Chapter 2 – The Open Systems Interconnection (OSI) Specifications

CompTIA Network+ Exam Objective 1.1

• Focus: Compare and contrast networking appliances, applications, and functions across the seven OSI layers.
• Layers examined: 1 Physical, 2 Data-Link, 3 Network, 4 Transport, 5 Session, 6 Presentation, 7 Application.

Internetworking Models & History

• Late 1970s: International Organization for Standardization (ISO) produced the Open Systems Interconnection (OSI) Reference Model.
  • Goal: Enable vendors to create interoperable network hardware & software.
  • Became the primary architectural model for modern networks.
  • Describes how data travels from an application on one computer, through media, to an application on another.
• Approach: Divide the end-to-end communication process into 7 logical layers, each providing services to the layer above and using services of the layer below.

Advantages of the Layered Reference Model

• Breaks a complex process into smaller, simpler components → easier design, development & troubleshooting.
• Promotes multi-vendor interoperability through standardized functions at each layer.
• Encapsulates change: modifications in one layer do not ripple to others.
• Encourages industry-wide standardization & fosters innovation.
• Allows heterogeneous hardware & software to coexist and communicate.

OSI Layers (Top → Bottom)

1. Application (7)
2. Presentation (6)
3. Session (5)
4. Transport (4)
5. Network (3)
6. Data-Link (2)
7. Physical (1)

Layer-by-Layer Functional Detail

Upper Layers (7–5)

• Application (L7)
  • Provides the user interface & high-level services (file, print, messaging, DB access, custom apps).
• Presentation (L6)
  • Data translation, character set conversion, compression, encryption/decryption.
  • Ensures data from the application layer of one system is readable by the application layer of another.
• Session (L5)
  • Establishes, manages & terminates dialogues.
  • Keeps data streams from different applications separate (e.g., you can browse the web while transferring a file).

Transport Layer (L4)

• Provides end-to-end connection services.
• Can be connection-oriented (reliable) or connectionless (best-effort).
• Key tasks:
  • Segmentation & reassembly.
  • Error detection & recovery (retransmit if necessary).
  • Flow control (prevent congestion; see windowing below).

Network Layer (L3)

• Provides logical addressing & routing.
• Determines best path through the internetwork (handled by routers).
• Devices: Routers, multilayer switches.

Data-Link Layer (L2)

• Responsible for framing, MAC addressing, and access to the physical media.
• Error detection (Frame Check Sequence – FCS) but not correction.
• Two sublayers:
  • Logical Link Control (LLC) – defined by IEEE $802.2$; multiplexes protocols, error checking.
  • Media Access Control (MAC) – defined by IEEE $802.3$ (Ethernet) & $802.11$ (Wi-Fi); controls access to shared medium.

Physical Layer (L1)

• Moves bits (1s & 0s) across the medium.
• Defines electrical/optical voltage, timing, pin-outs, cable specs & data rates.
• Devices: Hubs, repeaters, cables, connectors.

Transport Layer: Reliability Concepts

• Connection-oriented session (TCP-like)
  1. Three-way handshake – Sender → $\text{SYN}$, Receiver → $\text{SYN\,/\,ACK}$, Sender → $\text{ACK}$ (connection established).
  2. Data segments transferred.
  3. FIN / ACK pairs close the session gracefully.
• Reliable transport guarantees:
  • Every segment is acknowledged.
  • Lost segments are retransmitted.
  • Segments are sequenced on arrival.
  • Flow is controlled so buffers don’t overflow.

Flow Control Mechanisms

• Stop-and-Wait (simple)
  • Receiver advertises “STOP!” when its buffer is full and “GO!” when ready.
• Windowing (sliding window)
  • Sender transmits multiple segments (window size).
  • Receiver acknowledges the last in-order segment (e.g., $\text{Ack 4}$). Lost segment triggers retransmission.
  • Window size can grow/shrink dynamically to match network capacity.

Acknowledgement Example (Lost Segment)

• Sender transmits segments 1–7; segment 5 lost.
• Receiver acks up to 4 (last good).
• Sender retransmits 5, then continues.

Routing & Layer-3 Example Topology

• Networks:
  • $192.168.1.0/24$
  • $192.168.2.0/24$
  • $192.168.3.0/24$
• Router A interfaces: $192.168.1.1$ (Fa0/1) & $192.168.2.1$ (S0/0).
• Router B interfaces: $192.168.2.2$ (S0/0) & $192.168.3.1$ (Fa0/0).
• Sample routing tables (cost = metric):
  • Router A
  • $192.168.1.0/24$ → Fa0/1 (metric 0)
  • $192.168.2.0/24$ → S0/0 (metric 0)
  • $192.168.3.0/24$ → S0/0 (metric 1)
  • Router B
  • $192.168.3.0/24$ → Fa0/1 (metric 0)
  • $192.168.2.0/24$ → S0/0 (metric 0)
  • $192.168.1.0/24$ → S0/0 (metric 1)
• Broadcast domains
  • Each router interface is its own broadcast domain.
  • Routers break up broadcasts by default and provide WAN connectivity.

Data Encapsulation & Protocol Data Units (PDUs)

• Application, Presentation, Session layers → Data
• Transport (L4) → Segments
• Network (L3) → Packets / Datagrams
• Data-Link (L2) → Frames
• Physical (L1) → Bits

Ordered encapsulation example (top-down):

1. Application data
2. Add Transport header (e.g., TCP) → Segment
3. Add Network header (e.g., IP) → Packet
4. Add Data-Link header & trailer (incl. FCS) → Frame
5. Convert frame to electrical/optical pulses → Bits on the wire

Ethical / Practical Implications & Real-World Relevance

• Layered design lets vendors update hardware (e.g., faster Ethernet at Layer 1) without changing higher-layer software.
• Security features (encryption/compression) are isolated in the Presentation layer, allowing uniform application across many protocols.
• Troubleshooting uses the model as a systematic approach: start at the physical layer and move up (or vice-versa) to isolate faults.

Key Numbers & Standards to Memorize

• OSI layer count: $7$.
• IEEE sublayer standards: $802.2$ (LLC), $802.3$ (Ethernet MAC), $802.11$ (Wi-Fi MAC).
• Common private IP space example: $192.168.0.0/16$, with /24 subnets used in diagrams.
• Control flags in TCP handshakes: SYN, ACK, FIN.

Exam Preparation Checklist

• Be able to name, order, and describe all 7 OSI layers.
• Map common devices & protocols to their corresponding layers (e.g., router → L3, switch → L2, hub → L1, TCP → L4, SSL/TLS → L6).
• Explain connection-oriented vs connectionless transport, including flow control & reliability.
• Draw and interpret simple routing tables and subnet diagrams.
• Identify the purpose of LLC vs MAC sublayers and the associated IEEE standards.
• Describe the data encapsulation process & the names of PDUs at each layer.