CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 16 Introduction to Computer Networks

CMPE 150- Introduction to Computer Networks 2 Announcements Midterm on –In class, closed books/notes. Homework 3 is up. –Due on 11, Lab next week: discussion/review sessions for the midterm.

CMPE 150- Introduction to Computer Networks 3 Today MAC (cont’d).

CMPE 150- Introduction to Computer Networks 4 Channel Allocation Problem How to allocate single shared, broadcast channel among several stations/users. If no arbitration, several stations/users may transmit at the same time: COLLISIONS! A.k.a., “floor control”.

CMPE 150- Introduction to Computer Networks 5 Multiplexing Static: –FDM. –TDM. Dynamic: –Statistical TDM.

CMPE 150- Introduction to Computer Networks 6 Dynamic Multiplexing Dynamic allocation. In particular, statistical TDM. –Dynamically allocates time slots on demand. Increased channel utilization. But…

CMPE 150- Introduction to Computer Networks 7 Types of MAC Control: –Distributed. –Centralized. How they coordinate medium access: –Round-robin. –Scheduled-access. –Contention-based.

CMPE 150- Introduction to Computer Networks 8 Round-Robin MAC Centralized: polling. Distributed: token passing.

CMPE 150- Introduction to Computer Networks 9 Scheduled Access MAC Time divided into slots.

CMPE 150- Introduction to Computer Networks 10 Contention-Based MAC No control. Stations try to acquire the medium. Distributed in nature.

CMPE 150- Introduction to Computer Networks 11 MAC Protocols Contention-based –ALOHA and Slotted ALOHA. –CSMA. –CSMA/CD. Round-robin : token-based protocols. –Token bus. –Token ring.

CMPE 150- Introduction to Computer Networks 12 Contention-Based MACs ALOHA family. CSMA family.

CMPE 150- Introduction to Computer Networks 13 The ALOHA Protocols U of Hawaii in early 70’s. Packet radio networks. “Free for all”: whenever station has a frame to send, it does so. –Station listens for maximum RTT for an ACK. –If no ACK, re-sends frame for a number of times and then gives up. –Receivers check FCS and destination address to ACK.

CMPE 150- Introduction to Computer Networks 14 Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

CMPE 150- Introduction to Computer Networks 15 Collisions Invalid frames may be caused by channel noise or Because other station(s) transmitted at the same time: collision. Collision happens even when the last bit of a frame overlaps with the first bit of the next frame.

CMPE 150- Introduction to Computer Networks 16 Pure ALOHA: Performance Vulnerable period for the shaded frame.

CMPE 150- Introduction to Computer Networks 17 ALOHA’s Performance (Cont’d) S = G e -2G, where S is the throughput (rate of successful transmissions) and G is the offered load. S = S max = 1/2e = for G=0.5.

CMPE 150- Introduction to Computer Networks 18 Slotted Aloha Doubles performance of ALOHA. Frames can only be transmitted at beginning of slot: “discrete” ALOHA. Vulnerable period is halved. S = G e -G. S = S max = 1/e = for G = 1.

CMPE 150- Introduction to Computer Networks 19 ALOHA Protocols: Performance Throughput versus offered traffic for ALOHA systems.

CMPE 150- Introduction to Computer Networks 20 ALOHA Protocols: Summary Simple. But, poor utilization… –When?

CMPE 150- Introduction to Computer Networks 21 CSMA Protocols

CMPE 150- Introduction to Computer Networks 22 Carrier Sense Multiple Access The capacity of ALOHA or slotted ALOHA is limited by the large vulnerability period of a packet. By listening before transmitting, stations try to reduce the vulnerability period to one propagation delay. This is the basis of CSMA (Kleinrock and Tobagi, UCLA, 1975).

CMPE 150- Introduction to Computer Networks 23 CSMA Station that wants to transmit first listens to check if another transmission is in progress (carrier sense). If medium is in use, station waits; else, it transmits. Collisions can still occur. Transmitter waits for ACK; if no ACKs, retransmits.

CMPE 150- Introduction to Computer Networks 24 CSMA Protocol transmit no wait for a round-trip time positive ack? yes compute random backoff integer k no delay packet transmission k times Packet ready Channel Busy? yes

CMPE 150- Introduction to Computer Networks 25 CSMA (cont’d) Collisions can occur only when 2 or more stations begin transmitting within short time. If station transmits and no collisions during the time leading edge of frame propagates to farthest station, then NO collisions.

CMPE 150- Introduction to Computer Networks 26 CSMA Flavors After detecting carrier, a station can persist trying to transmit after the channel is idle again. 1-persistent CSMA (IEEE 802.3) –If medium idle, transmit; if medium busy, wait until idle; then transmit with p=1. –If collision, waits random period and starts again. Non-persistent CSMA: if medium idle, transmit; otherwise wait a random time before re-trying. –Thus, station does not continuously sense channel when it is in use. P-persistent: when channel idle detected, transmits packet in the first slot with p. –Slotted channel, i.e., with probability q = p-1, defers to next slot.

CMPE 150- Introduction to Computer Networks 27 CSMA vesrsu Aloha Comparison of the channel utilization versus load for various random access protocols.

CMPE 150- Introduction to Computer Networks 28 CSMA/CD CSMA with collision detection. Problem: when frames collide, medium is unusable for duration of both (damaged) frames. For long frames (when compared to propagation time), considerable waste. What if station listens while transmitting?

CMPE 150- Introduction to Computer Networks 29 CSMA/CD Protocol 1. If medium idle, transmit; otherwise If medium busy, wait until idle, then transmit with p=1. 3. If collision detected, transmit brief jamming signal and abort transmission. 4. After aborting, wait random time, try again.

CMPE 150- Introduction to Computer Networks 30 CSMA/CD Performance Wasted capacity restricted to time to detect collision. Time to detect collision < 2*maximum propagation delay. Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission.

CMPE 150- Introduction to Computer Networks 31 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

CMPE 150- Introduction to Computer Networks 32 Ethernet

CMPE 150- Introduction to Computer Networks 33 Ethernet IEEE 802. family. Standards for LANs and MANs. Ethernet defined in the IEEE standard. PHY, MAC, and LLC.

CMPE 150- Introduction to Computer Networks 34 Where in the Stack? (a) Position of LLC. (b) Protocol formats.

CMPE 150- Introduction to Computer Networks 35 Ethernet MAC CSMA/CD. Binary exponential back-off.

CMPE 150- Introduction to Computer Networks 36 Ethernet Frame Frame formats. (a) DIX Ethernet, (b) IEEE