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成帧
编码将比特转换为信号，但网络适配器还必须在比特流中找到每一帧的边界，对此有三种经典答案：在 PPP 等面向字节的协议中采用哨兵字符加字节填充，在 HDLC 等面向比特的协议中采用特殊比特模式加比特填充，以及在基于时钟的 SONET 中通过重复模式来定位固定大小的帧，每种方案对边界标记出现在数据本身时的处理方式各不相同。
1. From a stream of bits to frames
From a stream of bits to frames
2. Sentinels and stuffing
Sentinels and stuffing
3. Bit-oriented framing
Bit-oriented framing
4. Clock-based framing
Clock-based framing
5. Why framing exists
Encoding turns bits into signals. Why is a separate framing mechanism still needed on top?
6. Character stuffing
In a byte-oriented sentinel scheme, how does the sender transmit a payload byte that equals the end-of-text sentinel?
7. The count field's blast radius
What is the characteristic failure of count-based framing when the header's length field is corrupted in transit?
8. Bit stuffing in HDLC
HDLC marks frame boundaries with 01111110 and uses bit stuffing. What does the sender stuff, and what does the receiver do on seeing five 1s?
9. SONET's boundary without stuffing
How does SONET locate frame boundaries without any bit stuffing?
10. Choosing a scheme
A designer wants frames whose on-the-wire length never depends on the payload's contents, so that timing is perfectly predictable. Which family of framing serves that requirement, and why?
11. The PPP flag
The Point-to-Point Protocol and HDLC both delimit frames with the same distinguished byte. Give its 8-bit value as a binary string.
12. Five then a zero
In HDLC bit stuffing, after how many consecutive 1 bits in the payload does the sender insert a 0?
13. The STS-1 frame size
A SONET STS-1 frame is drawn as 9 rows of 90 bytes each. How many bytes long is one frame?
14. The escaping convention
In byte-oriented sentinel framing, what is the name of the technique that precedes each in-payload sentinel byte with a special escape byte?