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Github主页https://github.com/A-BigTree
项目链接https://github.com/A-BigTree/college_assignment
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Purpose of constructing computer networks
Facilitate information exchange
Functionality of computer networks
- Information/resource sharing
- Change our way of life
Why should we understand computer networks
- The era of information
- Computer networks shape our society and ourselves
The structure of computer networks
Computer network structure
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information. Where at least one process in one device is able to send/receive data to/from at least one process residing in a remote device, then the two devices are said to be in a network.
Physical structure: a “nuts and bolts” view
Protocols define the format,order of messages sent and received among network entities, and actions taken on msg transmission, receipt
Functional structure
Residential access nets(家庭接入网)
cable-based access(基于电缆接入)
频分复用(FDM):在不同频带传输的不同信道
frequency division multiplexing (FDM): different channels transmitted in different frequency bands
HFC:混合光纤同轴电缆;不对称:高达40 Mbps–1.2 Gbs下行传输速率,30-100 Mbps上行传输速率
HFC: hybrid fiber coax; Asymmetric: up to 40 Mbps – 1.2 Gbs downstream transmission rate, 30-100 Mbps upstream transmission rate
电缆、光纤网络将家庭连接到ISP路由器
network of cable, fiber attaches homes to ISP router
电缆调制解调器接入网络
Cable modem access network
电力线宽带接入网
Power line broadband access network
digital subscriber line (DSL)(数字用户线)
使用现有电话线连接中央办公室DSLAM
use existing telephone line to central office DSLAM
mobile access networks (WiFi, 4G/5G)(移动接入网络)
institutional access networks(机构介入网络)
mix of wired, wireless link technologies, connecting a mix of switches and routers
- Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps;
- WiFi: wireless access points at 11, 54, 450 Mbps;
- Mesh of interconnected routers
- Packet-switching: hosts break application-layer messages into packets
- forward packets from one router to the next, across links on path from source to destination;
- each packet transmitted at full link capacity;
T packet transmission delay = L bits R bits/sec T_{\text{packet transmission delay}}=\frac{L_{\text{bits}}}{R_{\text{bits/sec}}} Tpacket transmission delay=Rbits/secLbits
Host sending function:
- Takes application message;
- Breaks into smaller chunks, known as packets, of length L bits;
- Transmits packet into access network at transmission rate R
- link transmission rate, aka link capacity, aka link bandwidth
导向介质:
非导向介质:
电磁频谱中携带的信号
无物理“线”
广播和“半双工”(发送方到接收方)
传播环境影响:
链接类型:
对于包(packet)数据长度为L,传输速率为R,通信链路条数为N,则:
t trans = N L R t_{\text{trans}}=N\frac{L}{R} ttrans=NRL
如果链路的到达速率(bps)在一段时间内超过链路的传输速率(bps):
分配给的终端资源,保留给源和目标之间的“调用”(缓存,链路传输速率)
End-end resources allocated to, reserved for “call” between source and destination
动机不同:
PSTN:稳定、恒定、稳定的流量;
计算机网络:突发流量;
数据包交换为计算机网络显示了更高的效率;同时允许更多的用户去使用网络;
数据包交换的特点:
To evaluate the performance of computer networks
路由器缓冲区中的数据包队列
d nodal = d proc + d queue + d trans + d prop d_{\text{nodal}}=d_{\text{proc}}+d_{\text{queue}}+d_{\text{trans}}+d_{\text{prop}} dnodal=dproc+dqueue+dtrans+dprop
d p r o c d_{proc} dproc: nodal processing
d q u e u e d_{queue} dqueue:queueing delay
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Github主页https://github.com/A-BigTree
项目链接https://github.com/A-BigTree/college_assignment
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