数据通信与网络02-基本概念-1

2.2 Topology
Topology refer to the way a network is laid out, either physically or logically. ※Two or more devices connect to a link; two or more links form a topology.
Fig2.7 Tree topology
2.2.3 Tree :variation of a star
Advantages Central hub is active hub contains repeater(中继器:regenerate bit patterns before sending them out; strengthens transmissions and increases the distance a signal can travel) Isolate and prioritize communications from different computers.
2.1 Line Configuration
Line configuration refers to the way two or more communication devices attach to a link. A link is the physical communication pathway that transfers data from one device to another.
2.1.1 Point-to-Point
两个设备间的专用的链路可以采用：导线、电缆、微波或 卫星等。
2.1.2 Multipoint(多点连接)

A multipoint (also called multidrop) line configuration is one in which more than two specific devices share a single link.
Chapter 2 BASIC CONCEPTS
第二章 基本概念
General concept
2.1 2.2 2.3 2.4 2.5

Line configuration（线路配置） Topology（拓扑结构） Transmission mode（传输模式） Categories of networks（网络类型） Internetworks（网际互联）
2.2.5 RING（环形拓扑）

each device in the ring incorporates a repeater
2.2.5 Ring
特点： 每个节点仅与其两个直接相连的邻居节点之间 有点到点(point-to-point)的连接； 信号按照某个方向单向传输(unidirectional traffic)； 每个节点都包含一个中继器(repeater)，当需 要为其它节点转发信号时，中继器再生信号后 发送给下一个节点。
2.2.3 Tree :variation of a star

the majority devices connect to a secondary hub that in turn is connected to the central hub, the secondary hub may be active(有源) or passive(无源)
2.2.1 Mesh
Advantages:




The use of dedicated links guarantees that each connection can carry its data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices robust. If one link becomes unusable, it does not incapacitate the entire system. privacy or security. When every message sent travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages. Finally, point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution.
2.2.2 Star（星型拓扑）

Each device has a dedicated point to point link only to a central controller (usually called a hub (集线器))
2.2.2 Star
Advantages and Disadvantages Advantages of a Star Topology Easy to install and wire. No disruptions to the network when connecting or removing devices. Easy to detect faults and to remove parts. Disadvantages of a Star Topology Requires more cable length than a linear topology. If the hub or concentrator fails, nodes attached are disabled. More expensive than linear bus topologies because of the cost of the concentrators.
2.2.4 BUS（总线拓扑）

one long cable act as a backbone to link all the devices
2.2.4 BUS
Advantages and Disadvantages Advantages of a Linear Bus Topology Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology. Disadvantages of a Linear Bus Topology Entire network shuts down if there is a break in the main cable. Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down.
Link
2.1.2 Multipoint
Share a single link：两个以上的设备共享一 条链路 The manner of shared channel capacity: spatially(空分) ,temporally(时分) Spatially shared line configuration:多个设 备同时使用链路 Time shared line configuration:用户排队 使用链路

2.2 TOPOLOGY
There are five basic topologies possible : mesh, star, tree, bus and ring.
Topology
Mesh
Star
Tree
Bus
Ring
2.2 TOPOLOGY
The relationship of devices to be linked: Peer to peer(对等式) :设备平等的共享链路。 ring and mesh topology primary-secondary(主从式): 主设备控制通信过程。 star and tree ※Bus topology is equally convenient for either

Link
2.1 Line Configuration
Line configuration
Point-to-point
Βιβλιοθήκη Baidu
Multipoint
Categories of Line Configuration
2.1.1 Point-to-Point(点到点连接)

A point-to-point line configuration provides a dedicated link between two devices
2.2.1 Mesh（网状拓扑）
2.2.1 Mesh



In a mesh topology , every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. A fully connected mesh network therefore has n(n-1)/2 physical channels to link n devices. Every device on the network must have n-1 input/output (I/O) ports
Link
1.The entire capacity of channel is reserved for transmission between those two device. 2.The line configuration use wire, cable, microwave or satellite link to connect the two ends.
2.2.1 Mesh
Disadvantages: The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required.
2.2.1 Mesh
Example The Corporation has a fully connected mesh network consisting of 8 devices. Calculate the total number of cable links needed and the number of port each device Solution (The formula for the number of links for a fully connected mesh is n (n -1 )/2, where n is the number of devices.) Number of links= n (n -1)/2 = 8 (8-1) / 2 = 28 Number of ports per device = n - 1 = 8 - 1 = 7