I. LAN Transmission MEDIA
the commonly used transmission medium in local area network is coaxial cable, twisted pair, fiber and wireless communication channel. In the past coaxial cable cost-effective is the best.
At present, the high-speed local area network uses twisted-pair as the transmission medium, uses the optical fiber transmission in the long-distance transmission, the local area network which has the mobile site
, wireless communication technology is used.
two. Topological structure
A: Star-shaped structure
This kind of structure is one of the most widely used in the local area network, which is used almost all in enterprise networks. Star networks are almost Ethernet (Ethernet) networks dedicated to
, it is because each workstation node device in the network is connected by a network centralized device (such as a hub or switch), each node is named as a star-shaped distribution. This type of network
at present, the most used transmission medium is twisted-pair, such as the common five types of wire, super five twisted pair and so on.
The basic features of this topology network are as follows:
(1) Easy to achieve: it is used in the transmission medium is generally used in general twisted pair, the transmission medium is relatively inexpensive, such as the current five types of genuine twisted pair of 1.5 per meter
yuan around, and coaxial cable The cheapest also about 2.00 yuan a meter, fiber optic cable that is not to mention. This topology is mainly used in the IEEE 802.2, IEEE 802.3 standard Ethernet LAN;
(2) node expansion, Mobile Convenient: node expansion only need to pull a line from the hub or switch and other centralized devices, and to move a node only need to put the corresponding node device
move to a new node, and not as a ring-type network, "Take one and move the global";
(3) easy to maintain; a node failure will not affect the connection of other nodes, can arbitrarily remove the fault node;
(4) The use of broadcast information transmission mode: Any node to send information in the entire network of nodes can be received, which in the network has some hidden danger, but this in the LAN to make
with little influence;
(5) Fast network data transmission: This can be seen from the current 1000Mbps to 10G Ethernet access speed.
B: Ring-shaped structure
The network form of this structure is mainly used in the token network, in which the devices are connected directly through the cable, and finally form a closed loop, the whole network sends the information
is to pass in this ring, which is often referred to as "Token Ring network".
in fact, in most cases, this topology of the network will not be all computers are really connected to the physical ring type, generally, the ends of the ring are through an impedance matching device to
The implementation of the ring closed, because in the actual network process due to geographical constraints inconvenient really do the two ends of the ring physical connection.
The topology of the network mainly has the following characteristics:
(1) This network structure is generally only applicable to the IEEE 802.5 token network (Token Ring network), in which the "token" is transmitted sequentially in a ring-type connection. The use of
the transmission medium is usually coaxial cable.
(2) This kind of network realization is also very simple, the investment is minimal. Can be seen from its network structure, in addition to the network of workstations is the transmission medium-coaxial cable, and
Some connection equipment, no expensive node-focused devices, such as hubs and switches. But because of this, this kind of network can achieve the most simple function, can only be used as
General file service mode;
(3) faster transmission speed: In the token network allows 16Mbps transmission speed, it is much faster than the normal 10Mbps ethernet. Of course, with the wide application of Ethernet and Ethernet
The development of technology, the speed of Ethernet has also been greatly improved, at present, generally can provide 100Mbps speed, far higher than 16Mbps.
(4) Maintenance difficulties: From its network structure can be seen, the entire network of nodes are directly connected, so that any one node failure will cause the entire network interruption, paralysis,
maintenance is very inconvenient. On the other hand, because the coaxial cable is a pin-type contact mode, it is very easy to cause poor contact, network interruption, and so look up not
often difficult, it is believed that the people who have maintained this network will have deep experience.
(5) Poor expansion performance: Because of its ring-shaped structure, it is decided that its expansion performance is far better than the star structure, if you want to add or move nodes, you must interrupt the entire network
Connector at both ends of the ring.
C: Total line structure
This network topology is relatively simple, all the devices in the total linetype are directly connected with a transmission medium called Common bus, which is usually coaxial cable (including coarse cable
and thin cable), but now also has the use of optical cable as a total line transmission medium, such as ATM network, Cable Modem used by the network, etc. are the total LINETYPE network structure.
This structure has the following characteristics:
(1) Low-cost network: From the structure can not need additional interconnection equipment, is directly connected through a bus, so the network cost is low;
(2) This kind of network because each node is the common bus bandwidth, so in the transmission speed will with the access network user's increase and decrease;
(3) network user expansion is more flexible: need to expand the user only need to add a connector, but the number of users can be connected limited;
(4) Easy to maintain: a single node (each computer or hub can be considered a node) failure does not affect the normal communication of the entire network. But if the bus breaks,
The entire network or the corresponding backbone network method is broken.
(5) The disadvantage of this network topology is that only one end user can send the data at a time, and the other end users must wait to get the right to send.
Three. Media access control method
The method of effectively allocating the frequency bands of transmission media to the nodes on the network is called the media access control method. The method of media access control is one of the most important basic technologies of local area network.
Have a decisive impact on architecture, work processes, and network performance. The method of media access control is mainly to solve the problem of the algorithm or mechanism of media use right, so that many users can reasonably
and conveniently share the communication medium resources, so as to realize the reasonable distribution of the network transmission channel.
The main contents of the media access control method are two aspects: one is to determine the specific moment that each node on the network can send the information to the media, and the other is to solve how to
Shared media access and utilization are controlled. There are three kinds of media access control methods commonly used: carrier-listening multi-access CSMA/CD method and ring structure with collision detection in bus structure
Token Ring) Access control method and token bus access control method for the card.
Determine the main technical elements of the LAN
