274 Multiaccess Communication Chap.4 TDM for multiple sessions on a single point-to-point link,namely increased delay and underutilization of the medium. In Section 4.5.I we shall find that delay can be reduced and utilization increased by sharing the medium on a demand basis.This is more difficult than demand sharing (i.e.,statistical multiplexing)on a point-to-point link because the earth stations are not aware of the instantaneous traffic requirements of other earth stations.If several stations transmit at the same time (and in the same frequency band),their signals are garbled and incorrectly received. 4.1.2 Multidrop Telephone Lines Another example of a multiaccess channel is a multidrop telephone line.Such lines connect one primary node with a number of secondary nodes:the signal transmitted by the primary node goes over one pair of wires and is received by all the secondary nodes. Similarly,there is a return pair of wires which carries the sum of the transmitted signals from all the secondary nodes to the primary node.Conceptually,this is like a satellite channel.The secondary nodes,or earth stations,share the path to the primary node,or satellite.whereas the primary node,or satellite.broadcasts to all the secondary nodes. or earth stations.Most communication on a multidrop phone line is intended to go from primary to secondary.or vice versa,whereas most communication on a satellite channel is relayed by the satellite from one earth station to another.Conceptually,this difference is not very important,since the major problem is that of sharing the channel from the secondary nodes to the primary,and it makes little difference whether the messages are removed at the primary node or broadcast back to all the secondary nodes. The traditional mode of operation for multidrop telephone lines is for the primary node to poll (i.e..request information from)each secondary node in some order.Each secondary node responds to its poll either by sending data back to the primary station or by indicating that it has no data to send.This strategy avoids interference between the secondary nodes,since nodes are polled one at a time,but there is a certain amount of inefficiency involved,both in the time to send a poll and the time to wait for a response from a node with no data. 4.1.3 Multitapped Bus A third example of a multiaccess channel is a bus with multiple taps.In this case,each node can receive the signal sent by each other node,but again,if multiple nodes transmit at the same time.the received signal is garbled.We shall discuss this example later in the context of Ethernet.but for now.we observe that conceptually this channel is very similar to a satellite channel.Each node can communicate with each other node,but if nodes transmit simultaneously.the received signal cannot be correctly detected.The fact that nodes can hear each other directly here,as opposed to hearing each other via relay from the satellite.has some important practical consequences.but we will ignore this for now.274 Multiaccess Communication Chap. 4 TDM for multiple sessions on a single point-to-point link, namely increased delay and underutilization of the medium. In Section 4.5.1 we shall find that delay can be reduced and utilization increased by sharing the medium on a demand basis. This is more difficult than demand sharing (i.e., statistical multiplexing) on a point-to-point link because the earth stations are not aware of the instantaneous traffic requirements of other earth stations. If several stations transmit at the same time (and in the same frequency band), their signals are garbled and incorrectly received. 4.1.2 Multidrop Telephone Lines Another example of a multiaccess channel is a multidrop telephone line. Such lines connect one primary node with a number of secondary nodes; the signal transmitted by the primary node goes over one pair of wires and is received by all the secondary nodes. Similarly, there is a return pair of wires which carries the sum of the transmitted signals from all the secondary nodes to the primary node. Conceptually, this is like a satellite channel. The secondary nodes, or earth stations, share the path to the primary node, or satellite, whereas the primary node, or satellite. broadcasts to all the secondary nodes, or earth stations. Most communication on a multidrop phone line is intended to go from primary to secondary, or vice versa, whereas most communication on a satellite channel is relayed by the satellite from one earth station to another. Conceptually, this difference is not very important, since the major problem is that of sharing the channel from the secondary nodes to the primary, and it makes little difference whether the messages are removed at the primary node or broadcast back to all the secondary nodes. The traditional mode of operation for multidrop telephone lines is for the primary node to poll (i.e .. request information from) each secondary node in some order. Each secondary node responds to its poll either by sending data back to the primary station or by indicating that it has no data to send. This strategy avoids interference between the secondary nodes, since nodes are polled one at a time, but there is a certain amount of inefficiency involved, both in the time to send a poll and the time to wait for a response from a node with no data. 4.1.3 Multitapped Bus A third example of a multiaccess channel is a bus with multiple taps. In this case. each node can receive the signal sent by each other node, but again, if multiple nodes transmit at the same time. the received signal is garbled. We shall discuss this example later in the context of Ethernet. but for now, we observe that conceptually this channel is very similar to a satellite channel. Each node can communicate with each other node, but if nodes transmit simultaneously. the received signal cannot be correctly detected. The fact that nodes can hear each other directly here, as opposed to hearing each other via relay from the satellite. has some important practical consequences. but we will ignore this for now