Goals of MAC Protocols

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1 Goals of MAC Protocols MAC Protocols arbitrate access to a common shared channel among a population of users 1. Fair among users 2. High efficiency 3. Low delay 4. Fault tolerant 1

2 Ideal Mulitple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple 2

3 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 tightly coordinate shared access to avoid collisions 3

4 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 4

5 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands time 5

6 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique code assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) 6

7 CDMA Encode/Decode 7

8 CDMA: two-sender interference 8

9 Medium Access Sub-layer Network Layer Logical Link Control Sublayer Medium Access Sublayer Data Link Layer Physical Layer 9

10 Medium Access Sub-layer (contd.) Medium access (MAC) sublayer is not important on point-to-point links The MAC sublayer is only used in broadcast or shared channel networks Examples: Satellite, Ethernet, Cellular 10

11 Logical Link Control Sublayer Logical Link Control (LLC) sublayer provides mechanisms for reliable communications acknowledgement, etc 11

12 IEEE 802 Standards 12

13 MAC Sub-layer: Contents Fixed Assignment Protocols Demand Assignment Protocols Contention Access Protocols IEEE 802 LANs DQDB and FDDI 13

14 Fixed Assignment Protocols Static and predetermined allocation of channel access: independent of user activity Idle users may be assigned to the channel, in which case channel capacity is wasted Examples: TDMA, FDMA, WDMA 14

15 Demand Assignment Protocols Allocate channel capacity to hosts on a demand basis (i.e., only to active users) Requires methods for measuring the demand for the channel Polling Reservation schemes Token Passing Scheme 15

16 Polling A central controller interrogates each host and allocates channel capacity to those who need it Good for systems with: Short propagation delay Small polling messages Non-bursty traffic 16

17 Reservation Schemes Hosts independently reserve the channel for period of time Reservations are usually piggybacked on data messages passing along the channel Good for systems with : short propagation delay no central controller node non-bursty traffic 17

18 Token Passing Scheme A token always circulates around a ring net. A user grabs a token to transmit data Will discuss details later in the Token Ring LAN 18

19 Contention Access Protocols Single channel shared by a large number of hosts No coordination between hosts Control is completely distributed Examples: ALOHA, CSMA, CSMA/CD 19

20 Contention Access (contd.) Advantages: Short delay for bursty traffic Simple (due to distributed control) Flexible to fluctuations in the number of hosts Fairness 20

21 Contention Access (contd.) Disadvantages: Low channel efficiency with a large number of hosts Not good for continuous traffic (e.g., voice) Cannot support priority traffic High variance in transmission delays 21

22 Contention Access Methods Pure ALOHA Slotted ALOHA CSMA 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA CSMA/CD 22

EPL 657 Wireless Networks

EPL 657 Wireless Networks Some fundamentals: Multiplexing / Multiple Access / Duplex Infrastructure vs Infrastructureless Panayiotis Kolios Recall: The big picture... Modulations: some basics 2 Multiplexing