Understanding Multi-Layer Network Models: OSI & Internet

Multi-Layer Network Models

Most Important Network Models: OSI & Internet

Open Systems Interconnection Model (OSI)

Created by ISO as a framework.

7 Layers

Internet Model

Created by DARPA in 1970.

Made to solve problems of internetworking.

5 Layers

Based on Transmission Control Protocol (TCP/IP)

OSI Model Layers

7 – Application Layer: Set of utilities used by application programs.

6 – Presentation Layer: Formats data for presentation to the user.

5 – Session Layer: Initiates, maintains, and terminates each logical session between sender & receiver.

4 – Transport Layer: End-to-end issues, segmenting the message for networks, maintaining logical connections between sender & receiver.

3 – Network Layer: Making routing decisions.

2 – Data Link Layer: Message delineation, error control.

1 – Physical Layer: Defines how individual bits are formatted to be transmitted.

Internet Model Layers

5 – Application: Used by application programs.

4 – Transport: Establishes end-to-end connections, translates domain names into numeric addresses, segments messages.

3, 2, 1: Same as OSI.

Message Organization Terminology

(Layer, Organization)

(L) Transport = (O) Segment, Network = Packet, Data Link = Frames, Physical = Bits

Data Encapsulation

Data = Application

UDP Header, UDP Data = Transport

IP Header, IP Data = Internet

Frame Header, Frame Data, Frame Footer = Link

Layer 2: Ethernet

In charge of error-free data flow from one network node to another.

Organizing data frames sent to Layer 1 for transmission.

Adds Cyclic Redundancy Check (CRC) for data.

2 Sub-layers: Logical Link Control (LLC), Media Access Control (MAC)

LLC – Establishes/Maintains Links Between Talking Devices

Multiplexes Layer 3 protocol to Layer 2.

IEEE 802.2 standard.

All devices establish Logical Link Control (LLC) on the same network segment or LAN.

MAC – Provides Shared Access to Network Media Directly to the Adapter

Common standards: 802.3 (CMA/CD Ethernet), 802.4 (Token Bus), 802.5 (Token Ring), 802.12 (Demand Priority).

MAC is the address of the network adapter at the Data Link Layer.

LAN switches operate at this layer.

Basic Media Access Methods

  • Controlled: Determines if communication is okay, wired Ethernet.
  • Polling: Checks each device in turn if it wants to talk (Token Ring).

Error Control: Errors, Causes, and Prevention

  • White Noise: Movement of electrons – signal increase.
  • Impulse Noise: Sudden increase in electrons – protect wires.
  • Cross Talk: Guard band too narrow, increase guard/shield.
  • Echo Signal: Reflection, tune equipment.
  • Attenuation: Loss of signal from blockage, clear obstacles.
  • Intermodulation: Signals from circuits.

Error Control Detection

  • Parity Checking: Simple, has a parity bit (odd & even). Odd = odd, even = even.
  • Checksum: Data sent is summed, each byte is added, values must be correct or there’s an error.
  • Cyclic Redundancy Check (CRC): Check value, uses sophisticated equations, many versions, works well.

Ethernet Example: Concept Overview

NIC, Media

Ethernet = CSMA/CD, one device transmits at a time, the receiving device processes data.

NIC = All network devices access, requires a unique MAC address, transmits bits at Layer 1.

  • Repeater: Extends the length of the network, forwards data, collisions occur when two networks access one network.
  • Bridge: Used to combine Ethernet segments, listens for MAC addresses, forwards based on filter decisions.
  • Switch: Multiport bridge.
  • Hub: Multiport repeater, all data received on one port is retransmitted to all other ports.

Interconnection Devices

Repeater, Hub (Crossover Cables), Bridge, Switch, Router

Ethernet Address

  • IG 1 bit: Organizationally Unique Identifier (OUI) – 22 bits.
  • Unit ID: 3 bytes (24 bits).
  • IG: Group address, GL: Global/Local.

Manufacture ID is the Unit ID.

Address Types

  • Individual Station
  • Multicast
  • Broadcast

Ethernet Frame 802.3

  • Preamble: Used for synchronization, 64 bits.
  • Destination Address: Ethernet address of the destination host.
  • Source Address: Ethernet address of the source host.
  • Length/Type: Payload length or frame type. 1500 or less is length, otherwise it’s a frame type.
  • Payload: Encapsulated data area, 46-1500 bytes.
  • Pad: Meaningless data to fill the frame to the minimum size.
  • CRC: Cyclical Redundancy Check.

Reducing Collisions

Decrease data size or the number of computers, break down the network into smaller segments.

Interconnection Devices

A-G-A, T-G-T, N-R-N, D-B-D, P-R-P

Application Layer Protocols

  • Telnet: Terminal emulator – emulation via network, allows software to emulate a command line.
  • SSH: Secure Shell – Provides an encrypted and validated channel, TCP port 22, remote login, file copy, tunnel protocol.
  • FTP: File Transfer Protocol – used to transfer files, uses TCP ports 20 and 21, provides user login, not encrypted, RFC 1579.
  • TFTP: Trivial File Transfer Protocol – transfers files, uses UDP port 69, file transfer only, lower overhead, RFC 1783.
  • SNMP: Simple Network Management Protocol – simple management, manages network devices, uses UDP port 161.
  • HTTP: Hypertext Transfer Protocol – transfers files between clients and servers, optimized for small transfers, port 80.
  • HTTPS: HyperText Transfer Protocol Secure – transport layer security, encrypts communication, TCP port 443.
  • NTP: Network Time Protocol – synchronizes time from a server, accurate time base, RFC 5905.
  • DNS: Domain Name System – resolves qualified domain names to IP addresses, distributed database, client DNS queries, name resolution uses TCP and UDP port 53.
  • DHCP/BOOTP: Dynamic Host Configuration Protocol – provides automatic IP configuration, server solutions, uses UDP port 67, server maintains a database, client requests an IP lease.
  • APIPA: Automatic Private IP Addressing – automatically configured on IPs without DHCP, IP addresses are not routable, no internet access, RFC 3927.

Connection Types

  • Connections: Establishes a connection before data transfer.
  • Connectionless: Just sends data to the receiver without establishing a connection.


  • TCP: Transmission Control Protocol, connection-oriented, uses packets, acknowledges byte stream.
  • IP: Internet Protocol.
  • ICMP: Internet Control Message Protocol.
  • ARP: Address Resolution Protocol.
  • IGMP: Internet Group Messaging Protocol.

IP Addressing

Hosts need an IP address.

IPv4 configuration is represented as two 32-bit numbers.

  • Class A: 1-126
  • Class B: 128-191
  • Class C: 192-223
  • Class D: 224-239
  • Class E: 240-255

Maximum number of hosts per network: 65,534.

Example: a.167.77.214, b.65534, c.254

Subnetting breaks down networks into smaller segments.

Cisco Systems

– Router, swithces, concentrations, ip telephony, firewall, video, wireless, broadband, network man, soft

Fixed Configureation- routers/ switches, cant be changed because of installation
modular: Routers with slots, accept a number of adaptars, fddi, dsl hssi, synchornous serial