- Forward


A Layered View of Communications Networks
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

Print

Motivation
Back SMYC Forward
  • 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
Back SMYC Forward
  • 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:
    • layered
Advantages and Disadvantages of Layered Architectures
Back SMYC Forward
  • 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.)
Back SMYC Forward
Nerd Humor - Networking Problems
/imgs
(Courtesy of xkcd)
The Open Systems Interconnection (OSI) Layers
Back SMYC Forward
  • When?
    • Created in 1978
  • Who?
    • The International Standards Organization (ISO)
  • Why?
    • Create standards that would promote multi-vendor interoperability
The Open Systems Interconnection (OSI) Layers (cont.)
Back SMYC Forward
  • 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
Back SMYC Forward
  • 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
Back SMYC Forward
  • 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.)
Back SMYC Forward
  • 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
Back SMYC Forward

One Interpretation

osi-and-internet-layers
Layers in Asynchronous Transfer Mode (ATM)
Back SMYC Forward
  • 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
Back -