1. Distributed Systems Module 1 -Basic networking Teaching unit 1 – LAN standards Ernesto Damiani University of Bozen-Bolzano Lesson 3 – Introduction to Ethernet

2. Shared access networks They assume multiple nodes connected to the same physical link –Bus, ring and wireless structure –Broadcast: the transmission sent by a node is received by all the others They need some methods to control the access (MAC protocols) –Equity –Performances

3. Multiple access methods Fixed assignment –Channel partitioning so each node gets a slice of the bandwidth –Essentially, circuit switching, thus inefficient Contention-based –Nodes contend for bandwidth and recover from collisions  Examples: Aloha, Ethernet Reservation-based or token-based –They take turns using the channel  Examples: Token Ring

4. Ethernet history (1) It was developed by Bob Metcalfe and others at Xerox PARC in early 70’s It comes from Aloha packet-radio network It was standardized by Xerox, DEC, and Intel in 1978

5. LAN standards They define MAC and physic layer connection –IEEE 802.3 (CSMA/CD - Ethernet) standard; originally 2 Mbps, then 10 Mbps –IEEE 802.3u standard for 100 Mbps Ethernet –IEEE 802.3z standard for 1,000 Mbps (1 Gbps) Ethernet

6. CSMA/CD Ethernet’s MAC (Media Access Control) policy CS = Carrier Sense – It transmits only if medium is idle MA = Multiple Access CD = Collision Detection – It stops transmission immediately if a collision is detected

7. Ethernet overview (1) The most popular packet-switched LAN technology Maximum bus length: 2500m –500 segments with 4 repeaters

8. Ethernet overview (2) Bus and Star topologies are used to connect hosts –Hosts are attached to the network by Ethernet transceiver, hub or switch  The transceiver detects the line state and sends/receives signals  Hubs are used to facilitate shared connections –All hosts on an Ethernet are competing for access to the medium  Switches break this model

9. Ethernet overview (3) Ethernet by definition is a broadcast protocol –Any signal can be received by all hosts –Switching enables individual hosts to communicate Network layer packets are transmitted over an Ethernet by encapsulating Frame format FINE