Bilingual Course
Network Communications
网络通信
(For Master Students in the Department of Electronic Engineering)
Chapter 6 The Data Communications Interface
Instructor,Dr,Tianshuang Qiu
School of the Electronic and Information Engineering
Fall of 2004
网络通信 2
§ 6-1 Asynchronous and Synchronous
Transmission
说明
– 本节主要讨论信号的串行传输
– 假定每个信号元素表示一个 bit
两个主要问题
– 噪声等传输损伤对接收数据的影响，可能导致出错 ;
– 时序问题，若发 /收不精确一致，也可能出错，且这种时间上的不一致会积累,
网络通信 3
1,异步和同步传输
（ 1）异步传输
1) 概念
In asynchronous transmission,each character of data is treated
independently,Each character begins with a start bit that alerts the
receiver that a character is arriving,The receiver samples each bit in
the character and then looks for the beginning of the next character,
2) 方法
The strategy with this scheme is to avoid the timing problem by not
sending long,uninterrupted streams of bits,Instead,data are
transmitted one character at a time,where each character is five to
eight bits in length,Timing or synchronization must only be
maintained within each character; the receiver has the opportunity to
resynchronize at the beginning of each new character.
3） 工作过程
（ Please see Figure 6.1 in Page 176,E6,Next slide）
4) 优缺点优点：仅需一条信道，且传输距离较远；
缺点：额外开销较大（约 20%）。
网络通信 4
网络通信 5
Description on Figure 6.1
不传送字符时，发 /收之间的线路处于“空闲”状态；
字符的传送从“起始位（ start bit）”开始（二进制 0），其后跟着组成字符的 5—8bit数据；
数据字符的传送从最低位开始；
在数据 bit之后，通常跟有一个奇偶校验位（ odd or even parity
bit），校验位的值由发送器设置。其作用是使整个字符（包括校验位本身）的二进制 1的总数为偶数（称为偶校验）或奇数
（称为奇校验）；
最后一个元素是“停止位（ stop element）”。
发送器不停地发送停止位，直到出现下一个起始位。
网络通信 6
Effect of Timing Error
The timing requirements for this scheme are modest（适中的）,For
example,IRA (International Reference Alphabet) character are
typically sent as 8-bit units,including the parity bit,If the receiver is
5 percent slower or faster than the transmitter,the sampling of the
eight character bit will be displaced（移动） 45 percent and still be
correctly sampled,Figure 6.1(c) shows the effects of a timing error of
sufficient magnitude to cause an error in reception,In this example
we assume a data rate of 10,000 bits per second (10kbps); therefore,
each bit is 0.1 ms,or 100us,duration,Assume that the receiver is fast
by 6 percent,or 6us per bit time,Thus,the receiver samples the
incoming character every 94us,As can be seen,the last sample is
erroneous,
网络通信 7
两种错误：
– The last sampled bit is incorrectly received,
– The bit count may now be out of alignment,If bit 7 is a 1 and bit
8 is a 0,bit 8 could be mistaken for a start bit,This condition is
termed a framing error（组帧错误）,as the character plus start
bit and stop element are sometimes referred to as a frame,A
frame error can also occur if some noise condition causes the
false appearance of a start bit during the idle state
网络通信 8
（ 2）同步传输
1）概念
With synchronous transmission,a block of bits is transmitted in
a steady stream without start and stop codes,The block may be
many bits in length,To prevent timing drift between
transmitter and receiver,their clocks must somehow（某种方式） be synchronized,
One possibility is to provide a separate clock line between
transmitter and receiver,One side (transmitter or receiver)
pulses the line regularly with one short pulse per bit time,
The other side uses these regular pulses as a clock,This
technique works well over short distances as the data signal,
but over long distances the clock pulses are subject to the same
impairment （损伤） as the data signal,and timing errors can
occur,
网络通信 9
The other alternative is to embed the clocking information in the
signal,For digital signals,this can be accomplished with
Manchester or Differential Manchester encoding,
For analog signals,a number of techniques can be used; for example,
the carrier frequency itself can be used to synchronize the
receiver based on the phase of the carrier.
每个数据块以一个“前同步码（ preamble）”开始，用一个“后同步”码（ postamble）”结束。此外，数据中的控制信息中也有一部分是起同步作用的。
– 方法（如何让接收器判断数据块的开始和结束）
网络通信 10
嵌入式时钟编码举例网络通信 11
3）祯格式特点：
- 以前同步码开始，长度为 8bit；
- 后同步码使用相同的标志；
- 前同步码后紧跟一些控制字段；
- 再以后是数据字段；
- 数据字段后，还有控制字段。
- 例,对于 1000个字符的数据块，数据量为 1000*8=8000bit，若每个帧包含 48bit的额外开销，则 48/8000=0.6%.
- 优点：额外开销小；缺点：需要独立的时钟，或编码复杂。
网络通信 12
§ 6-2 Line Configuration
1,Topology
(1) Topology,The topology of a data link refers to the physical
arrangement of stations on a transmission medium,If there are only
two stations,the link is point-to-point,If there are more than two
stations,then it is multipoint topology,Traditionally,a multipoint link
has been used in the case of a computer (primary station) and a set of
terminals (secondary stations).
(2) Half Duplex,With half duplex transmission,only one of two
stations on a point-to-point link may transmit at a time.
(3) Half Duplex,Half duplex is also referred to as two-way alternate,
suggestive of the fact that two stations must alternate in transmitting,
This can be compared to a one-lane（ 单车道 ）,two-way（ 双向 ）
bridge.
网络通信 13
Examples
Point-to-point
Multipoint
Host
Terminal
Terminal
Terminal(Primary)
(Secondary)
Primary
Secondary
网络通信 14
§ 6-3 Interfacing
Why
– Most digital data processing devices have limited data
transmission capability,（例如：只能产生 NRZ-L这样的数据；
且传输距离有限）
– Consequently,it is rare for such a device (terminal,computer) to
attach directly to a transmission or networking facility.
DTE and DCE
– The devices,including terminals and computers,are referred to
as data terminal equipment (DTE),A DTE makes use of the
transmission system through the mediation of data circuit-
terminating equipment (DCE),An example of DCE is a modem,
网络通信 15
Examples
网络通信 16
DCE的用途
– The DCE is responsible for transmitting and receiving bit,one at
a time,over a transmission medium or network.
– The DCE must interact with the DTE,In general,this requires
both data and control information to be exchanged,This is done
over a set of wires referred to as interchange circuits,The two
DCEs in both sides need:
Understand each other;
With the same coding scheme;
With the same data rate;
网络通信 17
– DCE的主要特性
（ With Four important characteristics）
Mechanical,actual physical connection of the DTE to the
DCE（包括插头、插座、导线等）,
Electrical,with the voltage level and timing of the voltage
changes（使用相同的编码和电平来表示相同的事情，且信号持续时间相同，这些特性可确定数据率和传输距离）,
Functional,specify the functions that are performed by
assigning meanings to each of the interchange circuits（功能可分为数据的、控制的、时序的和电气的等）,
Procedural,specify the sequence of events for transmitting
data,based on the functional characteristics of the interface.
网络通信 18
V.24/EIA-232-F Interface
V.24是应用最广泛的 ITU-T标准；
V.24仅规定功能（ functional）和过程（ procedural）方面的接口；
V.24的电气（ electrical ）和机械（ mechanical）方面的接口参考其它标准；
EIA-232-F与 V.24的对应关系：
– 机械规约,ISO2110
– 电气规约,V.28
– 功能规约,V.24
– 过程规约,V.24
EIA-232最早由美国电子工业协会于 1962年发布，又称 RS-232。
目前的第 7版于 1997年发布。
网络通信 19
V.24/EIA-232引脚规定网络通信 20
机械特性
– 25芯插头，由 ISO定义；
– 用于连接 DTE与 DCE（但许多实际应用中所使用的交换电路远少于
25线）
电气特性
– 定义 DTE与 DCE之间的传输信号；
– 均使用数字信号（二进制的数据）；
– 以公共地为基准,<-3V为逻辑 1,>+3V为逻辑 0（即 NRZ-L编码） ;
– 信号速率 <20kbps;
– 传输距离 <15m
功能特性
– 25芯分为数据、控制、时序及接地等几类；
– 每个方向上有一条数据线（ Full duplex）；
– 两条辅助数据线路，半双工时使用；
– 控制线路 16条；
– 环回测试（ loopback test）,is a useful fault isolation tool,
网络通信 21
环回测试,Suppose that a user at a personal computer is communicating with a
server by means of a modem connection and communication suddenly ceases,
The problem could be with the local modem,the communication facility,the
remote modem,or the remote server,A network manager can use loopback
tests to isolate the fault,Local loopback checks the functioning of the local
interface and the local DCE,Remote loopback tests the operation of the
transmission channel and the remote DCE,(See Figure 6.6,E6)
发送器接收器本地DCEDTE
发送器接收器本地DCEDTE
发送器接收器远程DCE
网络通信 22
过程特性
– 定义了在实现特定应用时各种电路的执行程序
– 工作过程
V.24/EIA-232 Dial-up Operation（拨号）
网络通信 23
网络通信 24
ISDN Physical Interface
ISDN (Integrated Services Digital Network) 概况
– 是现存公用电话网和模拟电信网络的全数字替代网络 ;
– 能够在一个网络上支持广泛的话音和非话音应用 ;
– 能够支持各种应用，包括交换和非交换的连接，其中交换包括电路交换和分组交换以及二者的结合；
– 被引入 ISDN的新业务应当能够与 64kbps交换数字连接相兼容；
– ISDN将会具有智能，用于提供业务属性、维护以及网络管理功能。
– 采用分层体系结构，可以被映射到 OSI模型中；
– 实现 ISDN中允许配置的多样性网络通信 25
ISDN的连接模型网络通信 26
ISDN接口网络通信 27
ISDN的物理接口
物理连接
– TE（终端设备）,NT（网络端接设备），与 DTE和 DCE对应；
– 没有特定的功能电路，发送 /接收电路既传送数据，也传送控制信号；
– 控制信息以报文的形式发送
– 可以通过接口传送电能
电气特性
– 使用平衡传输（ balances transmission），采用双绞线，信号以电流的形式经过一根导线，并从另一根导线返回，组成环路；
– 平衡模式的容错能力更强，噪声影响较小；
– 编码方式取决于数据率,192kbps（基本速率），采用伪三进制；
1.544Mbps（主速率），采用 B8ZS交替信号反转码（ AMI），
2.048Mbps（主速率），采用 HDB3的 AMI。二者性能相似。
网络通信 28
Homework of Chapter 6
6.2
Due by Next Monday
End of This Chapter
Network Communications
网络通信
(For Master Students in the Department of Electronic Engineering)
Chapter 6 The Data Communications Interface
Instructor,Dr,Tianshuang Qiu
School of the Electronic and Information Engineering
Fall of 2004
网络通信 2
§ 6-1 Asynchronous and Synchronous
Transmission
说明
– 本节主要讨论信号的串行传输
– 假定每个信号元素表示一个 bit
两个主要问题
– 噪声等传输损伤对接收数据的影响，可能导致出错 ;
– 时序问题，若发 /收不精确一致，也可能出错，且这种时间上的不一致会积累,
网络通信 3
1,异步和同步传输
（ 1）异步传输
1) 概念
In asynchronous transmission,each character of data is treated
independently,Each character begins with a start bit that alerts the
receiver that a character is arriving,The receiver samples each bit in
the character and then looks for the beginning of the next character,
2) 方法
The strategy with this scheme is to avoid the timing problem by not
sending long,uninterrupted streams of bits,Instead,data are
transmitted one character at a time,where each character is five to
eight bits in length,Timing or synchronization must only be
maintained within each character; the receiver has the opportunity to
resynchronize at the beginning of each new character.
3） 工作过程
（ Please see Figure 6.1 in Page 176,E6,Next slide）
4) 优缺点优点：仅需一条信道，且传输距离较远；
缺点：额外开销较大（约 20%）。
网络通信 4
网络通信 5
Description on Figure 6.1
不传送字符时，发 /收之间的线路处于“空闲”状态；
字符的传送从“起始位（ start bit）”开始（二进制 0），其后跟着组成字符的 5—8bit数据；
数据字符的传送从最低位开始；
在数据 bit之后，通常跟有一个奇偶校验位（ odd or even parity
bit），校验位的值由发送器设置。其作用是使整个字符（包括校验位本身）的二进制 1的总数为偶数（称为偶校验）或奇数
（称为奇校验）；
最后一个元素是“停止位（ stop element）”。
发送器不停地发送停止位，直到出现下一个起始位。
网络通信 6
Effect of Timing Error
The timing requirements for this scheme are modest（适中的）,For
example,IRA (International Reference Alphabet) character are
typically sent as 8-bit units,including the parity bit,If the receiver is
5 percent slower or faster than the transmitter,the sampling of the
eight character bit will be displaced（移动） 45 percent and still be
correctly sampled,Figure 6.1(c) shows the effects of a timing error of
sufficient magnitude to cause an error in reception,In this example
we assume a data rate of 10,000 bits per second (10kbps); therefore,
each bit is 0.1 ms,or 100us,duration,Assume that the receiver is fast
by 6 percent,or 6us per bit time,Thus,the receiver samples the
incoming character every 94us,As can be seen,the last sample is
erroneous,
网络通信 7
两种错误：
– The last sampled bit is incorrectly received,
– The bit count may now be out of alignment,If bit 7 is a 1 and bit
8 is a 0,bit 8 could be mistaken for a start bit,This condition is
termed a framing error（组帧错误）,as the character plus start
bit and stop element are sometimes referred to as a frame,A
frame error can also occur if some noise condition causes the
false appearance of a start bit during the idle state
网络通信 8
（ 2）同步传输
1）概念
With synchronous transmission,a block of bits is transmitted in
a steady stream without start and stop codes,The block may be
many bits in length,To prevent timing drift between
transmitter and receiver,their clocks must somehow（某种方式） be synchronized,
One possibility is to provide a separate clock line between
transmitter and receiver,One side (transmitter or receiver)
pulses the line regularly with one short pulse per bit time,
The other side uses these regular pulses as a clock,This
technique works well over short distances as the data signal,
but over long distances the clock pulses are subject to the same
impairment （损伤） as the data signal,and timing errors can
occur,
网络通信 9
The other alternative is to embed the clocking information in the
signal,For digital signals,this can be accomplished with
Manchester or Differential Manchester encoding,
For analog signals,a number of techniques can be used; for example,
the carrier frequency itself can be used to synchronize the
receiver based on the phase of the carrier.
每个数据块以一个“前同步码（ preamble）”开始，用一个“后同步”码（ postamble）”结束。此外，数据中的控制信息中也有一部分是起同步作用的。
– 方法（如何让接收器判断数据块的开始和结束）
网络通信 10
嵌入式时钟编码举例网络通信 11
3）祯格式特点：
- 以前同步码开始，长度为 8bit；
- 后同步码使用相同的标志；
- 前同步码后紧跟一些控制字段；
- 再以后是数据字段；
- 数据字段后，还有控制字段。
- 例,对于 1000个字符的数据块，数据量为 1000*8=8000bit，若每个帧包含 48bit的额外开销，则 48/8000=0.6%.
- 优点：额外开销小；缺点：需要独立的时钟，或编码复杂。
网络通信 12
§ 6-2 Line Configuration
1,Topology
(1) Topology,The topology of a data link refers to the physical
arrangement of stations on a transmission medium,If there are only
two stations,the link is point-to-point,If there are more than two
stations,then it is multipoint topology,Traditionally,a multipoint link
has been used in the case of a computer (primary station) and a set of
terminals (secondary stations).
(2) Half Duplex,With half duplex transmission,only one of two
stations on a point-to-point link may transmit at a time.
(3) Half Duplex,Half duplex is also referred to as two-way alternate,
suggestive of the fact that two stations must alternate in transmitting,
This can be compared to a one-lane（ 单车道 ）,two-way（ 双向 ）
bridge.
网络通信 13
Examples
Point-to-point
Multipoint
Host
Terminal
Terminal
Terminal(Primary)
(Secondary)
Primary
Secondary
网络通信 14
§ 6-3 Interfacing
Why
– Most digital data processing devices have limited data
transmission capability,（例如：只能产生 NRZ-L这样的数据；
且传输距离有限）
– Consequently,it is rare for such a device (terminal,computer) to
attach directly to a transmission or networking facility.
DTE and DCE
– The devices,including terminals and computers,are referred to
as data terminal equipment (DTE),A DTE makes use of the
transmission system through the mediation of data circuit-
terminating equipment (DCE),An example of DCE is a modem,
网络通信 15
Examples
网络通信 16
DCE的用途
– The DCE is responsible for transmitting and receiving bit,one at
a time,over a transmission medium or network.
– The DCE must interact with the DTE,In general,this requires
both data and control information to be exchanged,This is done
over a set of wires referred to as interchange circuits,The two
DCEs in both sides need:
Understand each other;
With the same coding scheme;
With the same data rate;
网络通信 17
– DCE的主要特性
（ With Four important characteristics）
Mechanical,actual physical connection of the DTE to the
DCE（包括插头、插座、导线等）,
Electrical,with the voltage level and timing of the voltage
changes（使用相同的编码和电平来表示相同的事情，且信号持续时间相同，这些特性可确定数据率和传输距离）,
Functional,specify the functions that are performed by
assigning meanings to each of the interchange circuits（功能可分为数据的、控制的、时序的和电气的等）,
Procedural,specify the sequence of events for transmitting
data,based on the functional characteristics of the interface.
网络通信 18
V.24/EIA-232-F Interface
V.24是应用最广泛的 ITU-T标准；
V.24仅规定功能（ functional）和过程（ procedural）方面的接口；
V.24的电气（ electrical ）和机械（ mechanical）方面的接口参考其它标准；
EIA-232-F与 V.24的对应关系：
– 机械规约,ISO2110
– 电气规约,V.28
– 功能规约,V.24
– 过程规约,V.24
EIA-232最早由美国电子工业协会于 1962年发布，又称 RS-232。
目前的第 7版于 1997年发布。
网络通信 19
V.24/EIA-232引脚规定网络通信 20
机械特性
– 25芯插头，由 ISO定义；
– 用于连接 DTE与 DCE（但许多实际应用中所使用的交换电路远少于
25线）
电气特性
– 定义 DTE与 DCE之间的传输信号；
– 均使用数字信号（二进制的数据）；
– 以公共地为基准,<-3V为逻辑 1,>+3V为逻辑 0（即 NRZ-L编码） ;
– 信号速率 <20kbps;
– 传输距离 <15m
功能特性
– 25芯分为数据、控制、时序及接地等几类；
– 每个方向上有一条数据线（ Full duplex）；
– 两条辅助数据线路，半双工时使用；
– 控制线路 16条；
– 环回测试（ loopback test）,is a useful fault isolation tool,
网络通信 21
环回测试,Suppose that a user at a personal computer is communicating with a
server by means of a modem connection and communication suddenly ceases,
The problem could be with the local modem,the communication facility,the
remote modem,or the remote server,A network manager can use loopback
tests to isolate the fault,Local loopback checks the functioning of the local
interface and the local DCE,Remote loopback tests the operation of the
transmission channel and the remote DCE,(See Figure 6.6,E6)
发送器接收器本地DCEDTE
发送器接收器本地DCEDTE
发送器接收器远程DCE
网络通信 22
过程特性
– 定义了在实现特定应用时各种电路的执行程序
– 工作过程
V.24/EIA-232 Dial-up Operation（拨号）
网络通信 23
网络通信 24
ISDN Physical Interface
ISDN (Integrated Services Digital Network) 概况
– 是现存公用电话网和模拟电信网络的全数字替代网络 ;
– 能够在一个网络上支持广泛的话音和非话音应用 ;
– 能够支持各种应用，包括交换和非交换的连接，其中交换包括电路交换和分组交换以及二者的结合；
– 被引入 ISDN的新业务应当能够与 64kbps交换数字连接相兼容；
– ISDN将会具有智能，用于提供业务属性、维护以及网络管理功能。
– 采用分层体系结构，可以被映射到 OSI模型中；
– 实现 ISDN中允许配置的多样性网络通信 25
ISDN的连接模型网络通信 26
ISDN接口网络通信 27
ISDN的物理接口
物理连接
– TE（终端设备）,NT（网络端接设备），与 DTE和 DCE对应；
– 没有特定的功能电路，发送 /接收电路既传送数据，也传送控制信号；
– 控制信息以报文的形式发送
– 可以通过接口传送电能
电气特性
– 使用平衡传输（ balances transmission），采用双绞线，信号以电流的形式经过一根导线，并从另一根导线返回，组成环路；
– 平衡模式的容错能力更强，噪声影响较小；
– 编码方式取决于数据率,192kbps（基本速率），采用伪三进制；
1.544Mbps（主速率），采用 B8ZS交替信号反转码（ AMI），
2.048Mbps（主速率），采用 HDB3的 AMI。二者性能相似。
网络通信 28
Homework of Chapter 6
6.2
Due by Next Monday
End of This Chapter
