Networking Protocols, OSI (Open Systems Interconnection) Layers, Topologies & Transmission Media – Comprehensive Study Notes

Protocol Architecture

  • Layered structure spanning hardware (H/W) and software (S/W)
  • Supports data exchange across distributed apps (e-mail, file transfer, etc.)
  • Every layer offers its own rules (protocols)
  • Two dominant architectures
    • TCP/IP architecture
    • OSI model

Protocol: Definition & Key Elements

  • Protocol ≜ set of rules governing data communication
  • Specifies what, when, how information is exchanged
  • Three elemental facets
    • Syntax – structure / format of data
    • Semantics – meaning of each section of bits
    • Timing – when & how fast data is sent (e.g. 100 Mbps100\,\text{Mbps})
Syntax Details
  • Example bit layout
    • First 8 bits → sender address
    • Second 8 bits → receiver address
    • Remainder → message stream
Semantics & Timing Recap
  • Semantics interprets each field/value
  • Timing regulates speed, sequencing, and readiness between sender & receiver

Protocol Standards

  • Provide vendor-independent development models
  • Two categories
    • De facto (by fact)
    • Widespread use, not officially legislated
    • • Proprietary – wholly owned
    • • Non-proprietary – community/public domain
    • De jure (by law)
    • Legislated by recognized bodies, e.g.
      • ISO (International Standards Organization)
      • ANSI (American National Standards Institute)
      • IEEE (Institute of Electrical & Electronics Engineers)

OSI Model Overview

  • ISO framework covering all aspects of network comms → Open Systems Interconnection (OSI)
  • “Open” ⇒ any two systems can interoperate, independent of underlying H/W or S/W
  • OSI is a model, not a protocol; goal = flexible, robust, interoperable architectures
  • Seven ordered layers (bottom→top)
    1. Physical
    2. Data Link
    3. Network
    4. Transport
    5. Session
    6. Presentation
    7. Application
Peer-to-Peer Process & Interfaces
  • Inside one host: each layer uses services of the layer below
  • Across hosts: layer x communicates with its peer layer x using a matching protocol
  • Interface between adjacent layers defines services/information offered upward

Layer-by-Layer Functions

1 Physical Layer
  • Transmits individual bits between nodes
  • Responsibilities
    • Physical characteristics of interfaces/media (type of cable, connectors)
    • Bit representation & signal encoding (electrical / optical)
    • Data rate definition (bits per second)
    • Line configuration (device ↔ medium attachment)
    • Physical topologies (ring, star, etc.)
    • Transmission modes: simplex, half-duplex, full-duplex
2 Data Link Layer
  • Node-to-node frame delivery
  • Core functions
    • Framing (bit stream → frames)
    • Physical addressing (source/destination MAC)
    • Flow control (sender ≯ receiver rate)
    • Error control (detect/retransmit damaged/lost frames; trailer fields)
    • Access control (shared-link arbitration)
3 Network Layer
  • Source-to-destination packet delivery across multiple networks
  • Converts frames ↔ packets
  • Functions
    • Logical addressing (IP) unchanged across networks; physical may vary
    • Routing (path determination for packets)
4 Transport Layer
  • Process-to-process / end-to-end message delivery
  • Responsibilities
    • Service-point (port) addressing
    • Segmentation & reassembly (sequence numbers)
    • Connection control: connection-oriented vs connectionless
    • Flow & error control end-to-end
    • Congestion control (network load management vs flow control, which is receiver-centric)
5 Session Layer
  • Dialog control & synchronization between hosts
  • Modes: half-duplex vs full-duplex dialogs
  • Synchronization via checkpoints (e.g., every 100 pages in a 2000-page transfer → resend only from last checkpoint after failure)
6 Presentation Layer
  • Syntax/semantics of info exchanged
  • Services
    • Translation (character-set interoperability)
    • Encryption / decryption (plain ↔ cipher text)
    • Compression (multimedia bit-reduction)
7 Application Layer
  • Direct user services & network access
  • Example OSI services/protocol families
    • X.500 – directory
    • X.400 – message handling
    • FTAM – file transfer & management
    • NVT – network virtual terminal

TCP/IP Architecture

  • Internet’s native stack, predates OSI; layers don’t align 1-to-1
  • Layer grouping (top→bottom)
    • Application (e.g., NFS, SMTP, FTP, TELNET, DNS, SNMP, TFTP, RPC)
    • Transport (TCP, UDP)
    • Internet / Network (IP, ICMP, IGMP, ARP, RARP)
    • Data Link
    • Physical
  • Protocols relatively independent; can be mixed & matched per system need

Network Topologies

  • Physical/logical arrangement of links; geometric relationship of all comms links
  • Six major types
    1. Mesh
    • Every device connects point-to-point to every other device
    • Channels required: n(n−1)2\frac{n(n-1)}{2}; e.g. n=5⇒10n=5 \Rightarrow 10 links
    • Pros: eliminates traffic bottlenecks, robust, private & secure, fault isolation
    • Cons: extensive cabling, complex installation
    1. Star
    • Dedicated links from devices to a central hub/controller
    • Data routed via hub
    • Pros: cheaper than mesh, fewer cables, robust (node failure ≠ network failure)
    • Cons: hub is single point of failure; every device must connect to hub
    1. Tree (Hierarchical star variation)
    • Nodes attach to a central (active) hub; secondary (passive) hubs branch further
    • Pros: scalable (more devices per controller), allows prioritized communication
    • Cons: hub failure collapses network; complex install
    1. Bus
    • Multipoint “backbone” cable with drop lines & taps
    • Pros: easy install, minimal cabling, redundancy-free
    • Cons: hard to reconfigure (add/remove nodes), backbone fault stops all traffic
    1. Ring
    • Each device links to two neighbors forming a closed loop; unidirectional signal flow, repeaters per node
    • Pros: simple reconfiguration, fault isolation
    • Cons: uni-direction; one node failure breaks ring
    1. Hybrid – combinations (e.g., star-bus, star-ring)

Transmission Media

Guided vs Unguided
  • Guided (wired): signal confined to a physical path
    • Twisted Pair (UTP, STP)
    • Coaxial Cable
    • Optical Fiber
  • Unguided (wireless): signal propagates through air (radio, microwave, infrared, etc.)
Guided Media Details
Twisted Pair Cable
  • Two insulated copper conductors twisted together
  • UTP (unshielded) vs STP (shielded)
    • UTP: 2-pair (RJ-11) or 4-pair (RJ-45); easy, flexible, cheap; up to 100 m; lower bandwidth; less interference protection
    • STP: metal foil/braid shield; prevents EM noise & crosstalk; higher capacity; costlier & manufacturing-complex
Coaxial Cable
  • Central copper conductor, dielectric insulator, outer metallic shield, plastic jacket
  • Uses BNC connectors; bandwidth high (10 Mbps traditional Ethernet); long-distance telco & CATV; high noise immunity
Optical Fiber
  • Glass/plastic core surrounded by cladding
  • Propagation modes
    • Multimode step-index
    • Multimode graded-index
    • Single-mode
  • Bandwidth > 2 Gbps2\,\text{Gbps}; reach ≈50 km w/o regeneration
  • Pros: light weight, immune to EMI, high speed, analog & digital
  • Cons: expensive, installation expertise, unidirectional (need 2 fibers for duplex)
Unguided Media Details
Radio Waves
  • Omnidirectional; penetrate buildings; used indoor/outdoor; AM/FM, cordless phones, paging
  • Unicast, multicast, broadcast capabilities
  • Bluetooth: short-range (2.4–2.48 GHz) radio technology for personal devices
Microwaves
  • Line-of-sight, narrowly focused; GHz range; hundreds Mbps per channel
  • Cannot penetrate buildings; antennas must be aligned
  • Terrestrial microwave
    • 2–40 GHz; parabolic dish towers ≤30 mi apart; susceptible to interference
  • Satellite microwave
    • Space relay ~3600 KM3600\,\text{KM} geosynchronous orbit (rotational match with Earth)
    • Earth station → satellite (uplink ~6 GHz) → amplified → downlink (~4 GHz) to another station
    • High manufacturing/launch cost; weather-dependent
Infrared (IR)
  • 300 GHz – 400 THz; inexpensive optical transceivers
  • Short-range (~1 km indoor); cannot penetrate walls; high bandwidth; sun IR causes interference outdoors