12.1 Chapter 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

12.2 왜 multiple access control? 11 장에서의 data link layer 는 호스트 (host) 간에 point-to-point link 로 연결된 경우를 전제로 하였다. 하지만 동일한 링크를 여러 호스트가 공유하여 프레임을 전송할 경우, 해결해야 할 문제가 더 있다. 두 개 이상의 호스트들이 동시에 전송을 할 경우 충돌 (collision) 이 발생하여 전송을 할 수 없다.

12.3 따라서 전송 채널을 공유하여 전송할 경우 어느 순간에 하나의 호스트 만 전송을 하도록 하거나, 만약 충돌이 발생했을 경우 다시 전송할 수 있도록 해야 한다. 이것을 multiple access control(MAC) 이라고 한다. 꽝

12.4 Figure 12.1 Data link layer divided into two functionality-oriented sublayers

12.5 Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter

RANDOM ACCESS In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. ALOHA Carrier Sense Multiple Access (CSMA) Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Topics discussed in this section:

12.7 Figure 12.3 Frames in a pure ALOHA network

12.8 Figure 12.4 Procedure for pure ALOHA protocol

12.9 Figure 12.5 Vulnerable time for pure ALOHA protocol

12.10 Figure 12.6 Frames in a slotted ALOHA network

12.11 Carrier Sensing Multiple Access (CSMA) ALOHA 방식에서 개선할 점은 “ 상대방이 전송을 하고 있으면 나는 전송을 하지 않고 상대방의 전송이 끝날 때 까지 기다린다.” 는 것이다. 따라서 CSMA 에서는 어느 누가 현재 channel 을 사용하고 있는지 ( 전송하고 있는지 ) 확인을 한 후에 (carrier sensing) 전송을 한다.

12.12 Figure 12.8 Space/time model of the collision in CSMA

12.13 Figure 12.9 Vulnerable time in CSMA

12.14 CSMA/CD (collision detection) 전송을 하기 전에 아무도 현재 channel 을 사용하지 않는다는 것을 확인해도 프레임이 전파되는 시간 때문에 충돌이 발생할 수 있다. 따라서 전송을 하는 동안에는 혹시 충돌이 발생했는지 확인을 하고 (collision detection), 충돌이 발생했다면 전송을 중단하고 나중에 다시 전송을 해야 한다.

12.15 Figure Collision of the first bit in CSMA/CD

12.16 Figure Collision and abortion in CSMA/CD

12.17 A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal, as we see later) is 25.6 μs, what is the minimum size of the frame? Example 12.5 Solution The frame transmission time is T fr = 2 × T p = 51.2 μs. This means, in the worst case, a station needs to transmit for a period of 51.2 μs to detect the collision. The minimum size of the frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet.

12.18 Figure Flow diagram for the CSMA/CD

12.19 Figure Energy level during transmission, idleness, or collision

12.20 Figure Behavior of three persistence methods

12.21 Figure Flow diagram for three persistence methods

CONTROLLED ACCESS In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. We discuss three popular controlled-access methods. Reservation Polling Token Passing Topics discussed in this section: 어느 순간에 하나의 스테이션 만 전송을 하도록 한다.

12.23 Figure Reservation access method Reservation slots

12.24 Figure Select and poll functions in polling access method

12.25 Token passing 전송할 프레임 있음 data copy Token frame Data frame

12.26 Figure Logical ring and physical topology in token-passing access method

CHANNELIZATION Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we discuss three channelization protocols. Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA) Topics discussed in this section:

12.28 We see the application of all these methods in Chapter 16 when we discuss cellular phone systems. Note

12.29 Figure Frequency-division multiple access (FDMA)

12.30 In FDMA, the available bandwidth of the common channel is divided into bands that are separated by guard bands. Note

12.31 Figure Time-division multiple access (TDMA)

12.32 In TDMA, the bandwidth is just one channel that is timeshared between different stations. Note

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

12.34 Figure Simple idea of communication with code

12.35 Figure Chip sequences

12.36 Figure Data representation in CDMA

12.37 Figure Sharing channel in CDMA

12.38 Figure Digital signal created by four stations in CDMA

12.39 Figure Decoding of the composite signal for one in CDMA

12.40 Figure General rule and examples of creating Walsh tables