Distributed systems Module 1 -Basic networking Teaching unit 1 – LAN standards Ernesto Damiani University of Bozen-Bolzano Lesson 4 – Ethernet frame

Ethernet frame (1) The preamble is a sequence of 7 bytes, each sets to “ ” –It is used to synchronize the receiver before actual data are sent Addresses –Unique, 48-bit unicast address assigned to each adapter  For example: D5 - B1 - B2  Each manufacturer has own specific sequence of addresses –Broadcast: all 1’s –Multicast: the first bit is 1

Ethernet frame (2) Type field is a demultiplex key This key is used to determine which higher level protocol the frame should be sent to The body may contain up to 1500 data bytes

Ethernet’s MAC Algorithm (1) In Aloha, transmission decisions are taken without considering what other nodes are doing –Ethernet uses CSMA/CD; it listens to the line before and during the transmission –If line is idle (no carrier sensed)  it sends packet immediately  upper bound message size of 1500 bytes  it must wait 9.6us among back-to-back frames

Ethernet’s MAC Algorithm (2) If line is busy (carrier sensed) –It waits until it is idle and transmit the packet immediately If collision is detected –It stops sending and enters the jam signal –It tries again later

CSMA/CD FINE