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
机和 35mm 胶卷等。缺乏国际标准的领域包括合适录像机和录像带（美国是 NTSC VHS，欧
洲是 PAL），手提电话，电灯和灯泡（不同的国家使用不同的电压），影印机和纸（美国为
8.5*11 英寸，其他地方为 A4）等。
我的内容管理
收起
我的收益 登录查看自己的收益
我的积分
登录查看自己的积分
我的C币
登录后查看C币余额
我的收藏 
我的下载 
下载帮助 
评论0