1 Pertemuan 13 Teknik Akses Jaringan - Random Matakuliah: H0174/Jaringan Komputer Tahun: 2006 Versi: 1/0

2 Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : Menjelaskan teknik akses ke jaringan secara random

3 Outline Materi ALOHA Slotted ALOHA Reservation ALOHA

4 Medium Sharing Techniques Allocation Principles Channel partitioning (Multiple Access) –Divide channel into smaller pieces (time slots, frequency) –Allocate piece to node for exclusive use Random access –Allow collisions and Recover from collisions Scheduled /Taking turns –Tightly coordinate shared access to avoid collisions

5 /Broadcast Networks N senders and receivers connected by a shared medium (copper wire, atmosphere, water) Shared local access to the same media Multiple Access Protocol Determine which host is allowed to transmit next to a shared medium Channel partition: TDMA, FDMA, CDMA Random access: ALOHA, CSMA/CD, CSMA/CA Local Area Network (LAN) Ethernet, Fast Ethernet, Gigabit Ethernet, Wireless Ethernet, or b/a, or WiFi

6 Multiple-access protocols

7 Medium Access Methods

8 General principles: No Carrier Sensing Carrier Sensing Wait for silence Then talk (transmit) Listen while talking Backoff if there is transmission. Repeat Protocols also add a random increasing timeout Random Access Protocols

9 No Carrier Sensing ALOHA –Useful in which uncoordinated users are competing for the use of a single shared channel. –Pure Aloha (no global time synchronization) and Slotted Aloha (global time synch. required)

10 ALOHA Network

11 Pure ALOHA PURE ALOHA –Stations transmit whenever they have information to send at all times. –Collision will occur when two hosts try to transmit packets at the same time –If first bit of a frame overlaps with last bit of an earlier frame then both will be destroyed –Colliding frames destroyed detected by listening to the channel –Such a system is called a contention system

12 –If no ACK by timeout, then wait a randomly selected delay to avoid repeated collisions, then retransmit –Too short : large number of collisions –Too long : underutilization –Frames are the same size –In LAN feedback is immediate. For satellite broadcast 270msec delay Pure ALOHA

13 Pure ALOHA - Collision No synchronization Send without awaiting for beginning of slot Collision of packets can occur when a packet overlaps another packet: –Packet sent at t 0 collide with packets sent at [t 0 -1] or at [t 0 +1] time Collision Wasted Time Colliding with B Packet C Packet B Packet A t0t0 t 0 -1t 0 +1 Collision Overlap with start of packet B Overlap with end of packet B

14 Rather than sending a packet at any time, send along time slot boundaries Collisions are confined to one time slot Slotted ALOHA time Collision Wasted Time Colliding with B Packet CPacket B Packet A t0t0 t 0 -1t 0 +1

15 Efficiency of ALOHA

16 Pertemuan 14 Teknik Akses Jaringan – Carrier Sense Matakuliah: H0174/Jaringan Komputer Tahun: 2006 Versi: 1/0

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

18 Outline Materi CSMA Back-off strategy

19 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

20 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

21 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

22 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

23 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

24 IEEE 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

25 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

26 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

27 Comparison of multiple access G (transmission attempts per packet time)