1. Data Communication and Computer Networks
Data link layer
The Data Link layer of a model typically has the following responsibilities:
1. Creates a frame
2. Creates an error-free logical connection
Error control
Flow control
3. Makes sure the receiver stays synchronized with the incoming data stream.
Data Communication and Computer Networks

2. Data Communication and Computer Networks
Flow Control
Ensuring the sending entity does not overwhelm the receiving entity:
Preventing buffer overflow.
Propagation time.
Transmission time.
Data Communication and Computer Networks

3. Data Communication and Computer Networks
Error Control
Detection errors
Lost frames
Damaged frames
Automatic repeat request
Data Communication and Computer Networks

4. Data Communication and Computer Networks
Network layer
Layer 3.
The network layer is responsible:
The delivery of individual packets from the original source to the final destination .
Logical addressing:
if the packet passes the network boundary we need another addressing system to help (source to destination) connection.
Routing :
route or switch the packet to final destination.
Source-to-destination delivery (End-to-End).
Data Communication and Computer Networks

5. Data Communication and Computer Networks
Internet Protocol
Provides information about how and where data should be delivered
Subprotocol that enables TCP/IP to internetwork
To internetwork is to traverse more than one LAN segment and more than one type of network through a router
In an internetwork, the individual networks that are joined together are called subnetworks
Data Communication and Computer Networks

6. Internet Protocol cont.
IP is an unreliable, connectionless protocol, which means it does not guarantee delivery of data
Connectionless
Allows protocol to service a request without requesting verified session and without guaranteeing delivery of data
Data Communication and Computer Networks

7. Position of IP in TCP/IP protocol suite
Data Communication and Computer Networks

8. Data Communication and Computer Networks
Datagram
A packet in the IP layer is called a datagram, a variable-length packet consisting of two parts: header and data.
The header is 20 to 60 bytes in length and contains information essential to routing and delivery.
Data Communication and Computer Networks

9. Data Communication and Computer Networks
IP datagram
Data Communication and Computer Networks

10. Data Communication and Computer Networks
IP datagram fields
VER - version numbers, 4 and 6
HLEN - header length in 4-byte words. Value of 5 means 20 byte header
DS (differentiated services or Service type ) -
Data Communication and Computer Networks

11. IP datagram fields cont.
Precedence bits - never used. Similar to PRI bits in IPv6.
TOS (Type of Service) bits -
If you want to send a packet
with a special type of service,
use one of the 5 bit sets.
Data Communication and Computer Networks

12. IP datagram fields cont.
If we call these 8 bits Differentiated Services (and not the older Service Type), then the first six bits are called code-points
Data Communication and Computer Networks