When a host transmits data across a network to another device, the data goes through encapsulation: it is wrapped with protocol information at each layer of the OSI model. Each layer communicates only with its peer layer on the receiving device. To communicate and exchange information, each layer uses Protocol Data Units (PDUs). These hold the control information attached to the data at each layer of the model. They are usually attached to the header in front of the data field but can also be in the trailer, or end, of it. Each PDU is attached to the data by encapsulating it at each layer of the OSI model, and each has a specific name depending on the information provided in each header. This PDU information is read only by the peer layer on the receiving device. After it’s read, it’s stripped off, and the data is then handed to the next layer up.
The following figure shows the PDUs and how they attach control information to each layer. This figure demonstrates how the upper-layer user data is converted for transmission on the network. The data stream is then handed down to the Transport layer, which sets up a virtual circuit to the receiving device by sending over a synch packet. Next, the data stream is broken up into smaller pieces, and a Transport layer header (a PDU) is created and attached to the header of the data field; now the piece of data is called a segment. Each segment is sequenced so the data stream can be put back together on the receiving side exactly as it was transmitted.
Each segment is then handed to the Network layer for network addressing and routing through the internetwork. Logical addressing (for example, IP) is used to get each segment to the correct network. The Network layer protocol adds a control header to the segment handed down from the Transport layer, and what we have now is called a packet or datagram. Remember that the Transport and Network layers work together to rebuild a data stream on a receiving host, but it’s not part of their work to place their PDUs on a local network segment-which is the only way to get the information to a router or host.
Data Link layer
Data Link layer is responsible for taking packets from the Network layer and placing them on the network medium (cable or wireless). The Data Link layer encapsulates each packet in a frame, and the frame’s header carries the hardware address of the source and destination hosts. If the destination device is on a remote network, then the frame is sent to a router to be routed through an internetwork. Once it gets to the destination network, a new frame is used to get the packet to the destination host.
To put this frame on the network, it must first be put into a digital signal. Since a frame is really a logical group of 1s and 0s, the Physical layer is responsible for encoding these digits into a digital signal, which is read by devices on the same local network. The receiving devices will synchronize on the digital signal and extract (decode) the 1s and 0s from the digital signal. At this point the devices build the frames, run a CRC, and then check their answer against the answer in the frame’s FCS field. If it matches, the packet is pulled from the frame, and what’s left of the frame is discarded. This process is called de-encapsulation. The packet is handed to the Network layer, where the address is checked. If the address matches, the segment is pulled from the packet, and what’s left of the packet is discarded. The segment is processed at the Transport layer, which rebuilds the data stream and acknowledges to the transmitting station that it received each piece. It then happily hands the data stream to the upper-layer application.
Once the Transport layer header information is added to the piece of data, it becomes a segment and is handed down to the Network layer, along with the destination IP address.
The Network layer adds a header, and adds the logical addressing (IP addresses), to the front of each segment. The packet also has a protocol field that describes where it came from (either UDP or TCP). This lets the Network layer hand the segment to the correct protocol at the Transport layer when the packet reaches the receiving host. It also has a protocol field that describes where it came from (either UDP or TCP), so it can hand it to the correct protocol at the Transport layer when it reaches the receiving host. The Network layer is responsible for finding the destination hardware address that dictates where the packet should be sent on the local network. It does this by using the Address Resolution Protocol (ARP). IP at the Network layer looks at the destination IP address and compares that address to its own source IP address and subnet mask. If it turns out to be a local network request, the hardware address of the local host is requested via an ARP request. It the packet is destined for a remote host, IP will look for the IP address of the default gateway (router) instead.
The packet, along with the destination hardware address of either the local host or default gateway, is then handed down to the Data Link layer. The Data Link layer will add a header to the front of the packet and the piece of data then becomes a frame. This is shown below.
The frame uses an Ether-Type field to describe which protocol the packet came from at the Network layer. Now, a cyclic redundancy check (CRC) is run on the frame, and the answer to the CRC is placed in the Frame Check Sequence field found in the trailer of the frame. The Frame is now ready to be handed down, one bit at a time, to the Physical layer, which will use bit timing rules to encode the data in a digital signal. Every device on the network segment will synchronize with the clock and extract the 1s and 0s from the digital signal and build a frame. After the frame is rebuilt, a CRC is run to make sure the frame is OK. If everything turns out to be all good, the hosts will check the destination address to see if the frame is for them.
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