C H 4 T HE M EDIUM A CCESS C ONTROL S UBLAYER 1 Medium Access Control: a means of controlling access to the medium to promote orderly and efficient use.

OSI M ODEL AND P ROJECT 802 2

T HE C HANNEL A LLOCATION P ROBLEM Static Channel Allocation in LANs and MANs FDM: small and constant users, heavy load of traffic of each. TDM:same problem. Poor performance. None of the static channel allocation methods work well with bursty traffic. Dynamic Channel Allocation in LANs and MANs 3

P URE ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times. 4 Multiple Access Protocols

P URE ALOHA (2) Vulnerable period for the shaded frame. 5

SLOTTED ALOHA Time in uniform slots equal to frame transmission time Need central clock (or other sync mechanism) Transmission begins at slot boundary Frames either miss or overlap totally Max utilization 36.8% 6

7 SLOTTED ALOHA

RELATIVE FORMULAS FOR THE ALOHA 8 Throughput or Channel Utilization Probability of collisionProbability of success Pure ALO HA Slott ed ALO HA

PURE ALOHA AND SLOTTED ALOHA Throughput versus offered traffic for ALOHA systems. 9

P ERSISTENT AND N ONPERSISTENT CSMA All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit with a probability. If two stations start at the same instant, collision Propagation time is much less than transmission time Wait reasonable time (round trip plus ACK contention) No ACK then retransmit 10

P ERSISTENT AND N ONPERSISTENT CSMA Comparison of the channel utilization versus load for various random access protocols. 11

CSMA/CD (WITH COLLISION DETECTION) If collision detected, jam then cease transmission rather than finish transmitting their frame After jam, wait random time then start again Half-duplex system Save time and bandwidth. Basis of Ethernet LAN. 12

CSMA/CD OPERATION 13

T OKEN R ING (802.5) MAC protocol Small frame (token) circulates when idle Station waits for token Changes one bit in token to make it SOF for data frame Append rest of data frame Frame makes round trip and is absorbed by transmitting station Station then inserts new token when transmission has finished and leading edge of returning frame arrives Under light loads, some inefficiency Under heavy loads, round robin makes efficiency and fair. 14

T OKEN R ING O PERATION 15

FDDI MAC P ROTOCOL Fiber Distributed Data Interface As for except: Station seizes token by aborting token transmission Once token captured, one or more data frames transmitted New token released as soon as transmission finished 16

E THERNET Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Ethernet Performance Switched Ethernet Fast Ethernet Gigabit Ethernet IEEE 802.2: Logical Link Control 17

13.18 Ethernet evolution through four generations

ETHERNET TOPOLOGY Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. 19

B ASEBAND C ONFIGURATION The size limitation is usually solved by using repeaters to divide the medium into smaller segments Repeaters relay digital signals in both directions, making the segments appear like one medium As repeaters recover the digital signal, they remove any attenuation 20

13.21 Figure A network with and without a bridge

13.22 Figure Collision domains in an unbridged network and a bridged network

13.23 Figure Switched Ethernet

FAST ETHERNET Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. MAC Sublayer Physical Layer Topics discussed in this section:

13.25 Figure Fast Ethernet topology

13.26 Figure Fast Ethernet implementations

13.27 Table 13.2 Summary of Fast Ethernet implementations

GIGABIT ETHERNET The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z. MAC Sublayer Physical Layer Ten-Gigabit Ethernet Topics discussed in this section:

13.29 In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable. Note

13.30 Figure Topologies of Gigabit Ethernet

13.31 Figure Gigabit Ethernet implementations

13.32 Table 13.3 Summary of Gigabit Ethernet implementations

G IGABIT E THERNET (2) 33 Gigabit Ethernet - Differences zCarrier extension zAt least 4096 bit-times long (512 for 10/100) zFrame bursting extended to 200m. zNew coding

Summary of Ten-Gigabit Ethernet implementations 34

IEEE standard for LANs 35

IEEE 802.2: L OGICAL L INK C ONTROL (a) Position of LLC. (b) Protocol formats. 36

37 E THERNET MAC S UBLAYER P ROTOCOL WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

38 PDU FORMAT WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

M INIMUM AND MAXIMUM LENGTH 39

13.40 Example of an Ethernet address in hexadecimal notation

LAN T RANSMISSION T ECHNOLOGIES Ethernet 10 Mbit/s Token Ring 4/16 Mbit/s Fast Ethernet 100 Mbit/s FDDI 100 Mbit/s Gigabit Ethernet 1 Gbit/s ATM 25 Mbit/s to 2.4 Gbit/s Only Ethernet versions are growing 41