Layered Network Design

Tags: #◇Computer Science #◇CSNETWK #◇Layered Network Design #◇Conceptual

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◇Overview

Network architecture relies on a structured, layered approach to reduce design complexity. A Network Architecture is a formal, logical structure defining how network devices interact, ensuring interoperability—the capability of hardware and software from different vendors to successfully communicate.

All communications are governed by protocols, which dictate the format, the order of messages sent and received, and the specific actions to take upon message transmission and receipt.

OSI Architecture

The Open Systems Interconnection (OSI) model was developed in 1978 by the ISO to standardize network architectures and enable non-proprietary interoperability. It is comprised of 7 layers, offering clear advantages: functional isolation, simpler design, and independent technology updates without affecting other layers.

1. Application Layer (Layer 7): Interfaces with end-user applications. Defines specific user-oriented services (e.g., HTTP for web, SMTP for email).
2. Presentation Layer (Layer 6): Translates, formats, encrypts, and compresses data. It concerns itself with syntax and semantics (e.g., ASCII conversion).
3. Session Layer (Layer 5): Manages communication sessions between nodes. Enforces rules of dialog (e.g., half-duplex vs. full-duplex), synchronizes data flow, and handles connection recovery.
4. Transport Layer (Layer 4): Ensures error-free and complete delivery of data end-to-end. Partitions data into smaller packets if necessary (e.g., TCP, UDP).
5. Network Layer (Layer 3): Handles end-to-end routing and switching between heterogeneous networks using logical addressing (e.g., IP).
6. Data Link Layer (Layer 2): Transfers data across a physical link, handling error detection, framing, and flow control. Resolves damaged, lost, or duplicate frames (e.g., Ethernet).
7. Physical Layer (Layer 1): Transmits raw bit streams over a physical medium, translating bits into signals over wires, connectors, etc.

Layers 7-5 are Software/Application-oriented. Layer 4 handles Delivery. Layers 3-1 are Hardware/Network-oriented.

TCP/IP Suite

The TCP/IP model is a 4-layer architecture developed by the US Department of Defense.

1. Application Layer: Equivalent to OSI Layers 5, 6, and 7. Interfaces with applications.
2. Transport Layer: Equivalent to OSI Layer 4. Features two distinct protocols:
   TCP (Transmission Control Protocol): Connection-oriented, provides reliable, error-checked, ordered data transmission. Retransmits missing data and prevents duplicates. Used in Telnet, FTP, SMTP.
   UDP (User Datagram Protocol): Connectionless, provides unreliable datagram service. Does not guarantee delivery, order, or check for duplicates. Used in DNS, TFTP.
3. Internet Layer: Equivalent to OSI Layer 3. Routes user messages across networks utilizing metrics and IP addresses to locate hosts.
4. Network Interface Layer: Equivalent to OSI Layers 1 and 2. Connects the host to the local hardware/physical medium.

Encapsulation and Decapsulation

Encapsulation involves packaging data with protocol information as it passes down through the layers. This generic packaged data is called a Protocol Data Unit (PDU). Protocol information consists of headers (address info) and trailers (error control).
Application Layer: Creates the core data/message ($M$).
Transport Layer: Encapsulates the message with a transport header ($H_t$) to create a Segment.
Network Layer: Encapsulates the segment with a network header ($H_n$) to create a Datagram.
Data Link Layer: Encapsulates the datagram with a link header ($H_l$) (and potentially a trailer) to create a Frame.

Decapsulation occurs at the destination, stripping the respective headers layer by layer as data moves upward.

◇Important Formulas

(No specific formulas were discussed in this module)

◇Worked Examples

(No quantitative worked examples were provided in this module)

◇References & Navigation

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• Related Notes: ◇Internetworking, ◇Security