i. The protocol stack used on the Internet is the Internet Protocol Suite. It is usually called TCP/IP.

ii. The TCP/IP model is based on a five-layer model for networking, from bottom (the link) to top (the user application).

iii. These are the physical, data link, network, transport, and application layers.

The TCP/IP Layers:

• The model consists of five separate but related layers.
• TCP/IP also defines how to interface the network layer with the data link and physical layers.
• In Fig1 the unstructured stream of bits represents frames with distinct content.

Fig1: TCP/IP encapsulation and headers

• The Physical Layer :

  i. The physical layer contains all the functions needed to carry the bit stream over a physical medium to another system.

  ii. There are other things that the physical layer must determine, or be configured to expect.

  iii. In Fig2 .The transmission framing bits are used for transmission media purposes only, such as low-level control.

Fig2: The physical layer

i. Data rate—This transmission rate is the number of bits per second that can be sent. It also defines the duration of a symbol on the wire.

ii. Bit synchronization—The sender and receiver must be synchronized at the symbol level so that the number of bits expected per unit time is the same.

iii. Configuration—In a multipoint configuration, a link connects more than two devices, and in a multisystem bus topology such as a LAN, the number of systems can be very high.

iv. Topology—The devices can be arranged in a number of ways. In a full mesh topology, all devices are directly connected and one hop away. Systems can also be arranged as a star topology, with all systems reachable through a central system. There is also the bus and the ring.

v. Mode—So far, we’ve only talked about one of the systems as the sender and the other as the receiver. This is operation in simplex mode, where a device can only send or receive. More realistic devices use duplex mode, where all systems can send or receive with equal facility.

• The Data Link Layer :

  • The data link layer performs framing, physical addressing, and error detection, this layer also performs access control.
  • In LANs, this media access control (MAC) forms a sublayer of the data link layer and has its own addressing scheme known (not surprisingly) as the MAC layer address.
  • In addition, the data link layer can perform some type of flow control.
  • The unit of communication at the data link layer is a frame.
  • Fig3, showing that data link layer frames have both header and trailer.

Fig3: The data link layer

• The Network Layer:

  i. At the network layer, TCP/IP supports the Internet Protocol (IP).

  ii. The Internet Protocol (IP)is the transmission mechanism used by the TCP/IP protocols

  iii. IP transports data in packets called datagrams, each of which is transported separately.

  iv. The network layer delivers data in the form of a packet from source to destination.

  v. The biggest difference between the network layer and the data link layer is that the data link layer is in charge of data delivery between adjacent systems, while the network layer delivers data to systems that are not directly connected to the source.

  vi. Fig4 shows the relationship between the network layer and the transport layer above and the data link layer below.

  vii. These data units are packets with their own destination and source address formats.

  viii. The network layer uses one or more routing tables to store information about reachable systems.

Fig4: The Network Layer

ix. The unit of communication at the network layer is a datagram.

• The Transport Layer :

  i. Process-to-process delivery is the task of the transport layer.

  ii. This process of dividing message content into packets is known as segmentation.

  iii. The network layer forwards each and every packet independently, and does not recognize any relationship between the packets.

  iv. The transport layer, in contrast, can make sure the whole message.

  v. This function of the transport layer involves some method of flow control and error control (error detection and error correction) at the transport layer, functions which are absent at the network layer.

  vi. There are two very popular protocol packages at the transport layer: TCP—This is a connection-oriented, “reliable” service that provides ordered delivery of packet contents.

  UDP—This is a connectionless, “unreliable” service that does not provide ordered delivery of packet contents.

  vii. Fig5 , showing how data are broken up if necessary and reassembled at the destination.

  viii. The unit of communication at the transport layer is a segment, user datagram, or apacket, depending on the specific protocol used in this layer.

Fig5: The transport layer

• The Application Layer :

  i. The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model.

  ii. The application layer allows a user to access the services of our private internet or the global Internet.

  iii. Many protocols are defined at this layer to provide services such as electronic mail, file transfer, accessing the World Wide Web, and so on.

  iv. The unit of communication at the application layer is a message.