A Layered View of Communications Networks

A Layered View of Communications Networks
An Introduction

Computer Science Department

bernstdh@jmu.edu

Motivation

An Observation:
- Networks can be and are considered at different levels of abstraction/resolution
A Problem that Arises:
- This makes it hard for multiple parties to develop components that interact correctly
Overcoming this Problem:
- Create a unified view of all networks so that components can interact with each other as needed

Layered Architectures

The Concept:
- Functional components are arranged in groups that provide a layer of services
- Each layer hides all (strict) or most (relaxed or transparent) of the layers below
A Good and Bad Visualization:

Advantages and Disadvantages of Layered Architectures

Advantages:
- Standards can be developed for each layer independently
- One protocol can be substituted for another (at the same layer) without affecting any other layers
Disadvantages:
- Inefficiencies may arise

Advantages and Disadvantages (cont.)

Nerd Humor - Networking Problems
/imgs

(Courtesy of xkcd)

The Open Systems Interconnection (OSI) Layers

The Open Systems Interconnection (OSI) Layers (cont.)

Layer 7 - Application:
- Involves user requests
- Sometimes said to provide semantics
Layer 6 - Presentation:
- Adds formatting/display information in an application/machine independent way (e.g., ASCII/EBCDIC/Unicode, big-endian/little-endian)
Layer 5 - Session:
- Provides session control (a.k.a. dialog coordination)
Layer 4 - Transport:
- Provides process-to-process data transfer
Layer 3 - Network:
- Provides host-to-host data transfer that is independent of the switching mechanism (a.k.a. routing/relaying)
Layer 2 - Data Link:
- Provides blocking/framing, synchronization, error control, and flow control on direct links
Layer 1 - Physical:
- Provides transmission of unstructured bit streams over the physical medium

The IEEE Layers

Similarities to OSI:
- The highest six layers are the same
Differences from OSI:
- The data link and phsyical layers are changed to the logical link control, media access control and physical layers

The Internet: A 5-Layer View

Layer 5 - Application:
- Includes Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP)
Layer 4 - Transport:
- Includes Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
Layer 3 - Network:
- Includes Internet Protocol (IP)
Layer 2 - Link:
- Includes Point-to-Point Protocol (PPP) and Ethernet
Layer 1 - Physical:
- Includes the different protocols for Ethernet on different physical media

The Internet: A 5-Layer View (cont.)

Be Careful:
- Sometimes Layers 1 and 2 are combined to create a four-layer view (with the lowest level called the Access layer)
The Data at Each Layer:
- The complete "message" at layer \(i\) is the payload at layer i-1 (e.g., an HTTP "message" which has its own header and payload becomes the payload for a TCP "message" which has its own header, which becomes the payload for an IP "message" that has its own header)
Terminology at Each Layer:
- At each layer, the terminology changes (frame at layers 1/2, datagram at layer 3, segment at layer 4, message at layer 5)

OSI Layers and the Internet

One Interpretation

Layers in Asynchronous Transfer Mode (ATM)

ATM Adaptation Layer:
- Convergence (CS) -- standard interface
- Segmentation and Reassembly (SAR) - between cells and packets
ATM Layer:
- Flow control, virtual circuit/path management, cell multiplexing/demultiplexing
Physical Layer:
- Transmission Convergence (TC) -- checksum generation/checking, cell generation
- Physical Medium Dependent (PMD) -- bit timing, physical network access

There's Always More to Learn