Data Communication and Computer Networking



Possible references:


Topic objective: The students should master the understanding of computer networks as layered, with rough knowledge of important aspects of respective layers. 


Data communication between computers can be seen as layer-structured.

Computers are networked through data communication.



The standard seven-layer OSI network model

1. Physical

Concerns transmission of unstructured bit stream through physical media; handles mechanical, electrical (+ optical), functional dan procedural characteristics for physical medium access

2. Data link

Provides reliable information transfer across physical links; transmits data blocks (frames) with the required synchronization, error control dan flow control

3. Network

Provides higher layers which are independent of data transfer technology and switching used for connecting systems; responsible for establishing, maintaining, and terminating connections

4. Transport

Provides reliable and transparent data transmission between two end points; provides end-to-end error recovery and flow control

5. Session

Provides control structure for communication between applications; establishing, managing, and terminating connections (sessions) between cooperating applications

6. Presentation

Provides application processes with independence from differing data representations (syntax)

7. Application

Provides access to OSI environment to users and also provides distributed information services




Data Communication


Data transmission

large bandwidth better digital signal higher data rate

defects -

signal attenuation with distance:  loss (in dB) = 10 log10 P1/P2

            attenuation distortion

            delay distortion


Transmission media:


Data coding

            Digital data, digital signal – various coding schemes: NRZ-L, NRZI, Bipolar-AMI,…

            Digital data, analogue signal – amplitude shift keying, frequency shift keying, phase shift keying

            Analogue data, digital signal – pulse code modulation: sampling and digitizing

            Analogue data, analogue signal – amplitude modulation, frequency modulation, phase modulation


Digital data communication

asynchronous transmission – for every character sent, there must be a start bit and a stop bit

synchronous transmission – character block/bit sent without start and stop code

error checking – parity checking, cycle redundancy checking (generalization to division by more than 2 in parity checking)

error correction – retransmission, or forward error correcting code (which reduces data rate at least to 50%)

interface – characteristics: mechanical (concerns physical connection), electrical (concerns voltage levels, timing), functional (concerns functions carried out), procedural (concerns sequence of events in transmitting data); example: EIA-232-D, RS-422-A, etc.


Data link control

            fulfills the requirements:

frame synchronization – start and end of frame should be clear

usage of various line configurations – point-to-point, multipoint, simplex, half duplex, duplex

flow control – at rate which is acceptable to recipient

error control – error correction due to transmission

addressing – node identity on multipoint line

control and data in same data link – need to differentiate between control and data

connection management – initialization, maintenance, termination require coordination and cooperation



frequency division

time division



Data communication network


Switched network:       Circuit switching – dedicated communication path between sender and receiver provided through network nodes as long as required – telephone system usually

Packet switching – data transmitted as sequence of packets, each directed independently through network nodes – computer network usually, well-known protocol: X.25

            issues: routing, traffic control

Broadcast network:      radio; satellite


LAN: Local Area Network – communication network limited to a small area e.g. in one building or in several nearby buildings


MAN: Metropolitan Area Networks – medium area communication network e.g. a town area


WAN: Wide Area Network – large area communication network e.g. a country


Network topology: ring, bus, tree, star


usetap (taps information from/to communication bus), repeater (amplifies signal online), hub (amplifies signal online and distributes it to other lines), bridge (bridges two subnetworks with similar protocol, and controls which signals are to be transferred across; operates on level 2 of OSI model), router (device between subnetworks possibly of different protocols, and routes packets to appropriate subnetworks; operates on level 3 of OSI model and uses protocol possessed by each router and host)




Computer Communication


Protocol and architecture

OSI layered model



SNA (Systems Network Architecture) IBM – also 7 layers


TCP/IP (Transmission Control Protocol/Internet Protocol) – hierarchical more than layered; importance given to inter-(possibly different) network connections; connectionless service (datagrams sent without agreed connection); approach to management functions as high protocol




TCP/IP Protocol Set





Transaction Service








Presentation Service






Data Flow Control







Transmission Control






Path Control


Internet (IP)


Network Access


Data Link


Data Link Control






Physical Control






            IP protocol used





Transport protocol

the gist of computer communication – end-to-end connection

Data broken up at initial end to data packets, which are sent (possibly through different paths) and rebuilt at final end.


2 main types:

connection oriented [flow control, error control, sequential transmission - reliable] (e.g. TCP)

connectionless or datagram service [more robust if reliability can be sacrificed for speed (e.g. in inwards data collection, outwards data distribution, demand-response (shared server to several distributed users), real-time applications (e.g.. voice, video transmission) ) – no flow control etc] (e.g. UDP)


Structure of TCP & UDP packets:

                                                    ←   32 bits wide   →

Source address

Destination address



Length of segment






User data




Format of TCP header:

Source port

Destination port

Sequence number

Acknowledgement number

Data offset










Immediate pointer

Option                                                                    Padding


Format of UDP header:

Source port

Destination port





Session protocol and service

– user-oriented connection service:

One session can be supported on one transport connection, also several sessions can be multiplexed onto one transport connection, or one session can be broken up among several transport connections

3 dialog modes: synchronous duplex, half duplex and simplex

e.g. recover lost data through backup by recovery unit


TCP/IP architecture – no specific session layersome session tasks carried out by TCP


Presentation facilities


[User application:       TCP/IP – specific application e.g. SNMP, FTP, SMTP, TELNET

                                    OSI – presentation layer and service elements in application layer]


A universal language for definition of representations: ASN.1 (Abstract Syntax Notation One) – based on module which contains list of type assignments, values and macro definitions


Network applications

Sit in the application layer & supported by presentation layer directly in OSI model (in TCP/IP, application depends on TCP or UDP)

e.g.      SNMPv2 (version 2) (can operate on TCP/IP or OSI)

            FTAM (File Transfer, Access, and Management) (OSI standard)

            X.400 (electronic mail standard by CCITT)





integrated services digital network (voice, data, multimedia,..)

- user interface standard

- implentation as set of digital switches and paths (with time-division multiplexing) (not analog space-division multiplexing using frequency)

- supports various types of traffic

- architecture, protocol


Broadband ISDN (B-ISDN) – architecture and protocol for ISDN with higher data-transmission rate than main rate for ISDN


Frame relaylike packet-switching but without error control (no longer needed with efficient modern networks) – can reach 2 Mbps compared to 64 kbps for packet switching.

Flow and error control if any, only at end-to-end and not hop-to-hop.


Cell relay – (ATM – asynchronous transfer mode)  - like frame relay but with packets of fixed lengths (‘cells’) (53 octets: 5 octets header, 48 octets data) (lessen processing requirement) – can reach 100-1000s Mbps; data rate for each channel can also defined dynamically


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