What Is a Pseudowire?

Pseudowire is a collective name for various simulation technologies in the communication field, and it is a point-to-point connection established between edge routers. The main function of the pseudowire is to simulate some common services such as Frame Relay, Time Division Multiplexing Service (TDM), Asynchronous Transmission Service (ATM), and Ethernet. The simulated service passes a hidden core MPLS (multiprotocol label) Switched) network, encapsulated into a common MPLS format.

Pseudowire is a collective name for various simulation technologies in the communication field, and it is a point-to-point connection established between edge routers. The main function of the pseudowire is to simulate some common services such as frame relay, time division multiplexing service (TDM), asynchronous transmission service (ATM), and Ethernet. Switched) network, encapsulated into a common MPLS format.

Chinese name

Pseudo line

Foreign name

Pseudo Wire

Definition

Point-to-point connection between edge router pairs

Hidden core

MPLS

The concept of pseudo-wire simulation

Understanding pseudowire technology requires an understanding of the concept of simulation.

Simulation is literally understood by most people, which is to imitate a class of things to achieve a realistic effect. E.g:

• The purpose of military exercises is to imitate the battlefield when war actually occurs, in order to achieve the purpose of training soldiers on the ground.

• In the course of learning to drive a motor vehicle, there will be several lessons in which we use a car cab simulation warehouse to simulate driving against an electronic screen.

• Pilots use simulated aircraft on the ground for flight simulation before actually flying.

• Many of the school's experimental courses use computer simulation software to simulate scientific experiments.

Pseudo-wire pseudo-wire simulation technology

Through the above examples, you can understand the meaning of simulation. Now let's talk about simulation in pseudo-wire technology.

The simulation in the pseudowire is to implement circuit simulation on a packet network. Circuits refer to traditional circuit-switched networks.

After the services on the circuit-switched network reach the packet network, the packet network simulates a line that can perfectly transmit the services of these circuit-switched networks. When the services on the circuit-switched network reach their destination, they leave the packet network and enter the new circuit-switched network. When these circuit-switched network services are transmitted through the packet network, they do not "know" that they have left the circuit-switched network, because the packet network completely emulates a circuit of a circuit-switched network.

Figure 1 Circuit simulation

Seeing this, I believe you can understand that the simulated circuit in the figure above is a packet network that simulates a virtual circuit-switched network circuit to transmit circuit-switched services. This circuit is not a real circuit-switched circuit and is "disguised" , So it is called a pseudo-line. Pseudo-wire technology can be compatible with existing time-division multiplexing equipment, and replaces traditional circuit-switched networks with high-efficiency and low-cost packet networks to transmit voice, video, and data, reducing the cost of information transmission.

Pseudo-wire pseudo-wire working process

How does pseudowire technology work?

Let's take a real-life example, for example, I want to send a parcel to my distant friend. I have 2 options:

I delivered it myself. In this case, it will take me a lot of time and energy and a lot of economic investment.

Sent by courier company. In this way, I don't need to waste my time and energy, just send the package to the courier company, and the remaining work is done by the courier company. I also saved a lot of money financially.

If you choose to use a courier company to ship and deliver the package. Then I just need to ask the courier to collect the package. The process is as follows:

1. The courier receives the parcel.

2. The courier does not care about the contents of the package, and is only responsible for delivering the package to the nearest station.

3. The parcel arrives at the courier station in the area and waits to be delivered to the municipal transfer center.

4. The courier site does not care about the content and destination of the package, and is only responsible for concentrating all the packages arriving at the station to the municipal transfer center.

5. After the parcel arrives at the municipal transfer center, it is distributed.

6. The transshipment center does not care what is inside the package, but only cares about the weight, volume, destination of the package and the level of service required for the package (ie, the speed of the package and delivery). After allocating according to these attributes, packages with the same attributes are sealed in a larger package. These large parcel attributes are owned by the courier company and have only simple destination attributes.

7. Large parcels are transported through the courier's transportation network to their destination.

8. The destination transfer center unpacks and delivers it to the area express station according to the customer address, and the courier completes the package delivery.

9. The parcel arrived in the hands of a friend.

Note that during this process, I gave the parcel to the courier, and the other courier delivered the parcel to my friend. In the meantime, my friend and I only contacted the courier, and I had no idea how the parcel was delivered in the middle. The package was exactly the same when it was handed over to me and when it arrived in a friend's hand. My friend and I were not clear about the details of the package being sealed in other packages during the delivery process.

Explain this example as a metaphor for the principle of pseudowire technology as follows:

• Parcels are traditional circuit-switched services.

• Couriers are ordinary lines in a circuit-switched network.

• Express sites are edge devices in a circuit-switched network.

• A transit center is an edge device in a packet network.

• The delivery company's delivery network is a virtual circuit simulation circuit, that is, a pseudo line

Figure 1 Working process of pseudowire

Circuit-switched services reach the edge devices of the packet network through the edge devices of the circuit-switched network. A virtual line is created between the edge device of the packet network and the edge device at the other end of the packet network. Circuit-switched services are repackaged into packet data packets and transmitted in the virtual line. After the packet data packet passes the tunnel, it is unpacked by the edge device of the packet network to restore the original circuit switched service, and the circuit switched service is sent to the circuit switched network.

In this process, the packet network does not care what the transmitted service is, but is only responsible for encapsulation and transmission. The circuit switching service itself has not changed. It is transparently transmitted in the packet network. The transmission line is a virtual circuit simulation circuit. The simulation circuit itself is not a real circuit switching circuit, so it is called a pseudo-wire.

Development and application of pseudowires

Circuit simulation was implemented on ATM switches in the early days, and then transferred to IP networks for circuit simulation (ATM switches are expensive and the development of IP networks is unstoppable). In IP packet networks, tunnels are used to load pseudowires, and various simulated services are transparently transmitted through the pseudowires. During the transmission, no changes are made to the simulation service itself, and the original service is restored after the transmission is completed.

The services that are usually simulated are Frame Relay, Time Division Multiplexing Service (TDM), Asynchronous Transmission Service (ATM), etc. The commonly used bearer network is an MPLS (Multiprotocol Label Switching) network.

Pseudowire related protocol

PWE3: Abbreviation for Pseudo wire emulation edge-to-edge. 3 has no special meaning. It only represents three E abbreviations for emulation edge-to-edge, which translates to edge-to-edge pseudo-wire simulation. PWE3 is an extension of the Martini protocol, and the basic signaling process is the same.

PWE3 is an end-to-end Layer 2 service bearer technology. It belongs to a point-to-point L2VPN. Through it, the original access method can be well integrated with the existing IP backbone network, reducing the repeated construction of the network and saving operating costs.

PWE3, as a kind of emulation of layer 1 and layer 2 services on a packet-switched network, helps operators to provide a single network with a variety of services (including TDM leased lines, FR, ATM, Ethernet ...) to carry. From the user's point of view, PW (Pseudo-Wire) is just like the user selects a non-shared link / circuit.