CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 17 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 Lab this week: discussion/review sessions for midterm.

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

CMPE 150- Introduction to Computer Networks 4 Types of MAC Control: –Distributed. –Centralized. How they coordinate medium access: –Round-robin. –Scheduled-access. –Contention-based.

CMPE 150- Introduction to Computer Networks 5 Round-Robin MAC Centralized: polling. Distributed: token passing.

CMPE 150- Introduction to Computer Networks 6 Scheduled Access MAC Time divided into slots. Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

CMPE 150- Introduction to Computer Networks 7 Scheduled-Access MAC: Example The basic bit-map protocol.

CMPE 150- Introduction to Computer Networks 8 Contention-Based MAC No control. Stations try to acquire the medium. Distributed in nature.

CMPE 150- Introduction to Computer Networks 9 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 10 Contention-Based MACs ALOHA family. CSMA family.

CMPE 150- Introduction to Computer Networks 11 ALOHA Protocols: Performance Throughput versus offered traffic for ALOHA systems.

CMPE 150- Introduction to Computer Networks 12 ALOHA Protocols: Summary Simple. But, poor utilization… –When?

CMPE 150- Introduction to Computer Networks 13 CSMA Protocols

CMPE 150- Introduction to Computer Networks 14 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 15 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 16 Collisions Can collisions still occur?

CMPE 150- Introduction to Computer Networks 17 CSMA Flavors 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 18 CSMA vesrsus Aloha Comparison of the channel utilization versus load for various random access protocols.

CMPE 150- Introduction to Computer Networks 19 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 20 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 21 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. Thus frames need to be at least “2*RTT” long.

CMPE 150- Introduction to Computer Networks 22 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

CMPE 150- Introduction to Computer Networks 23 Ethernet

CMPE 150- Introduction to Computer Networks 24 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 25 Where in the Stack? (a) Position of LLC. (b) Protocol formats.

CMPE 150- Introduction to Computer Networks 26 Ethernet MAC CSMA/CD. Binary exponential back-off.

CMPE 150- Introduction to Computer Networks 27 Ethernet MAC (Cont’d)

CMPE 150- Introduction to Computer Networks 28 Ethernet Frame Length At 10Mbps with 2,500 m maximum distance: –RTT ~ 50 microsec. –Thus, at least 500-bit frames. –It is actually 512 bits. –If fewer bits than that, add “padding”.

CMPE 150- Introduction to Computer Networks 29 Ethernet Frame Frame formats. (a) DIX Ethernet, (b) IEEE Destination address: “1” for “group” addresses. Type: mux/demux of network layer protocols. Data:max. of 1500 bytes.

CMPE 150- Introduction to Computer Networks 30 Binary Exponential Backoff Randomization after collision.

CMPE 150- Introduction to Computer Networks 31 BEB (Cont’d) After first collision, each station waits for 0 or 1 slot before trying again. After second collision, they pick either 0, 1, 2, or 3 slots at random to wait. After 3 rd. collision, number of slots to wait is between 0 and In general, after I collisions, wait is between 0 and 2 i – 1. After 10 collisions, randomization interval frozen at 1023 slots. After 16 collisions, error!

CMPE 150- Introduction to Computer Networks 32 Ethernet Performance

CMPE 150- Introduction to Computer Networks 33 Ethernet Cabling The most common kinds of Ethernet cabling.

CMPE 150- Introduction to Computer Networks 34 Ethernet Cabling (Cont’d) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

CMPE 150- Introduction to Computer Networks 35 Ethernet Topologies Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

CMPE 150- Introduction to Computer Networks 36 Switched Ethernet A simple example of switched Ethernet.

CMPE 150- Introduction to Computer Networks 37 Fast Ethernet The original fast Ethernet cabling.

CMPE 150- Introduction to Computer Networks 38 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

CMPE 150- Introduction to Computer Networks 39 Gigabit Ethernet (Cont’d) Gigabit Ethernet cabling.