Читать книгу Understanding Infrastructure Edge Computing - Alex Marcham - Страница 45

Layer 2 is known as the data link layer and is responsible for a few key functions of the network stack. One of these is medium access control (MAC), which is the process of determining the means by which a network endpoint may access the transmission medium to send data. In many networks, this is not as simple as merely transmitting immediately whenever layer 3 communicates to layer 2 that it has data to send. Consider a wireless network as an example: The radio spectrum used by these networks is a shared medium, where transmissions from one network endpoint are able to be received by many other network endpoints. If one endpoint were to transmit without checking to see if the “coast is clear” and transmitted at the same time as another endpoint, it is likely that both transmissions would be garbled, resulting in data being lost or being retransmitted. This is to be avoided wherever possible as it reduces the efficiency of the network considerably.

Protocols operating at layer 2 are also responsible for providing link local addressing, a form of network endpoint identification where endpoint interfaces are given locally unique identifiers that enable intranetwork communication but which are not intended to be used as globally unique; this means that they are not suitable for use between networks but can uniquely identify endpoints on the same network. Through various means such as broadcast and multicast data transmission, these link local addresses are often used to allow an endpoint to discover the globally unique addresses of the endpoints it needs to communicate with, or at least that of the next hop in the path to get there.