Chapter 2 Network Models 2-1 layered tasks

Duties of the data link layer

• Flow control
• It imposes a flow control mechanism , if the data rate at the receiver is less than produced by sender the data link layer imposes a flow control to avoid overwhelming the receiver
• Error control
• Add mechanisms to detect and retransmit damaged or lost frames.
• Prevent also duplication of frames.
• Error control is normally achieved through a trailer added to the end of frame.
• Access control
• When two or more devices than one devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at given time

• Figure 2.7 Hop-to-hop delivery

• A node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

• Example 2.1

• Most LANs use a (6-byte) physical address written as 12 Hex digit
• Every byte (2 Hex digits) is separated by a colon
• 07:01:02:01:2C:4B

NETWORK LAYER

• Responsible from the delivery of packets from the original source to the final destination across multiple networks
• If 2 systems are connected to the same link, there is no need for network layer

Duties of network layer

• Logical addressing
• Network layer adds unique ( IP or logical address) to the packet.
• IP addresses are necessary for universal communications that are independent of physical network
• No two host address on the internet can have the same IP address
• IP addresses 32-bit address that uniquely define a host connected to the Internet
• Routing
• Provide the routing mechanism for the router which route the packet to their final destination.
• Routers: devices used when independent networks are connected to create an internetworking ( network of networks)
• Fragmentation
• Division of a packet into a smaller units to accommodate a protocols MTU
• Fragmentation Offset
• A field in the IP header used to show relative position of the fragment with the respect to the whole datagram

• Figure 2.9 Source-to-destination delivery

• Each computer is connected to only one link and therefore has
• only one pair of addresses (logical & physical )
• Each router, however, is connected to 3 networks, therefore; each router
• has 3 pairs of addresses, 1 for each connection.

• Example

• MAC addresses will change from hop to hop, but the logical addresses will not