Datorntverk A lektion 7 Kapitel 13 Multiple access

Datornätverk A – lektion 7 Kapitel 13: Multiple access control. Local Are Networks. (CSMA/CD, Token Bus, Token Ring, Logical Link Control)

Chapter 13 Multiple Access

Figure 13. 1 Multiple-access protocols

Figure 13. 2 Evolution of random-access methods

Evolution of Contention Protocols Aloha Slotted Aloha CSMA/CD • Developed in 1970 to be used on radio LAN on Hawaiian islands. The access to the channel is random • Improvement to Aloha: Start transmission only at fixed time slots • Carrier Sense Multiple Access: Start transmission only if no transmission is ongoing • CD=Collision Detection: Stop ongoing transmission if collision is detected

Figure 13. 5 Collision in CSMA

Animeringar som illustrerar tystnadsdetektering i CSMA: ○ www. itm. miun. se/~mageri/animations/netbook/anim 06_2 -csma. mov ○ www. itm. miun. se/~mageri/animations/bjnil/anim 1 long. exe Animering som illustrerar kollisionshantering i CSMA/CD: ○ www. itm. miun. se/~mageri/animations/bjnil/anim 1. exe

Figure 13. 6 Persistence strategies

13. 7 CSMA/CD procedure

Figure 13. 8 CSMA/CA procedure

CSMA/CD • • • Sense for carrier. If carrier present, wait until carrier ends. Send packet and sense for collision. If no collision detected, consider packet delivered. Otherwise, abort immediately, perform “exponential back off” and send packet again. • CSMA/CD is used in traditional Ethernet LAN

Exponential Back-off • When a sender detects a collision, it sends a “jam signal”. ○ Jam signal is necessary to make sure that all nodes are aware of the collision ○ Length of the jam signal 48 bits • When collision is detected, the sender resends the signal after a random time ○ The random time is picked from an interval of 0 to 2 N x maximum propagation time ○ N is the number of attempted retransmission ○ Length of the interval increases with every retransmission

13. 3 Channelization FDMA TDMA CDMA

Note: In FDMA, the bandwidth is divided into channels.

Note: In TDMA, the bandwidth is just one channel that is timeshared.

Note: In CDMA, one channel carries all transmissions simultaneously.

Figure 13. 14 Chip sequences

Figure 13. 15 Encoding rules

Figure 13. 16 CDMA multiplexer

Figure 13. 17 CDMA demultiplexer