (switch, HUB, router, …)
• It is sufficient to be in WiFi range
• Protection possibillities:
– MAC addresses filtering (list of permitted MACs)
– transmission encryption
Wireless networks - encryption
• WEP (Wired Equivalent Privacy)
– from 1999, authenticity of workstations and transmission
encryption, algorithm RC4 and security key of 40 or 104
bits + 24 bit initiation vector
– vulnerability of RC4 => is not safe anymore
• WPA (WiFi Protected Access)
– substitution of WEP, in 2002, RC4 again but + 128 bit key and 48 bit
initiation vector
– fundamental improvement via dynamic key (TKIP)
– hard to break through
Wireless networks - encryption
• WPA 2 (WiFi Protected Access 2)
– in 2004
– protocol CCMP (used for 802.11i)
– AES encryption (Advanced Encryption Standard)
– high security
– needs higher computer performance => older devices
could not work with WPA2
Network Layer
Network Layer
• Is responsible for:
– routing and adressing in the network
– connection among networks (that are not adjacent)
– packet forwarding
• Allows to bridge different properties of networks
• More „intelligent“ networking layer
• It needs to distinguish individual computers:
– using IP addresses (in case of the Internet)
Network Layer
• Creates a virtual homogeneous network that has:
– unified way of addressing
– unified way of packet‘s format
– unified way of transferring service (nor reliable
connection-less datagram service)
• Information unit = packet
• Typical active network device/node is router
• Protocols:
– IP (v4, v6), ICMP, ARP, DDP, Ipses, IPX, many more
Network Layer
• IP address (v4):
– symbolic address of a computer
– identifies a computer in a network
– 32 bit number = four decimal numbers from 0 – 255
delimited by „.“
– each decimal number represents 8 bits
– example: 213.145.55.12
• IP address has two parts – first identifies a
network, second identifies a computer
Network Layer
• It is not allowed to use all ones (decimal 255) or
zeros (decimal 0)
• These addresses are decicated for special purposes:
– all ones = broadcast
– all zeros = for LAN
• Addresses 127.x.x.x are for loopback (localhost)
• How to convert decimal numbers to binary:
• e.g.: 192.168.252.111 is …
Network Layer
http://www.converter.cz/baster/baster.php
Transport Layer
• Provides transfer and communication between endnodes
(end-to-end, host-to-host)
• The aim is to provide quality of service that is
desired by higher level layers
• It provides:
– transparency, reliability, flow control, multiplexing
• Equals different properties and quality of networks
• Converts transport addresses to network ones (but
doesn‘t care about routing)
Transport Layer
• TCP (Transmission Control Protocol):
– provides reliable, ordered, and error-checked delivery
data between applications („no packet could be lost“)
– information unit = TCP segment
– transmission of files, e-mails, webpages etc.
• UDP (User Datagram Protocol):
–a simple connectionless transmission (minimalize
protocols usage)
– it is used in cases, where error checking and correction is
not necessary (or performed by the application)
Transport Layer
Feature Name UDP UDP Lite TCP Multipath TCP SCTP DCCP RUDP
Packet header size 8 bytes 8 bytes 20–60 bytes 50–90 bytes 12 bytes 12 or 16 bytes 6+ bytes
Transport layer packet entity Datagram Datagram Segment Segment Datagram Datagram Datagram
Connection oriented No No Yes Yes Yes Yes Yes
Reliable transport No No Yes Yes Yes No Yes
Unreliable transport Yes Yes No No Yes Yes Yes
Preserve message boundary Yes Yes No No Yes Yes Yes
Ordered delivery No No Yes Yes Yes No Yes
Unordered delivery Yes Yes No No Yes Yes Yes
Data checksum Optional Yes Yes Yes Yes Yes Optional
Checksum size (bits) 16 16 16 16 32 16 16
Partial checksum No Yes No No No Yes No
Path MTU No No Yes Yes Yes Yes Unsure
Flow control No No Yes Yes Yes No Yes
Congestion control No No Yes Yes Yes Yes Unsure
Explicit Congestion Notification No No Yes Yes Yes Yes
Multiple streams No No No Yes Yes No No
Multi-homing No No No Yes Yes No No
Bundling / Nagle No No Yes Yes Yes No Unsure
Session Layer
• Aim is to organize and synchronize dialog between
cooperating session layers
• Provides opening, closing and managing of session
connection
• Provides:
– authentication (identity check)
– authorization (access to files, operations, functions etc.)
– session restoration
• Protocols: AppleTalk (ADSP, ASP), RPC, SSL, SPDY, …
Presentation Layer
• Aim is to transform data into a form, which is used
by applications („syntax“ layer)
• Provides codes and alphabet conversions, graphic
order modification, Byte order adjustment and so on
• Deals with data structure but not with
interpretation
• Provides – encryption, conversions, compression,…
• Protocols: TLS, XML, Telnet, ASCII, MIME, MPEG etc.
Application Layer
• Aim is to allow applications to access
communication system in order to enable
cooperation among applications
• Shared protocols and interface methods
• Protocols:
– FTP, SMTP, DNS, DHCP, HTTP, IMAP, POP3, BitTorrent,
Bitcoin
– and much more…
Application Layer
• Supports applications and end-user processes
• Everything is application-specific
• Services as: File transfers, Emails
• Telnet and FTP entirely in this layer
•Examples: www browsers, NFS, SMTP, POP, HTTP,
DNS, FTP, Telnet
Computer Networks
Vít PÁSZTO
End…