Introduction to Data Communications
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Layer 3 - Network Layer

The Network layer is concerned with finding the shortest path to the destination. This usually means finding the fastest route through multiple networks to the destination. Routing algorithms are used to determine the "shortest" path. Shortest does not mean the physically shortest distance but fastest route.

The Network layer converts the segments into smaller protocol data units (PDUs) called packets that the network can handle. The Network layer is connectionless in that it does not guarantee that the packet will reach its destination. It is often referred to as "send and pray". The packet is sent out on the wire and we pray that it arrives.

Networks are identified by network addresses that are separate from node addresses. Since the Network layer is concerned with finding networks, it adds the source and destination addresses to the packet.

Fig. 7 Network Layer

Layer 2 - Data Link Layer

The Data Link layer has three primary jobs:

  1. Bus Arbitration: The Data Link layer controls "whose turn it is to talk?" on the medium.
  2. Framing of the bits: it puts the bits in proper order and fields
  3. Error detection and correction at the bit level

The Data Link layer is in charge of whose turn it is to talk on the wire. It will have a method for determining how its going to control the communication.

The Data Link layer has the job putting the bits in the correct order for sending out on the wire. The Data Link layer has the job of organizing the data in a frame. The frame consists of sections called fields. Each field will have a specific function such as Destination Address which identifies the host for which the data is being sent to. Other fields will be used for synchronizing source and destination clocks and others for error checking.

The Data Link layer resides in the firmware layer of the network interface card. Firmware is software that is burnt into a read only memory. The node will have a unique address that identifies it from all other nodes. This address is called a hardware address because it is burnt into the firmware. Ethernet network interface cards' unique address is typcially called a MAC address after a Data Link sub-layer called the Media Access Control (MAC) layer.

The Data Link layer takes the packets and puts them into frames of bits: 1s and 0s for transmission and assembles received frames into packets. The Data Link layer works at the bit level and is concerned about bit sequence. Error checking is at the bit level and frames with errors are discarded and a request for re-transmission is sent out.

Fig. 8 Data Link Layer

Layer 1 - Physical Layer

The Physical layer concerns itself with the transmission of bits and the network card's hardware interface to the network. The hardware interface involves the type of cabling (coax, twisted pair, etc..), frequency of operation (1 Mbps, 10Mbps, etc..), voltage levels, cable terminations, topology (physical shape of the network: star, bus, ring, etc..), etc.. Examples of Physical layer protocols are 100BaseT, 1000BaseT, Token Ring... See Fig. 9.

Fig. 9 Physical Layer

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007