Multiple access (kỹ THUẬT TRUYỀN số LIỆU SLIDE)

Multiple Access Outline  Multiple access mechanisms  Random access  Controlled access  Channelization Sublayers of Data Link Layer Multiple Access Mechanisms Random Access Random Access  Also called contention-based access  No station is assigned to control another ALOHA Network Frames in Pure ALOHA ALOHA Protocol Example  Calculate possible values of T when stations on an ALOHA B network are a maximum of 600 km apart Tp = (600 × 10 ) / (3 × 10 ) = ms  When K=1, T ∈ {0ms,2ms} B  When K=2, T ∈ {0ms,2ms,4ms,6ms} B  : 10 Token Passing 34 Channelization Channelization  Similar to multiplexing  Three schemes  Frequency-Division Multiple Access (FDMA)  Time-Division Multiple Access (TDMA)  Code-Division Multiple Access (CDMA) 36 FDMA 37 TDMA 38 CDMA  One channel carries all transmissions at the same time  Each channel is separated by code 39 CDMA: Chip Sequences  Each station is assigned a unique chip sequence  Chip sequences are orthogonal vectors  Inner product of any pair must be zero  With N stations, sequences must have the following properties:  They are of length N  Their self inner product is always N 40 CDMA: Bit Representation 41 Transmission in CDMA 42 CDMA Encoding 43 Signal Created by CDMA 44 CDMA Decoding 45 Sequence Generation  Common method: Walsh Table  Number of sequences is always a power of two 46 Example: Walsh Table  Find chip sequences for eight stations 47 Example: Walsh Table  There are 80 stations in a CDMA network What is the length of the sequences generated by Walsh Table? 48 ...Outline  Multiple access mechanisms  Random access  Controlled access  Channelization Sublayers of Data Link Layer Multiple Access Mechanisms Random Access Random Access  Also called... Channelization  Similar to multiplexing  Three schemes  Frequency-Division Multiple Access (FDMA)  Time-Division Multiple Access (TDMA)  Code-Division Multiple Access (CDMA) 36 FDMA 37 TDMA... station does when channel is idle or busy 21 Persistence Methods 22 CSMA/CD  Carrier Sense Multiple Access with Collision Detection  Station monitors channel while sending a frame 23 Energy