Tag Archives: OSI Model - Page 2

The Network Layer – Layer 3

The Network layer of the OSI Model basically defines logical addressing and the ways that packets are moved from source to destination on a network. The functions of the Network layer can be broken down as follows:

The Network Layer Layer 3 246x300 The Network Layer   Layer 3

  • Message addressing
  • Path determination between source and destination nodes on different networks
  • Routing messages between networks
  • Controlling congestion on the subnet
  • Translating logical addresses into physical addresses
  • Logical Address Construction

    Protocol Total Address Length Bits in Network Portion Bits in Host Portion
    TCP/IP 32 Class A – 8
    Class B – 16
    Class C – 24
    Class A – 24
    Class B – 16
    Class C – 8
    IPX 80 32 or less (only
    significant digits listed)
    48 bits (MAC address)
    AppleTalk 24 16 or less (indicates one
    or many in cable range)
    8 bits or less
    (cynamically assigned)

    Routing protocols support routed protocols. A routing protocol is used to pass messages between routers for maintaining and updating routing tables. Examples of routing protocols are RIP, IGRP, OSPF, EIGRP.
    Routed protocols are used to carry end-user traffic across the internetwork. Examples of routed protocols are IP and IPX Read more »

    The Data Link Layer – Layer 2

    The Data Link layer performs a number of separate activities, including:

    The Data Link Layer Layer 2 246x300 The Data Link Layer – Layer 2

  • Physical addressing
  • Network topoloty
  • Error notification
  • Access to the physical medium
  • Flow control
  • Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control.

    Physical addressing, is not to be confused with network or IP addresing. The physical address defines how devices are labeled in the data link layer. This physical address is most commonly called the Media Access Control (MAC) address. The MAC address is a unique number assigned by the manufacturer. This numbering system is actually administered by one of the networking governing bodies.

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    Tags: data link layer, link layer, What acronym is used to reference the data link sublayer that identifies the network layer protocol encapsulated in the frame?

    The Physical Layer- Layer 1

    The Physical Layer has two responsibilities; it sends bits and receives bits.

    The Physical layer specifications specify the electrical, meThe Physical Layer Layer 1 246x300 The Physical Layer  Layer 1chanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems.

    The physical layer is usually a combination of software and hardware programming and may include electromechanical devices.

    All wireing, power, cabling and connections are part of the physical layer. Without the physical layer functioning properly none of the upper layers will respond correctly.

    * transmits raw bit stream over physical cable
    * defines cables, cards, and physical aspects
    * defines NIC attachments to hardware, how cable is attached to NIC
    * defines techniques to transfer bit stream to cable

    Tags: Physical Layer, phy layers, layer 1 devices