1. Teknik Akses Jaringan Carrier Sense Pertemuan 12 Matakuliah: H0484/Jaringan Komputer Tahun: 2007

2. Bina Nusantara Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : Menjelaskan teknik akses ke jaringan dengan carrier sense

3. Bina Nusantara Outline Materi CSMA Back-off strategy

4. Bina Nusantara Carrier Sense Multiple Access ALOHA & Slotted ALOHA are inefficient because stations don’t take into account what other stations are doing before they transmit (Talk-before-listen) Sense for carriers (see if anyone else is transmitting) before begin transmitting Carrier Sense Multiple Access (CSMA) is Listen Before Talk In LAN’s it is possible for stations to detect what other stations are doing and reactively change. CSMA can improve performance These protocols are called carrier sense protocols. They are named 1-persistent CSMA, non-persistent CSMA, p- persistent CSMA

5. Bina Nusantara When station has data to send, it listens to channel Channel idle: station transmits packet Channel busy: station waits till channel is idle When channel becomes free, a host transmits its packet immediately (with probability 1) 1 - Persistent Packet A time Station B listens Packet B delay Station B sends

6. Bina Nusantara Propagation delay II – If propagation delay is small zero, collision may still occur – Station A transmits, Stations B, X, Y simultaneously realize that line is busy and wait – When line is free, stations B transmits then station X, Y simultaneously transmit. Collision. Collision in CSMA Packet A time Station B listens Packet B Packet Y delay Collision still possible over long propagation delays Packet X Station B sends

7. Bina Nusantara Before sending, station senses channel If no transmission, station starts sending. If channel is busy it does not continuously sense the channel in order to start transmitting Wait/sleep a random interval before sensing again As soon as channel is idle, then send a packet Random interval reduces collisions Higher throughput than 1-persistent CSMA when many senders Non Persistent Packet A time Station B listens Packet B Random delay Station B sends

8. Bina Nusantara Generalization of 1-persistent CSMA Typically applied to slotted channels Slot length is chosen as maximum propagation delay A station senses the channel, and If slot is idle, transmit with probability p, or defer with probability q=1-p If next slot is idle, transmit with probability p, or defer with probability 1-p, repeat… If channel is initially busy, it waits until the next slot then sense channel continuously until it becomes free and applies the above algorithm p-Persistent

9. Bina Nusantara IEEE 802.3 is a standard for a 1-persistent CSMA/CD LAN – If cable is busy, station waits until cable is idle. – If 2 or more stations simultaneously transmit on an idle cable, they will collide. – All colliding stations then terminate their transmission and, wait random time and then start process again. Ethernet is a specific implementation. CSMA with Collision Detection

10. Bina Nusantara Ethernet uses CSMA/CD Listen-while-talk protocol A station listens even while it is transmitting, and if a collision is detected, stops transmitting Ethernet - CSMA/CD Packet A time Station B listens Packet B delay Packet B size Not transmitted Station B sends Packet B Station B detects collision, Stops sending

11. Bina Nusantara Ethernet CSMA/CD requires a minimum size packet If packet B arrives at A and A is no longer transmitting, then Host A will Fail to detect the collision Thinks its packet got through Thinks the incoming packet is a new packet Therefore, to detect a collision: Minimum packet size >= 2*(prop. delay)*BW A Ethernet Packet A t d=propagation delay B Packet A Packet B Packet B t+d B transmit at t+d, just before packet A arrives. B sees collision and transmits a runt” packet Packet arrives at t+2d Minimum size Ethernet packet

12. Bina Nusantara Comparison of multiple access G (transmission attempts per packet time)