文档介绍：The Physical Layer
Highlights
Some concepts
Transmission medium
Message switch
Simplified munications Model
Fundamental Concepts (1)
信道(Channel):传送信息的媒体
数据率(Bit Rate):数据传输速率(bps)
波特(Baud):码元传输速率
1 Baud = (log2V) bps,其中V是信号的电平级数
传播速度(Propagation Speed):通信线路上,信号单位时间内传送的距离(米/秒)
带宽(Bandwidth):信号占用的频率范围(Hz)
信道容量(Channel Capacity):信道的最大数据率
吞吐量(Throughput):网络容量的度量,表示单位时间内网络可以传送的数据位数(bps)
误码率BER(Bit Error Rate):信道传输可靠性指标
P= 传送错的位数/ 传送总位数
Fundamental Concepts (2)
时延(Delay):从向网络中发送数据块的第一比特开始,到最后一位数据被接收所经历的时间
时延的组成:传播时延、发送时延、处理时延、排队时延
传播时延:信号通过传输介质的时间。
发送时延:设备发送一个数据块所需要的时间(数据块长度/信道带宽)
处理时延:交换机/路由器检查数据、选路的时间
排队时延:在交换机/路由器中排队等待的时间
Sources of Delay (1)
1. Nodal processing delay
check bit errors
determine output link
2. Queueing dealy:
time waiting at output link for transmission
depends on congestion level of router
Sources of Delay (2)
3. Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into link = L/R
4. Propagation delay:
d = length of physical link
s = propagation speed in medium
propagation delay = d/s
Simplex, Half-duplex, Full-duplex
Simplex
Transmission occurs in only one direction
eg. FM Radio broadcasting
Half-duplex
Transmission can be made in both direction, but at one time only one direction is available
eg. talkie and walkie (对讲机)
Full-duplex
Transmission can be made in both directions simultaneously
eg. Phone conversation
Parallel transmission vs. Serial transmission
Asynchronous Transmission vs. Synchronous Transmission