数据通信与网络知识点
第一章
1、Data communication:is the transfer of data form one device to another via some form of transmission medium.
2、数据通讯五个方面：protocol、message、medium、sender、receiver
protocol is a set of sules that govern data communication 。It represents an agreement between the communication devices。
the message is the information to be communication。it consists of text，number，picture，sound，or video-or any combination of there。
sender can be a computer，workstation，telephone handset，television，and so on。
the transmission medium is the physical path by which a message travels from sender to receiver。
the receive is the device that receives the massage，it can be a computer，
3、networks网络：A network is a set of devices(often refferred to as nodes)connected by media links.
4、network criteria网络指标：performance（性能）、reliable（可靠性）、security（安全性）
5、reliable三个方面：frequently of failure（故障时间间隔）、recovery time after a failure（故障修复时间）、catastrophe（灾难性事故预防）
6、protocol：a protocol is a set of rule that govern data communication，workstation，telephone handset，television，and so on。
三要素：syntax（语法）semantics（语言）timing（时序）
7、standard De focto（by fact or by convention）实施标准
standard De jure（by law or by regulation）法定标准
8、STANDARD ORGANIZATION 标准组织
standard creation committees 标准化委员会
ISO:the international standards orgnization国际化标准化组织
ANSI：the American national standard institute美国国家标准化协会
IEEE:the institute of electrical and electronic engineers 电气电子工程师协会
EIA：the electronic industries association电子工业联合会
Bellcore：the bellcore is an important source of draft standards to ANSI
第二章
1、line configuration：piont-to-piont、multipoint、
2、拓扑结构topology：mesh（网状）star（星形）bus（总线型）tree（树形）ring（环形）
3、共享链路两种方式：（1）peer-to-peer（对等方式）：网状、环形（2）primary-secondary（主从方式）：树形、星形
4、n个设备使用电缆条数：网状n(n-1)/2 星形n
5、传输方式transmisson mode:simplex（单工）、half-duplex（半双工）、full-duplex（全双工）
simplex： one station can transmit，the other can only receive。The communication is undirectional单向传输
half-duplex：each station can both transmit and receive，but not at same time双向传输，但不同时
full-duplex：both station can transmit and receive。可同时双向传输
6、网络的三种分类：
LAN：cocal area network。局域网
MAN：metropolitan area network。城域网
WAN：wide aera network 万维网
第三章
1、对OSI模型解释：OSI（open system interconnection）to faciletate the development of interact
ive system。
The OSI is not a protocol，it is a model for understanding and designing a network architecture。
2、7层模式：physical物理层、data link数据链路层、 network网络层、transportation传输层、session会话层、persentation表示层、application应用层
3、peer-to-peer process对等过程：the processes on sech machine that communicate at a given layer are called peer-to-peer processes。
4、物理层考虑的七个问题：（1）线路配置（2）数据传输模式（3）拓扑（4）信号（5）编码（6）接口（7）媒介
5、七层各层的功能：
（1）物理层：to transmit bits，to provide mechanical and electrical specifications。
（2）数据链路层：to organize bits into frams，to provide node-to-node delivery点到点。
（3）网络层：to move packets from source to destination，to provide internetworking。
（4）传输层：to provide end-to-end message delivery and error recovery。
（5）会话层：to establish、manage、and terminate session。
（6）表示层：to translate，encrypt and compress data。
（7）应用层：to allow access to network resource。
第四章
1、频率单位：HZ KHZ GHZ THZ
周期单位：s ms us ps
2、频谱frequency spectrum：the frequency spectrum of a signal is the combination of all sine wave signals that signal。
带宽bandwidth：bandwidth of a signal is the width of the frequency spectrum。
3、有效频谱：significant spectrum：is the portion of the signals spectrum that can adequately reproduce the original signal。
有效带宽：the significant bandwidth ：the bandwidth of the significant spectrum
介质带宽：medium bandwidth：the bandwidth of a signal which is sent on a transmission medium。
4、信道容量channel capacity：is the maximum bit rate a transmission medium can transfer。
决定因素：depends on the type of encoding technique and the signal-to-noise ratio of system
第五章
1、编码的四种类型：digital/digital、analog/digital、digital/analog、analog/analog
2、digital/digital encoding的三种类型：unipolar单极性编码、bipolar双极性编码、polar极性编码
3、bipolar encoding分类：AMI(bipolar alternate mark inversion)信号交替反转码
B8ZS（bipolar 8-zero substitution）8零替换编码
HDB3（high-density bipolar 3）高密度双极性3零编码
4、PAM：pulse amplitude modulation 脉冲振幅调制
PCM：pulse code modulation 脉冲编码制（A-D）
5、analog-PCM编码四步骤：（1）PAM：连续-离散 采样保持
（2）quantization量化：is a method of assigning integral valuse in a specific range
（3）binary encoding二进制编码
（4）digital/digital encoding
6、采样定理：the sampling rate must be at least 2 times the highest frequency
7、digital/analog en
coding四种方法：
（1）ASK：amplitude shift keying幅移键控
（2）FSK：frequency shift keying频移键控
（3）PSK：phase shift keying相移键控
（4）QAM：quadrature amplitude modulation正交调幅
8、bit rate比特率：is the number of bits per second（bps）
baud rate波特率：is the number of signal units per second（Bd）
波特率小于等于比特率
9、贷款公式
psk：BW=Nbaud
ASK:BW=（1+d)*Nbaud=Nbaud
FSK：BW=|fc1-fc0|+Nbaud
带宽BW=波特率=比特率
10、PSK constellation：A diagram called constellation or phase-state diagram is used to show the relationship between phase in PSK
11、波特率和比特率的关系：
ASK,FSK,2-PSK:baud rate=bit rate
4-PSK，4-QAM:bit rate=2baud rate
8-PSK，8-QAM：bit rate=3baud rate
16-QAM：bit rate=4baud rate
第六章
1、digital data transmission分类：parallel并行传输、serial串行传输（asynchronous异步，synchronous同步）
2、异步传输解释：In an asynchronous transmission mode we send one start bit（0）at the beginning and one or more step bit（1）at the end of each byte。there may be gaps between each byte。
3、串行同步：In synchronous transmission we send bits one after another without start/stop bits or gaps，it is the responsibility of the receive to group the bits into meaningful frame。
4、DTE：data terminal equipment（数据终端设备）
A DTE is any device that is a source of or desitination for binary digital data。
DCE：data circuit-termination equipment（数据电路终结设备）
A DCE is any device that transmits or receive data in the from of an analog or digital signal through a network
5、DTE-DCE standard try to define the mechanical，electrical，and functional characteristics of connection between the DTE and DCE
6、EIA-530和EIA-449的关系：EIA-530 standard is a version of EIA-449 that user DB-25 pins
7、modem调制解调器：is the most familiar type of DCE。modem stands modulator/demodulator
功能：A modulator converts a digital signal to analog signal（D-A）
A modulator converts a analog signal to digital signal（A-D）
8、智能调制解调器：intelligent modem contain software to support a number of additional functions，such as auto matic answering and dialing。
第七章
2、transmission media分类：guided和unguided
3、guided media分类：twisted-pari cable双绞线电缆，coaxial cable同轴电缆，fiber-optic cable光纤电缆
4、UTP：unshielded twisted-pari cable 非屏蔽性电缆
STP：shielded twisted-pari cable 屏蔽性电缆
5、光纤模式mode:multimode多模（step-index阶跃式，graded-index渐变式）和single-mode单模
6、光纤优点：noise resistance抗干扰强
less signal attenuation 抗衰减
higher bandwidth
缺点：cost 花费多
installation/maintenance安装维护麻烦
fragility易脆性
第八章
1、复用multiplexing定义：multiplexing is the set of technique that allows the simultaneous transmission of multiple signal across a single data link。
2、path和channel区别：a path refers to physical link（通道，马路）。channel refers to a portion of a path。（通路，路上的车道）
3、multiplexing复用分类：frequency-division multiplexing（FDM）频分复用，time-division multiplexing(TDM)时分复用（synchronous，asynchronous）
4、FDM理解：in FDM，each signal modulate a different carrier frequency。the modulated carriers are combined to from a new signal that is then send across the link。in FDM multiplexers modulate and combine signal while multiplexers decompose and demodulata。in FDM guard bands keep the modulataed signal from overlapping and interfering with one another。
5、同步DTM理解：in synchronous TDM each frame contain at least one time slot dedicated to each device，the order in which each device sends its data to the frame is unvaring，if a device has no data to send，its time slot is sent empty。in synchronous TDM，a bit may be added to the beginning of each frame for synchronization。
6、帧内时间片数与设备数关系：同步TDM：n个设备n和时间片，异步TDM：n个设备m个时间片数（m《n）
第九章
1、errors分类：single-bit error（单比特错误），multiple-bit error（多比特错误），burst error（突发错误）
single-bit error is when only one bit in the data unit has changed
multiple-bit error is when two or more nonconsecutive bits in the data unit have changed。
burst error means that two or more consecutive bits in the data unit have changed。
2、redundancy冗余定义：error detection uses the concept of redundancy which means adding extra bits for detecting error at the destination。
3、detection methods分类：
VRC:vertical redundancy cheek（垂直冗余校验）
LRC:longitudinal redundancy cheek （纵向冗余校验）
CRC：cyclical redundancy cheek （循环冗余校验）
第十章
1、data link control包括：line discipline、flow control、error control
line discipline：线路规程who should send now?方式：ENQ/ACK询问/应答，poll/select轮询/选择
flow control：流量控制：how much data may be sent？
error control：how can error be corrected？
2、line control包括：ENQ/ACK、Poll（data—primary）/select（data--secondary）
3、ENQ/ACK is used in peer to peer communication，where there are dedicated link between two devices。
4、flow control包括：stop and wait、sliding window
5、error control包括：stop-and-wait ARQ、sliding window ARQ(go-back-n、selective-reject)
6、stop-and-wait ARQ is a form of stop-and-wait flow control extended to include retransmission of data in case of lost or damaged frames。
7、in go-back-n ARQ，retransmission begins with the last unacknowledeged frame even if subsequen
t frames have arrived correctly。
8、in selective-reject ARQ only the unacknowledeged frame is retransmitted。
第十一章
1、data link protocol分为：synchronous protocols和asynchronous protocol
2、synchronous protocols分为：character-oriented protocols和bit-oriented protocols
3、In a character-oriented protocols the frame or packet is interpreted as a series of characters。In a bit-oriented protocols the frame or packet is interpreted as a series of bits。
4、Data transparency in data communication means we should be able to send any combination of bits as data。
5、Data transparency in BSC is achieved by a process called byte stuffing。
6、域名：I-frame：flag、address、control，information（user data coming from upper layer）、FCS、flag
S-frame：flag、address、control、FCS、flag
U-frame：flag、address、control，information（managment data）、FCS、flag
7、比特填充的作用：bit stuffing in HDLC is the process of adding one extra 0 whereever there are five conservate 1s in the data so that the receive does not mistake the data for a flag。
第十二章
1、802.1 internetworking=(network）、802.2 LLC=（data link）、(802.3 CSMA/CD、802.4 Token bus、802.5 token ring）=（physical）
2、LLC：logical link control is the upper sublayer of the data link layer。
3、MAC：media access control is the lower sublayer of the data link layer。
4、802.3 defines two categories of LAN：baseband digital和broadband analog。
5、CSMA/CD概念：carrier sense multiple with collision detection is the result of an evolution from MA to CSMA to CSMA/CD.
6、10base5:数据传输速率10Mbps，base基带，最大电缆长度500meters；
10base2：数据速率100Mbps，基带，每条电缆最大长度185meters；
1base5：数据速率1Mbps，基带，最大电缆长度500meters；
10base-T：数据速率10Mbps，基带，类型为双绞线电缆（概念：a star-topology LAN using unshielded twisted pair cable instead of coaxial cable。it support a data rate of 10 Mbps and has a maximum length of 100 meters。）
7、FFDI全称：fiber distributed data interface 光纤分布式数据接口
FFDI is a local area network protocol standardized by ANSI and the ITU-U(ITU-T x.3).It supports data rates of 100Mbps.
The copper version of FDDI is known as CDDI.
8、4B/5B原理：(a special encoding mechanism called four bits/five bits)
Each four-bit segment of data is replaced by five-bit code before being encoded in NRZ-I
4B/5B ensures that there cannot be a data sequence of more than three 0s transmitted across media in FDDI protocol.This takes care of the bit synchronization problems arising from strings of 0s in NRZ-I encoding.
第十三章
1、switching methods分为：circuit switching（电路交换）、packet switching（包交换）、message seitching（消息交换）
2、circuit switching包括：space-division switching（空分交换）和
time-division switching（时分交换）。
3、TSI（time-slot interchange）作用：In TSI time slots are sent out in an order based on the decision of a control unit.
4、packet switching分为：datagram approach数据报、virtual circuit approach虚拟电路
5、(in the datagram approach to packet switching，each packet is treated independently from all others，even when one packet represents just a piece of a multipacket transmission，the network treats it as though it existed alone。)
Datagram approach数据报法：All packets(or datagrames)belong to the same message but may go by different paths to reach their destination.(May be out of order.)
6、Virtual circuit approach应用于：SVC(switched virtual circuit)交换虚电路、PVC(permanent virtual circuit)永久虚电路
7、SVC:in this method,a virtual circuit is created whenever it is needed and exists only for the duration of the specific exchange.仅在这次通信交换的过程中存在
第十四章
1、X.25描述：
(1)X.25 is an interface between data terminal equipment and data circuit terminating equipment for terminal operation at the packet mode on public data network。 为公共数据网络在包模式下提供终端操作
(2)一句话：X.25 is a packet switching protocol used in a wide area network.广域网使用的包交换协议
(3)it uses a virtual circuit approach to packet switching(SVC or PVC) and uses asynchronous TDM to multiplex packet.异步时分（包复用）
2、X.25 protocol specifies three layer that correspond to the OSI model：、
physical：EIA-232,V-series,X-series,others
Data link:the link access procedure，balanced（LAPB）layer
Network:the packet level protocol（PLP）layer。
the latter two layer perfrom both flow and error control。
3、安全包序列中：D=1：the packet requires an acknowledgment from the final destination。包需应答信息
D=0：no acknowledgment is needed。不需应答信息
M=1：more packets are to follow。后面还有包
M=0：that packet is the last one of the message。是最后一个包
第十五章
1、connecting devices包括:networking(repeaters和bridge)和internetworking（routers和gateways）
2、作用范围：
repeaters：physical layers(1)
bridge：physical layer，data link layers(1.2)
routers：physical layer，data link，network layers(1.2.3)
gateways：layer 1-7
3、Gateway网关概述：gateways operate in all seven layers of the OSI model。they convert one protocol to another and can therefore connect two dissimilar network。
第十六章
1、数据链路层与传输层区别：
the data link layer provide service within a single network.一个网络
the transport layer operates across an internet.多个网络
2、transport layer duties传输层功能:
(1)end-to-end delivery (2)addressing (3)reliable delivery (4)flow control (5)multiplexing
3、Reliable delivery分类：
(
1)error control (2)sequence control (3)loss control (4)duplication control
4、Error control:
data link layer:node-to-node(begining and end of the link)
transport layer:end-to-end(including routers)
5、multiplexing分为：downward、upward in transport layer
6、三向握手three-way-handshake:
(1)connection request (2)connection confirmation (3)acknowledgement of confirmation
第十七章
1、services of the session layer功能：
(1)to voordinate connection and disconnection of dialogs between applications (2)to provide synchronization points for data exchange (3)to coordinate who sends when (4)to ensure that the data exchange is complete before the session closes(a greatful close)
2、主要同步点和次要同步点特点：
each major synchronization point must be acknowledged before the session can continue.主同步点必须被确认
the minor synchronization points do not need to be comfirmed.次要同步点在对话之间，不需确认
3、presentation layer表示层功能分类：
(1)translation (2)encryption (3)authentication (4)compression
4、加密(encryption algorithm)+ke 解密(decryption algorithm)+kd (plaintext原文-ciphertext密文)
5、data compression数据压缩分类：
(1)lossless methods (2)lossy methods
6、virtual terminal虚拟文件: 目的:真实文件的安全隔离security separation
与真实文件内容完全一样，发起方访问的是虚拟文件，保护真实文件安全
7、virtual terminal(VT)虚拟终端
作用:a virtual terminal is an imaginary terminal(a software model of a terminal)with a set of standard characteristics that every host understands.