1. Teknik Akses Jaringan - Non Carrier Sense Pertemuan 11 Matakuliah: H0484/Jaringan Komputer Tahun: 2007

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

3. Bina Nusantara Outline Materi ALOHA Slotted ALOHA Reservation ALOHA

4. Bina Nusantara 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. Bina Nusantara 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 802.11 b/a, or WiFi

6. Bina Nusantara Multiple-access protocols

7. Bina Nusantara Medium Access Methods

8. Bina Nusantara 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. Bina Nusantara 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. Bina Nusantara ALOHA Network

11. Bina Nusantara 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. Bina Nusantara – 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. Bina Nusantara 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. Bina Nusantara 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. Bina Nusantara Efficiency of ALOHA