Notes on data communication and network-network exchange technology

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If there are multiple devices in the network, how can we connect them for one-to-one communication? A good solution is switching ).

Exchange methods are classified as follows:

Packet Switching has been gradually eliminated. Today, we will focus on circuit switching and group switching.

Circuit Switching Network:

A Circuit Switching Network is a group of switches connected by physical links. The connection between two sites is a dedicated path composed of one or more links. Each connection uses a dedicated channel on the link.

Indicates a circuit switching network with four switches and four links:

Each link is divided into multiple channels by multiplexing technology (the analog and digital signals described in the previous article.

When a needs to communicate with B, it needs to send a connection request to B. This request is received by all switches, and channel reservation is made on each link and these channels are combined, create a dedicated path. This process is called the establishment phase.

After the path is created, data can be transmitted on this path.

After the transmission ends, the disconnected Party sends a disconnect request. After the switch receives the request, it releases the path.

Conclusion: circuit switching is implemented at the physical layer.

The efficiency of the circuit switching network is relatively low, but the delay is the smallest, and many resources are occupied.

Datagram network:

In group switching, no resources are allocated to the group and no bandwidth is reserved for the link. Resources are allocated on demand based on the principle of first-come-first-served.

It indicates how site a transfers four groups to site B.

The four datagram messages belong to the same packet, but are transmitted to the destination through different paths. In addition, different paths may have different latencies, resulting in inconsistent grouping order, or loss of groups due to lack of resources.

The datagram exchange network also becomes a connectionless network. The Router does not save the connection status, and does not need to establish or remove the connection. Then how is the datagram transmitted to the destination? The answer is the route table using the source and destination IP addresses.

Path Selection is actually the process of packet routing. The route table is dynamic and periodically modified. (For more information about routing, see Rip, IGRP, and VPN protocols ).

Conclusion: data exchange is at the network layer. The efficiency of the datagram network is higher than that of the circuit switching network. However, it may take some time to process the forwarded data packets. Virtual circuit network:

A virtual circuit network is a product of combining a circuit switching network and a datagram network. He has some of the following features:
1. In the data transmission phase, the establishment phase and the demolition phase are required.
2. Allocate resources as needed during the establishment phase.
3. like a datagram network, data is divided into groups. Each group's header contains an address, which has local permissions, instead of the end-to-end permissions (that is, the switch can know the channel of the next switch and the transfer group based on this address, but does not know which end the switch will be sent ).
When creating a virtual circuit, you need to create a virtual circuit table on each switch between the source host and the target host. This is the key to creating a virtual circuit. A virtual circuit table is generally composed of many virtual circuit records. A virtual circuit record consists of an input interface, an output interface, an input VCI (virtual circuit ID), and an output VCI. Note that the input interface and the input VCI in the virtual circuit of the vswitch uniquely identify a virtual circuit passing through the vswitch.
There are two methods to create a virtual circuit table. One way is to manually configure the virtual circuit table of each switch by the network administrator. The virtual circuit established in this way is generally called a permanent virtual circuit (PVC ). The administrator can also manually delete the virtual circuit table in the vswitch. Another method is to send a special packet to the target node from the source node. When the special packet passes through the switch in the network, a virtual circuit table is created inside the switch. The source node sends a special message through signaling ). The virtual circuit established in this way is called the switched
Virtual circuit, SVC ). A notable feature of the switched virtual circuit is that the host can dynamically create and delete this virtual circuit without the involvement of a network administrator.
To better understand the working principle of the virtual circuit, let's look at the example. In the figure, if host a has data to send to host B, the first step must be to establish a connection between host a and host B; then host a sends data to host B along the established virtual circuit.

If you need to establish a virtual circuit from host a to host B, you must allocate a VCI for the virtual circuit that is not used yet. The following describes how to establish a virtual circuit by taking an exchange circuit as an example. Assume that the VCI value from host a to vswitch 1 is 5, and the VCI value from vswitch 1 to vswitch 2 is 11; the value of VCI between vswitch 2 and vswitch 3 is 7, and the value of VCI between vswitch 3 and host B is 4. In this case, to establish a virtual circuit from host a to host B, host a first sends a connection request packet to vswitch 1, which contains the address of host B. After receiving the connection request message, vswitch 1 creates a new record in its virtual circuit table, which consists of the input interface, input VCI, output interface, and output VCI.
Consists of four parts. Vswitch 1 assigns a VCI value for the link between interface 2 from host a to vswitch 1 that has not been used so far. In this example, vswitch 1 is the input VCI that receives packets from host a from interface 2. The value is 5 (which means 0 ~ 4 ).
Then, switch 1 also needs to select an appropriate output interface for the connection request message to send it to the next switch or destination node (this involves routing selection issues, I will not go into details here ). In this example, assume that the communication between host a and host B passes through vswitch 1, vswitch 2, and vswitch 3. Therefore, after receiving the packet sent by host a to establish a connection request with host B and entering the virtual circuit table, switch 1 sends the connection request to switch 2, vswitch 2 receives a connection request message and completes a process similar to vswitch 1. In this example, vswitch 2 assigns a value of 11 to the input VCI of the packet from Interface 3. Similarly, vswitch 3 assigns a value of 7 to the input VCI of packets from interface 0.
Finally, the connection request message is sent to host B. Assuming that host B is willing to establish a connection with host a, it also assigns a value to the input VCI of the virtual circuit. In this example, It is 4, host B uses this input VCI to identify all packets from host.
Because the establishment of the virtual circuit requires confirmation, host B should return a connection confirmation message to host. The Connection Confirmation message is sent to switch 3 first, which contains the input VCI value (4) selected by host B for the virtual circuit ), when vswitch 3 receives the connection establishment confirmation message, it can fill in the complete records corresponding to this virtual circuit table. Then, Switch 3 sends the connection confirmation message to switch 2, and switch 2 completes the complete record of the virtual circuit. Then, vswitch 2 sends the connection confirmation message to vswitch 1, and vswitch 1 completes the complete record of this virtual circuit. Finally, switch 1 sends the connection confirmation message to host a, and finally establishes a virtual circuit between host a and host B.
In this way, each switch has information about the virtual circuit from host a to host B. Each switch has a complete record of the virtual circuit, as configured by the Administrator, however, the establishment of virtual circuits between host a and host B is automatic and does not require manual configuration by the network administrator. Now, host a and host B can transmit data, which is used in the same way as PVC.
When data is sent between host a and host B, host a sends a disconncet packet to host B. The connection revocation packet is first sent to switch 1, and switch 1 deletes records related to the virtual circuit from its virtual circuit table, then, the connection undo packet is sent to vswitch 2 (in this case, vswitch 1 finds the virtual circuit table and the next node of the virtual circuit is vswitch 2). Similarly, switch 2 also deletes records related to the virtual circuit from its virtual circuit table; so on until host B completes the removal of the virtual circuit.

Conclusion: before sending the first data packet, host a generally has to wait for at least one RTT time, that is, the connection request message sent by host a has arrived at host B, the Connection Confirmation message returned by host B is sent to host.

Although the connection request message sent by host A contains the address of host B in the virtual circuit setup phase, in the real data transmission phase, each data packet must carry only one VCI identifier, this reduces the link overhead.

If a virtual circuit goes through a switch or link failure, the virtual circuit will become invalid and a new virtual circuit must be created.

Note:

The virtual circuit here is not a concept of virtual circuit in TCP connection.

The virtual circuit mentioned here is a concept in the WAN.

The virtual circuit in TCP is just a concept, that is, there is a circuit that logically seems to exist, but in fact it uses different paths with different sessions. That is to say, there is no fixed path. It is flexible in the communication process. TCP uses retransmission and validation to ensure the reliability of the connection, which is achieved through software.