Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access o channel spectrum divided into frequency bands O each station assigned fixed frequency band O unused transmission time in frequency bands go idle 0 example: 6-station LAN, 1, 3, 4 have pkt, frequency bands 25.6 idle ewos time 8 9 5: Datalink layer 5a-21
5: DataLink Layer 5a-21 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
Channel Partitioning(CDMa CDMA(Code Division Multiple Access) o unique"code"assigned to each user; i.e., code set partitioning o used mostly in wireless broadcast channels(cellular, satellite, etc) o all users share same frequency but each user has own chipping"sequence(i.e, code) to encode data o encoded signal =(original data)X(chipping sequence) ecoding: inner-product of encoded signal and chipping sequence o allows multiple users to"coexist"and transmit simultaneously with minimal interference(if codes are "orthogonal") 5: Datalink layer 5a-22
5: DataLink Layer 5a-22 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”)
CDMA Encode/decode sender channel output Z I, m data bits 日 slot 1 slot o 日面 channel channel output output ot1 +slot0→ ∑Z slot 1 lot o received i received input receiver de 5: DataLink Lo 5a-23
5: DataLink Layer 5a-23 CDMA Encode/Decode
CDMA: two-sender interference senders data bits L dis-1 code,重 hannel. Z data L d=1 bits cde回 d2=22 d=1 d=1 slot 1 slot o received I received Input input receiver coR「 5: Datalink layer 5a-24
5: DataLink Layer 5a-24 CDMA: two-sender interference
Random Access Protocols When node has packet to send o transmit at full channel data rate R o no a priori coordination among nodes o two or more transmitting nodes ->"collision O random access MAC protocol specifies o how to detect collisions o how to recover from collisions(e.g, via delayed retransmissions O Examples of random access mac protocols o slotted alOHA O ALOHA O CSMACSMA/CD, CSMA/CA 5: DataLink Layer 5a-25
5: DataLink Layer 5a-25 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