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Unit 1 of 3 · M.Sc IT Sem 2

Unit 1: Network architecture and medium access

Computer Networks notes · PTU syllabus (PGCA1910)

3 min read10 topics10 exam questions
On this page
  1. Unit summary
  2. Uses, goals and applications of computer networks
  3. Network structure and architecture
  4. The OSI reference model
  5. The TCP/IP reference model and comparison with OSI
  6. Static and dynamic channel allocation
  7. ALOHA protocols
  8. Carrier sense multiple access (CSMA)
  9. CSMA with collision detection (CSMA/CD)
  10. Collision-free protocols
  11. Networking devices
  12. Key terms
  13. Quick revision
  14. Important questions

Unit summary

Networks are built in layers and share channels among many stations. This unit covers the uses, goals and applications of networks, network structure and architecture, the OSI and TCP/IP reference models and their comparison, static and dynamic channel allocation, ALOHA, CSMA and CSMA/CD, collision-free protocols, and networking devices — repeaters, bridges, routers, gateways and switches.

After this unit you can

  • Explain the uses, goals, structure and architecture of networks
  • Compare the OSI and TCP/IP models
  • Explain channel allocation and multiple access protocols
  • Explain the functions of networking devices

PTU syllabus topics

  • Uses
  • goals and applications of computer networks
  • network structure and architecture
  • OSI and TCP/IP reference models and their comparison
  • static and dynamic channel allocation
  • ALOHA protocols
  • CSMA and CSMA/CD
  • collision-free protocols
  • networking devices (repeaters, bridges, routers, gateways, switches)
ComparisonNetworking devices
Layer
Job

Repeater

Physical

Regenerates signals

Hub

Physical

Broadcasts to all ports

Bridge / switch

Data link

Forwards by MAC address

Router

Network

Forwards by IP address between networks

Gateway

All layers

Connects different protocols

1

Topic 1

Uses, goals and applications of computer networks

ClassificationUses of computer networks
Networks

  • Business applications

    Resource sharing, client–server systems, email, video conferencing, e-commerce

  • Home applications

    Internet access, social media, streaming, online learning, smart homes

  • Mobile users

    Phones, laptops, navigation, mobile banking, IoT

  • Social issues

    Privacy, security, misinformation, digital divide

Key termsGoals of networking
Resource sharing
Printers, files, internet connection
High reliability
Alternative sources and backups
Cost saving
Many cheap computers instead of one mainframe
Scalability
Add capacity gradually
Communication medium
Fast exchange among people
2

Topic 2

Network structure and architecture

  • Structure: hosts (end systems) connected through a subnet of transmission lines and switching elements (routers). Classified by transmission technology — broadcast (one shared channel) and point-to-point — and by scale — PAN, LAN, MAN, WAN and internetworks.
  • Network architecture: a set of layers and protocols. Each layer offers services to the layer above through interfaces, and peer layers on different machines communicate using protocols.
ProcessLayered communication
  1. 1Application data handed down
  2. 2Each layer adds its header (encapsulation)
  3. 3Physical layer sends bits
  4. 4Receiver's layers strip headers in reverse order
  5. 5Application receives the data
  • Design issues for layers: addressing, error control, flow control, multiplexing, routing, segmentation and reassembly, security.
3

Topic 3

The OSI reference model

The OSI (Open Systems Interconnection) model, from ISO, divides communication into seven layers.

LayerFunctionExample
7 ApplicationNetwork services to usersHTTP, FTP, SMTP, DNS
6 PresentationTranslation, encryption, compressionJPEG, SSL/TLS, ASCII
5 SessionEstablishes, manages and ends sessionsNetBIOS, RPC
4 TransportEnd-to-end delivery, segmentation, error and flow controlTCP, UDP
3 NetworkLogical addressing and routingIP, ICMP, routers
2 Data linkFraming, MAC addressing, error detectionEthernet, switches
1 PhysicalTransmission of raw bitsCables, hubs, signals

Exam tip

Mnemonic (top to bottom): "All People Seem To Need Data Processing".

4

Topic 4

The TCP/IP reference model and comparison with OSI

The TCP/IP model is the practical model used by the internet. It has four layers: Application (OSI layers 5–7), Transport, Internet (network) and Network Access (data link + physical).

ComparisonOSI vs TCP/IP
OSI
TCP/IP

Layers

7

4

Developed by

ISO, as a reference model

US Department of Defense, from real protocols

Approach

Model first, protocols later

Protocols first, model describes them

Use

Teaching and reference

Used on the internet

Transport

Connection-oriented and connectionless

TCP and UDP

5

Topic 5

Static and dynamic channel allocation

ComparisonChannel allocation
Static allocation
Dynamic allocation

Method

Fixed share for each user — FDM or TDM

Channel given to stations as they need it

Suits

Few users with steady traffic (telephone trunks)

Many users with bursty traffic (LANs)

Weakness

Wasted capacity when users are idle; new users cannot join

Collisions or coordination overhead

  • Assumptions of dynamic allocation: independent stations, a single shared channel, collisions detectable, continuous or slotted time, carrier sense or no carrier sense.
6

Topic 6

ALOHA protocols

ComparisonPure and slotted ALOHA
Pure ALOHA
Slotted ALOHA

Transmission

Any time a frame is ready

Only at the start of a time slot

Vulnerable period

Two frame times

One frame time

Throughput

S = G e^(−2G); maximum 18.4% at G = 0.5

S = G e^(−G); maximum 36.8% at G = 1

Clock synchronisation

Not needed

Needed

  • Developed at the University of Hawaii (1970) for radio links between islands; if a frame collides, the sender waits a random time and retransmits.
7

Topic 7

Carrier sense multiple access (CSMA)

ComparisonCSMA variants
Behaviour when the channel is busy
Trade-off

1-persistent

Keep sensing; send as soon as idle

Collisions when several wait

Non-persistent

Wait a random time before sensing again

Fewer collisions, longer delay

p-persistent (slotted)

When idle, send with probability p; otherwise defer to next slot

Balances delay and collisions

8

Topic 8

CSMA with collision detection (CSMA/CD)

ProcessCSMA/CD (wired Ethernet)
  1. 1Sense the carrier
  2. 2If idle, transmit; if busy, wait
  3. 3Listen while transmitting
  4. 4On collision, send a jam signal and stop
  5. 5Wait a random time (binary exponential backoff) and retry
ComparisonCSMA/CD and CSMA/CA
CSMA/CD
CSMA/CA

Used in

Classic wired Ethernet (IEEE 802.3)

Wireless LANs (IEEE 802.11 Wi-Fi)

Approach

Detect collisions and recover

Avoid collisions before they happen

Method

Listen while sending; jam and backoff

Wait IFS, random backoff, optional RTS/CTS, ACK for each frame

Why

Collisions can be detected on a cable

Wireless stations cannot hear collisions (hidden station problem)

  • Minimum frame size: in CSMA/CD the frame must last at least two propagation delays so a collision is detected while sending — 64 bytes for 10 Mbps Ethernet.
9

Topic 9

Collision-free protocols

Key termsCollision-free protocols
Bit-map protocol
Contention slots — each station sets its bit to reserve; stations then send in order
Token passing
A token circulates; only its holder sends (Token Ring, FDDI)
Binary countdown
Stations broadcast their addresses bit by bit; highest address wins
Limited-contention protocols
Adaptive tree walk — contention among smaller groups under load
10

Topic 10

Networking devices

ComparisonNetworking devices
OSI layer
Function

Repeater and hub

Physical

Regenerate and repeat signals to all ports

Bridge

Data link

Connects two LAN segments; filters frames by MAC address

Switch

Data link (layer-3 switches also route)

Multi-port bridge; forwards frames only to the destination port

Router

Network

Connects different networks; chooses paths by IP address

Gateway

Up to application

Translates between different protocols or architectures

  • Collision and broadcast domains: a hub forms one collision domain; each switch port is its own collision domain; routers separate broadcast domains.

Key terms

Protocol
Rules governing communication between peer layers
Channel allocation
Sharing a channel among users
ALOHA
Random access protocol without carrier sensing
Token passing
Collision-free access using a circulating token
Bridge
Device connecting LAN segments at the data link layer

Quick revision

  • Uses and goals; broadcast and point-to-point; PAN to WAN.
  • Layers, services, interfaces, protocols; design issues.
  • OSI seven layers; TCP/IP four layers; comparison.
  • Static vs dynamic allocation; pure (18.4%) and slotted (36.8%) ALOHA; 1-, non-, p-persistent CSMA; CSMA/CD.
  • Bit-map, token passing, binary countdown; repeater, hub, bridge, switch, router, gateway.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.State four goals of computer networks.
  2. Q2.What is encapsulation?
  3. Q3.Give the maximum throughput of pure and slotted ALOHA.
  4. Q4.Distinguish 1-persistent and non-persistent CSMA.
  5. Q5.What is binary countdown?
  6. Q6.Distinguish a bridge and a router.

Long-answer questions

  1. Q1.Explain the uses, structure and architecture of computer networks.
  2. Q2.Compare the OSI and TCP/IP reference models.
  3. Q3.Explain the ALOHA and CSMA protocols.
  4. Q4.Explain collision-free protocols and networking devices.

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