CHAPTER 18 The Network

18.3 Ethernet Frame

FIGURE 18.1 Ethernet Frame

In this model we define:

• Preamble Field used for synchronization, 64-bits

• Destination Address Ethernet address of the destination host, 48-bits

• Source Address Ethernet address of the source host, 48-bits

• Type Type of data encapsulated, e.g. IP, ARP, RARP, etc, 16-bits.

• Data Field Data area, 46-1500 bytes, which has
  Destination Address Internet address of destination host
  Source Address Internet address of source host

• CRC Cyclical Redundancy Check, used for error detection

The data to be sent is encapsulated by each layer, from the Application down to the Physical, and each adds it's own header information. When data is received each layer strips off it's header and then passes the packet up to the next layer. The Transport Layer makes sure that the source and destination, hosts and ports, can be identified, and includes a sequence number so that a file can be broken into multiple packets and recombined on the receiving end. The Internet Layer determines how the frames will be delivered, including fragmenting them to send along a path with a smaller maximum transmission unit (MTU) or recombining them for a larger MTU path. It determines the routing used to get to the destination. The Network Layer provides the encapsulation of the datagram into the frame to be transmitted over the network. It includes the ethernet addresses of the source machine and of the next hop towards the destination. These addresses are rewritten with each hop.