the Internet. The average person in the developed world may have dozens of
Internet hosts by then.
25. 如果网络容易丢失分组，那么对每一个分组逐一进行确认较好，此时仅重传丢失
的分组。而在另一方面，如果网络高度可靠，那么在不发差错的情况下，仅在整个文件
传送的结尾发送一次确认，从而减少了确认的次数，节省了带宽；不过，即使有单个分
组丢失，也需要重传整个文件。
26. Small, fixed-length cells can be routed through switches quickly, and completely
in hardware. Small, fixed-size cells also make it easier to build hardware that handles many
cells in parallel. Also, they do not block transmission lines for very long, making it easier to
provide quality-of-service guarantees.
27. The speed of light in coax is about 200,000 km/sec, which is 200 meters/sec.
At 10 Mbps, it takes 0.1 sec to transmit a bit. Thus, the bit lasts 0.1 sec in
time, during which it propagates 20 meters. Thus, a bit is 20 meters long
here.
28. The image is 1024 768 3 bytes or 2,359,296 bytes. This is 18,874,368
bits. At 56,000 bits/sec, it takes about 337.042 sec. At 1,000,000 bits/sec, it
takes about 18.874 sec. At 10,000,000 bits/sec, it takes about 1.887 sec. At
100,000,000 bits/sec, it takes about 0.189 sec.
29. Think about the hidden terminal problem. Imagine a wireless network of five
stations, A through E, such that each one is in range of only its immediate
neighbors. Then A can talk to B at the same time D is talking to E. Wireless networks have
potential parallelism, and in this way differ from Ethernet.
30. One disadvantage is security. Every random delivery man who happens to be
in the building can listen in on the network. Another disadvantage is reliability.
Wireless networks make lots of errors. A third potential problem is battery
life, since most wireless devices tend to be mobile.
31. 优点1：如果每个人都使用标准，那么每个人都可以与其他任何人交流；优点2：
广泛使用标准将导致规模经济，比如生产大规模集成电路芯片。缺点1：为了取得标准化
所需要的政治妥协经常会导致差的标准；缺点2：一旦标准被广泛采用了，要对它再做改
变就会非常困难，即使发现了新的更好的技术或方法，也难以替换。
32. 具有国际标准的系统的例子包括CD 播放器和CD 盘片，随声听和录音磁带，照相
