Overview Jaringan Komputer (3) Kuliah 4 Overview Jaringan Komputer (3)

ARQ Protocols Automatic Repeat Request. Protocols that wait for ACK before sending more data. ACKs now are used for flow AND error control. What can happen? At receiver: frame arrives correctly, frame arrives damaged, frame does not arrive. At sender: ACK arrives correctly, ACK arrives damaged, ACK does not arrive.

ARQ Protocols Sender: Receiver: Send frame 0. Start timer. If ACK 0, arrives, send frame 1. If timeout, re-send frame 0. Receiver: **Waits for frame. If frame arrives, check if correct sequence number. Then send ACK for that frame. Go to (**)

Sliding Window Protocols Window: number of “outstanding” frames at any given point in time. Every ACK received, window slides…

Sliding Window Protocols A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) After the first frame has been received. (d) After the first acknowledgement has been received.

A One-Bit Sliding Window Protocol Normal case. (b) Abnormal case. The notation is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet.

A Protocol Using Go Back N Pipelining and error recovery. Effect on an error when (a) Receiver’s window size is 1. (b) Receiver’s window size is large.

A Sliding Window Protocol Using Selective Repeat (a) Initial situation with a window size seven. (b) After seven frames sent and received, but not acknowledged. (c) Initial situation with a window size of four. (d) After four frames sent and received, but not acknowledged.

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination

MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions “recover” from collisions “Taking turns” Nodes take turns, but nodes with more to send can take longer turns

Random Access Protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Slotted ALOHA Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success

Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization

Throughput versus offered traffic for ALOHA systems. Pure ALOHA (3) Throughput versus offered traffic for ALOHA systems.

Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others!

CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist

CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

Collision-Free Protocols The basic bit-map protocol.

Collision-Free Protocols (2) The binary countdown protocol. A dash indicates silence.

Adaptive Tree Walk Protocol The tree for eight stations.

IP ADRESSING & ROUTER

IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link router’s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface, not host, router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 223.1.3.1 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1

IP Addressing IP address: network part (high order bits) host part (low order bits) What’s a network ? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 LAN 223.1.3.1 223.1.3.2 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address)

IP Addressing How to find the networks? Detach each interface from router, host create “islands of isolated networks 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.3.2 223.1.3.1 223.1.3.27 223.1.2.6 Interconnected system consisting of six networks 223.1.2.1 223.1.2.2

IP Addresses class A B C D 32 bits network host 10 network host 110 1.0.0.0 to 127.255.255.255 A network host B 128.0.0.0 to 191.255.255.255 10 network host 192.0.0.0 to 223.255.255.255 C 110 network host 224.0.0.0 to 239.255.255.255 D 1110 multicast address 32 bits

IP addressing: Subnetting The technique used to allow a single network address to span multiple physical networks is called subnet addressing, subnet routing, or subnetting. 128.10.1.0 Rest of Internet R All traffic to 128.10.0.0 128.10.2.0 A single Class B network with two physical networks

IP addressing: Subnetting Internet part Local part Subnet scheme Physical network Internet part Host

IP addressing: CIDR classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network portion of address 11001000 00010111 00010000 00000000 network part host 200.23.16.0/23

Tugas Baca & Presentasi Bab Nrp 9 01,04 10 05,09, 11 12,13,14,15 12 16,17,19 13 20,21,26 14 28,29,30,701 15 31,32,33 Referensi: William Stallings: Wireless Communications & Networks (second edition)