Bridge function The function of the bridge is similar to a repeater in extending the network span, but it can provide intelligent connection services, that is, forwarding and filtering based on which network segment the frame end address is in. The bridge's understanding of the network segment of the site is achieved by "self-learning". All bridges provide connection services at the data link layer. Depending on the type of LAN they are connected to, there are 4 types of bridges: transparent bridges, conversion bridges, encapsulated bridges, and source routing bridges. Description: The bridge uses the physical layer and data link layer protocols of LAN1 (token ring network) to read the end addresses of all frames sent by the LAN1 workstation. The bridge ignores the frames addressed to the LAN1 workstation and filters them out. Encapsulated bridge 1 uses the physical layer and data link layer protocols used by LAN1 to read the MAC end addresses of all frames sent by devices on LAN1;
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When using a bridge to connect two LANs as shown in Figure 2, the bridge must first check the end address of the MAC frame from network segment 1. If the frame is destined for a station on segment 1, the bridge does not forward the frame to segment 2, but filters it out; if the frame is destined for a station on segment 2, the bridge Then forward it to network segment 2. This shows that if there is a pair of users on LAN1 and LAN2 communicating at the same time on this network segment, it is obviously achievable. Because the bridge has played a role of isolation. It can be seen that the bridge has the effect of increasing the network bandwidth under certain conditions.
Compared with the repeater, the storage and forwarding function of the bridge has advantages and disadvantages. The advantages are:
The use of bridges for interconnection overcomes physical limitations, which means that the total number of data stations and network segments that make up the LAN can be easily expanded.
. The bridge incorporates store and forward functions to make it suitable for connecting two LANs using different MAC protocols. Therefore, a mixed network environment in which different LANs are connected together is formed.
The relay function of the bridge only depends on the MAC frame address, so it is completely transparent to high-level protocols.
. The bridge divides a larger LAN into segments, which is helpful to improve reliability, availability and security.
The main disadvantages of the bridge are:
.Because the bridge receives and buffers frames before performing forwarding, it will introduce more delay than the repeater.
.Because the bridge does not provide flow control function, it may be overloaded when the traffic is heavy, resulting in frame loss.
The advantages of the bridge over the disadvantages are the reasons for its widespread use.
Types of bridges
(1) Transparent bridge The so-called "transparent bridge" means that it is completely transparent to any data station, and users cannot perceive its existence and cannot address the bridge. All routing decisions are determined by the bridge itself. When the bridge is connected to the network,
It can automatically initialize and configure itself.
The port where the LAN segment is connected to the bridge is called the bridge port. A basic bridge has only two ports, while a multi-port bridge can have multiple ports connected to the LAN.
Each bridge port is composed of a MAC integrated circuit chip corresponding to a specific LAN type and related port management software. The port management software is responsible for initializing the chip and managing the buffer when it is powered on. In general, the available memory is logically divided into several fixed sizes and units, called buffers. Buffer management involves passing the free buffer pointer to the integrated circuit chip in order to be ready to receive frames. Also transfer the frame buffer batch pin to the chip, and then forward the frame.
All bridges operate in an indiscriminate manner, which means that the bridge receives incoming frames at each of its ports and buffers them. When the frame is received by the MAC chip on a port and placed in the allocated buffer, the port management software prepares the chip to receive the new frame, and then passes the pointer of the buffer including the received frame to the bridge protocol entity for processing. If the bridge arrives on two or more frames at the same time on its port and needs to forward these frames from the same port, the transfer of the buffer pointer between the port management software and the bridge protocol entity software is achieved through a set of queues.
The forwarding and filtering of the bridge can be illustrated by Figure 3 (b). The bridge 1 connecting LAN1 and LAN2 in the figure has two ports, and the bridge connecting LAN2 and LAN3 also has two ports. The forwarding data base in the two bridges indicates from which port the reachable station is forwarded. When the bridge receives a frame, it can determine whether to filter or forward the frame by looking up the forwarding data base. Because the bridge operates on the MAC sublayer of the data link layer, this forwarding data base can be established by checking the station address in the MAC frame. The process of establishing a forwarding data base based on the MAC frame address is called a "self-learning" process.
(2) The conversion network to conversion bridge is a special form of transparent bridge. It uses different protocol LANs to provide network connection services at the physical layer and data link layer. Figure 4 shows the conversion bridge connecting the token ring network and the Erhwrnet network.
Conversion bridges provide connection services by processing envelopes associated with each LAN type. The processing provided by the conversion bridge is relatively simple because the token ring and Ethernet envelope are similar. However, the frame lengths of the two LANs are different, and the conversion bridge cannot segment the long frames, so when using this bridge, the frame lengths sent by the interconnected LANs must be acceptable by both LANs.
The bridge accepts the frames sent to the LAN2 workstation and forwards these frames to LAN2 using the physical layer and data link layer protocols used by LAN2. The bridge performs the same processing on the frames sent by the LAN2 workstation.
(3) Encapsulated bridges Encapsulated bridges are usually used to connect FDDI backbone networks. Figure 5 shows this connection structure. The encapsulated bridge is used to connect 4 Ethernets to the FDDI backbone network.
Unlike a conversion bridge, an encapsulated bridge puts the received frame in an envelope used by the FDDI backbone network, and forwards the encapsulated frame to the FDDI backbone network, and then passes it to other encapsulated bridges. The envelope is removed and sent to a predetermined workstation.
To explain its working process, suppose that the workstation on LAN1 wants to send a message to a device on LAN3. The process is as follows:
Encapsulated bridge 1 accepts frames addressed to other LANs, places these frames in FDDI envelopes, and sends this envelope to the FDDI backbone network;
Encapsulated bridge 1 filters out all frames addressed to devices on LAN1;
Encapsulated bridge 2 receives all frames, removes the envelope, and checks the MAC frame address, because the MAC frame address is not local
On LAN2, these frames are filtered out;
Encapsulated bridge 3 receives all frames, removes the envelope, and checks the MAC frame address, because the MAC frame address is local
LAN3, the encapsulated bridge 3 uses the physical layer and data link layer protocols of LAN3 to send frames to the predetermined device of LAN3;
The operation of the encapsulated bridge 4 is the same as the encapsulated bridge 2;
The encapsulated bridge 1 evacuates frames from the FDDI backbone network from the FDDI double ring.
(4) Source routing bridge Source routing bridge is mainly used to interconnect the token ring network, but in theory can be used to connect any type of
LAN.
A basic difference between bridges is that source routing bridges require information sources (not the bridge itself) to provide the routing information needed to pass frames to the destination.
When using source routing to select a bridge, the bridge does not need to save the forwarding data base. The basis for forwarding and filtering frames is the data included in the frame envelope. If the source wants to write the route to the destination when sending data, it must first be obtained through the "route inquiry process".
Route inquiry can be implemented in several ways, one of which will be described below. Referring to the structure of Fig. 6, 5 token ring networks are connected by 3 source routing bridges. Assume that the LAN1 station has a message to send to the station on LAn5. lAN1
The station on the top initiates the path discovery process by sending "probing" packets. The inquiry packet uses a unique envelope that only the source routing bridge can recognize. Once each source routing bridge receives the inquiry packet, it enters the connection and the name of the connection that received the inquiry packet into the routing information field. The bridge then spreads the packet to all connections except those that received the packet.
Therefore, multiple copies of the same inquiry message may appear on the LAN, and the receiver of the inquiry frame will also receive multiple copies, one copy for each possible path from the source to the destination. Each received frame includes a series table consisting of connection / bridge names, which lists possible paths from source to destination.
The receiver of LAN5 may receive multiple inquiry messages, so choose a path according to the fastest and most direct principle, and send a response back to the sender of LAN1. This response lists the specific path between the source and the destination that consists of the intermediate bridge and LAN connection.
After the source of LAN1 finds this path, it stores it in the memory for its subsequent use. These messages are included in different types of envelopes that can be recognized by the source routing bridge. After receiving this envelope, the bridge only needs to scan the table composed of the connection and the bridge to obtain the forwarding information.
Advantages
Suitable for multiple operating wavelengths (1260nm – 1650nm); unstinted.
Equal splitter ratios for all branches.
Compact configuration; smaller size; small occupation space.
Good stability across all ratios.
