Group 10

  1. The purpose and function of the data link layer in preparing communication for transmission on specific media

PURPOSE

  • adds physical addressing

FUNCTION

  • provides methods for exchanging data frames between devices over a common media

WAN Topologies


Point-to-Point - Permanent link
between two endpoints.

Hub and Spoke

  • A central site interconnect branch sites using point to points links.

Mesh

  • Provides high availability, but
    requires that every end system be interconnected to every other
    system Administrative and physical
    costs can be significant.

LAN Topologies

Star - End devices are connected to a central
intermediate device. Use Ethernet switches.

Extended Star - Additional Ethernet switches
interconnect other star topologies.

Bus - Used in legacy networks. All end
systems are chained to each other and terminated in some form on each end.
Switches are not required to interconnect the end devices. Bus topologies using coax cables
were used in legacy Ethernet networks because it was inexpensive and easy to set
up.

Ring - End systems are connected to their
respective neighbor forming a ring. Unlike the
bus topology, the ring does not need to be
terminated. Ring topologies were used in egacy Fiber Distributed Data Interface (FDDI)
and Token Ring networks.

Data Link Frame

Frame start and stop indicator
flags - Identifies the beginning and end limits of the frame

Addressing - Indicates the
source and destination nodes.

Type - Identifies the Layer 3
protocol in the data field.

Control - Identifies special flow
control services such as QoS.

Data - Contains the frame
payload (i.e., packet header segment header, and the data).

Each frame type has
three basic parts:
◦ Header ◦ Data ◦ Trailer

Structure of the frame
and the fields contained in the
header and trailerdepend on Layer 3
protocol.

Differentiate the roles of the MAC address and the IP address.

There are two primary addresses assigned to a device on an
Ethernet LAN:


  • Physical address (the Ethernet MAC address)
  • Logical address (the IP address)

When the destination IP address is on a remote network, the destination MAC address will be the address of the host’s default gateway.

As an example, PC-A sends an IP packet to the file server on the same network. The Layer 2 Ethernet frame contains:

  • Source MAC address
  • Destination MAC address

The Layer 3 IP packet contains:

  • Destination IP address
  • Source IP address

In the figure, PC-A is sending an IP packet to a web server on a remote network.

  • The destination IP address is that of
    the File Server.
  • The destination MAC address is that of
    Ethernet interface of R1.