Networking Technologies

Chapter 1: Computer Networks and the OSI Model

 

 

Objectives:

Chapter 1, Volume 2, is about relating the OSI model to networks. The objectives important to this chapter are on page 1-1:

  1. Understanding protocols.
  2. Understanding the tasks of each model layer.
  3. Understanding the physical and logical topologies used in real networks.
  4. Reviewing the connectivity devices associated with various layers.

Concepts:

The chapter begins with a discussion of the need for rules in communications. In networking, the rules are called protocols. The word "protocol" can also means a specific utility that is used which follows a set of rules.

Page 1-4 reviews the names of the seven layers of the ISO-OSI model, reviews a mnemonic sentence for them, and introduces the idea that three major tasks are covered by various layers:

  • Physical Connections - covered by layers 1 and 2
  • Reliable transport of data - covered by layers 3 and 4
  • Services - covered by layers 5, 6 and 7

The chapter continues by discussing an overview/review of the concepts from the layer chapters of the previous volume. A few new ideas are thrown in:

  • Layer 1, Physical - Protocols define structure, physical specifications for media, rules for transmitting bits.
  • Layer 2, Data Link - Bits are formed into frames, headers give address information. Sub-layers:
    • MAC - Rules to access the media, logical topologies, hardware addressing. The book discusses other addresses here, although they belong on other layers: Network addresses belong in the Network layer headers, Service addresses can be placed in the Data Link or Network layer headers.
    • LLC - Frame synchronization, connection services and error control
  • Layer 3, Network - Datagram packets are routed to other networks. Connection services are introduced.
  • Layer 4, Transport - End-to-end control and error checking. Reliability. In some networks, this may be the last or next-to-last layer.
  • Layers 5, 6 and 7: Session, Presentation and Application- Some networks group all these functions into one group of Upper Layer Protocols. The Session layer functions control dialog between nodes, the Presentation layer functions format data and bits, and the Application layer functions share network services.

The text informs us that the protocols to be discussed next are often associated with layers 1 through 4, and that protocols serving Upper Layer functions often serve multiple layers at once.

Topologies are reviewed, starting on page 1-13. The five physical topologies and two logical topologies from volume one are shown in graphics and discussed.

Starting on page 1-15, the text discusses connectivity devices.

  • Repeaters - repeat all bits, no judgment is used.  Physical layer devices.
  • Active Hubs - same as above, but for UTP instead of coax. Physical layer devices.
  • Network Interface Cards - hardware addresses are used. Data Link layer devices.
  • Bridges - filter frames based on hardware addresses. Data Link layer devices. It is observed that the LAN segments that are linked by a bridge must use the same protocol stack.
  • Switches - filter frames to single ports instead of single LAN segments. Data Link layer devices.
  • Routers - choose routes from information about paths to other networks. Network layer devices.
  • Gateways - translate formats between dissimilar networks. Can be called a Network layer device or an Upper Layer device.