Wide area network WAN - Next part

As we know, the local area network (LAN) is used to connect devices together. The data transfer rate in the local network is therefore quite high. WAN networks, on the other hand, connect remote devices geographically and therefore WAN technology is different from LAN technology. WAN uses transmission method, hardware and protocol other than LAN. The data transfer rate in WAN is also much lower when compared to LAN. We will examine the overview of WAN technologies under several angles

1. Overview of communications in WAN
2. WAN services
3. WAN hardware
4. Methods of data packaging in the WAN

Overview of communications in WAN

The WAN uses the transmission infrastructure of a third-party service provider, mainly telephone companies, to provide long-distance connection services. The most common configuration of a WAN includes components as shown below. A message is initiated from the client and sent by a device called DTE to the WAN service provider. DCE devices in the service provider's central office will 'push' the packet to the WAN, then go through the switches to reach the destination. Similar devices on the receiving end will end the journey.

Figure 1: Typical WAN

Data terminal equipment (DTE - Data Terminal Equipment): The device on the edge of the WAN link has the function to send and receive data. DTE is located at the subscriber's location, which is the connection point between the subscriber's LAN and the service provider's WAN. DTE is usually a router, but in some cases it can be a computer or a multiplexer. DTEs at the top of this side will perform communication with the corresponding DTE device on the other end.

Boundary Point (Damarcation Point): The point of connection between the telephone company telephone line and the subscriber line. Boundary points are also called network interfaces or point of presence. Typically, customers will be responsible for all equipment inside the boundary point and the telecommunications company will be responsible for all devices on the other side.

Terminal cable (Local Loop ): Cable connects from Boundary point to Central office of telephone company. Usually it is a twisted pair cable (UTP), but it can also be a combination of twisted pair cable, fiber optic cable and other types of media.

Central Office (Central Office): The nearest switchboard, is also the closest WAN service provider to the subscriber. The central office provides entry points for calls going to the 'WAN cloud' and provides points for calls from the WAN cloud to phone users. In addition, it acts as a a network switch point to transfer data packets to other central offices. It also provides a unidirectional direct current for the end-to-end cabling system to establish the circuit.

Equipment to close the data circuit (DCE - Data Circuit-terminating Equipment)

Communication device with both DTE and WAN cloud. DCE is usually a router of a service provider with the function of relaying data between a customer and a WAN cloud. In a narrow sense, DTE is any device that provides pulse signals for DTE. DCE can also be a device similar to DTE (usually a router) except that each type of device plays a separate role.

WAN (WAN cloud) cloud: A series of trunking, switchboard and central office constitute the transmission infrastructure of the telephone company. It is shown in the image as a cloud because there is a physical structure that changes frequently and only those responsible for network administration know where the data will go at the switchboards. For customers, it is important that data has been transferred over the line to the destination.

Packet-switching exchange switchboards: Switching switchboards on packet switching networks of telecommunication companies. PSE are intermediate points in the WAN cloud.

Data transmitted on LAN is mainly sent from a digital device (computer) to another digital device via direct connection. Meanwhile, because some WANs use the same telephone network available, data transmission can use one or a combination of the following methods:

Analog signal transmission

Analog signals are usually expressed in wave form. The intensity and frequency of analog signals change constantly, so it can accurately represent continuous or sound movements or multi-state movements. The intensity and frequency of the signal increase and decrease corresponding to the pitch and intensity of the sound. Analog signals are often used to represent real-time data. Radio, phone and media often use analog signals.

Figure 2: Analog signal transmission

Digital signal transmission

Instead of continuous flow, digital signals use only two states, 0 and 1, to represent data bits. This is the ideal signaling method for computer networks. Computers will need a modem, a device that converts a computer's digital signal into an analog signal to transmit data through a similar phone line.

Figure 3: Digital signal transmission

Note : Previously, PSTN telephone network was the same network completely. Similar signals from the telephone to the telecommunications company and will continue to be transferred to systems that use analog signals to reach the destination. Today, today's phone systems use a combination of two methods. Most of the switching networks (swithced network) of telecommunication companies have been digitized, the last leg connecting most households and some businesses still use the same signal. The diagram below shows the two digital computers that can be connected via WAN with both digital components and similar components. When a computer sends a signal over a WAN, the modem converts the digital signal into an analog signal to transfer the signal to the telephone company. The telephone company modem will again convert the data into digital form to transmit over the switching network. The signal is converted back into the analog signal at the destination of the telecommunications company to transfer to the modem of the computer receiving the data. Finally, this modem will convert the analog signal into a digital form for the computer.

Figure 4: PSTN network uses a combination of two signaling methods

Types of connections in the WAN

When a message moves through the WAN cloud, the way it moves from one point to another on its path will vary depending on the physical connection and protocol used. WAN connections are usually classified into the following types:

Dedicated Connection (Dedicated Connection)

This is a permanent connection, directly connecting a device to another device. Dedicated connections are stable and fast but can be expensive. Hiring a line from a WAN service provider means you have to pay for the connection even if you don't use it. Furthermore, because dedicated lines establish direct connections between only two points, the number of lines required will exponentially increase the number of locations to be connected. For example, if you want to connect 2 locations, you need a line but want to connect 4 locations, you will need up to 6 lines.

Figure 5: Dedicated connection

Features of dedicated connection:

1. Always available
2. Use leased line subscribers of WAN service providers
3. More expensive than other WAN solutions
4. Use separate connections between points
   

Use dedicated connection when:

1. High traffic data is transferred over LAN
2. Need to connect regularly
3. There are few places to connect with

Circuit- switched network

The switching network gives you an alternative to a leased line (dedicated connection), allowing you to use shared lines. Switching networks work in two directions, allowing both dial-in and dial-out connections to be established.

Figure 6: Switching network

When you use switching networks:

1. The computer sends the dial data to the line and the connection is established
2. The computer receives the data sent to confirm and lock the line
3. The computer sends data transmission data over the connection is established
4. After completing the data transfer, the connection is released to other users

Switched circuit using SVC - switched virtual circuit. A dedicated data link is established at the beginning of the communication process through a series of electronic switches. This private road will remain until the end of the communication process.

Public telephone system is a switching network. When you make a call, PSTN uses switches to create a physical, direct and dedicated connection for the duration of the call. When you stop the call, the switches release the line for other users. Computers connected via the network work in the same way. When the computer dials to the network, the network path is first set up so that data will then be transferred through this temporary dedicated path.

Packet-switched network (packet-swiched)

Packet-switching networks do not require a separate leased line or temporary dedicated road. Instead, the path of the message is dynamically set when data is transferred over the network. Packet switching connection is a regular connection. That means you don't need to worry about establishing a connection or keeping the line alone. Each packet includes the necessary information to reach the destination.

Figure 7: Packet switching network

The packet switching network has the following characteristics:

1. The message is divided into small units, called packages
2. Packages are transferred independently over the network (can follow different paths)
3. Packages are rearranged in the original order at the destination
4. The sending device and the default receiving device see the connection as permanent (no dialing required)

The packet switching network using permanent virtual circuits (PVC- permanent virtual circuit). Although PVC is like a direct, dedicated connection, the way each packet travels across the network may be different.

Wide area network services

PSTN

Public switched telephone network is the oldest and largest scale network that can be used for WAN communications. Features of PSTN include:

1. This is a switching network, with a global scope
2. The interface with PSTN is similar, so computers use modems to connect to PSTN
3. Speed ​​on PSTN is often limited to 56 Kbit / s
4. You can use PSTN when on demand or rent a separate circuit

Figure 8: PSTN telephone network

Private line (Leased Line)

For some companies, the benefit of a leased line can be much higher than the cost. Private leased lines are independent roads and have higher speeds than regular PSTN lines. However, it is quite expensive so usually only large companies use it. Other features of the leased line include:

1. Provide regular connection, stable quality
2. You can spend more to upgrade your own leased line

Figure 9: Private leased line

X.25

X.25 was born in the 1970s. Its original purpose was to connect large servers (mainframes) to remote stations. The advantage of X.25 compared to other WAN solutions is that it has built-in error checking mechanism. Select X.25 if you have to use the same line or the line quality is not high.

Figure 10 Network X25 on unstable transmission media

X.25 is ITU-T's standard for WAN communication using packet switching technology over the telephone network. The term X.25 is also used for the Physical layer and Data link layer protocols to create the X.25 network. According to the original design, X.25 uses the same line to create a packet switching network, although the X.25 network can also be built on a digital network. Currently, the X.25 protocol is a set of rules that determine how to establish and maintain connections between DTEs and DCEs in a public data network (PDN - public data network). How it relates to DTE / DCE devices and PSE (Packet-swiching exchange) will transmit data.

1. You need to pay subscription fee when using X.25 network
2. When using the X.25 network, you can make a connection to the PDN via a dedicated line
3. The X.25 network operates at 64 Kbit / s (on the same line)
4. Packet size (called frame) in X.25 network is not fixed
5. X.25 protocol has a very strong error checking and correcting mechanism, so it can work relatively stable on low quality analog phone line system.
6. X.25 is currently widely used in many countries around the world where the numbers are not popular as well as the line quality is low.

Frame Relay

Frame Relay is more efficient than X.25 and is gradually replacing this standard. When using Frame Relay, you pay the line rental fee to the nearest node on the Frame Relay network. You send data over your line and the Frame Relay network will route it to the node closest to the receiver and transfer the data to the recipient's line. Frame Relay is faster than X.25

Frame Relay is a standard for communication in WAN networks that converts packets through high quality digital lines. A Frame Relay network has the following features:

1. There are many similarities when deploying an X.25 network
2. There is an error checking mechanism but there is no mechanism for error correction
3. Data transmission speeds can be up to 1.54 Mbit / s
4. Allows multiple different packet sizes
5. Can connect as a backbone connection to the LAN
6. Can be deployed through various types of connections (56K, T-1, T-3)
7. Works at the Physical Layer and Data Link Layer in the OSI model.
   

Figure 11: Frame Relay network on stable transmission aspect

When registering using Frame Relay service, you are committed to the service level called CIR (Committed Information Rate). CIR is the maximum data transfer rate committed you receive on a Frame Relay network. However, when network traffic is low, you can send data at a faster rate than CIR. When traffic is high, priority will be given to customers with high CIR levels.

ISDN (Integrated Services Digital Network)

One of the purposes of ISDN is to provide WAN access to households and businesses using copper telephone lines. For that reason, the first ISDN deployment plans have proposed replacing existing analog lines with digital lines. Currently, the transition from analog to digital is going strong in the world. ISDN improves performance compared to WAN access via dial and has lower cost than Frame Relay.

Figure 12: ISDN line

ISDN sets standards for the use of analog phone lines for both digital data transmission and similar data transmission. The characteristics of ISDN are:

1. Allow broadcasting multiple types of data (voice, video, graphics .)
2. Data transmission speed and connection speed are higher than traditional dial-up connections

ATM

Asynchronous Transfer Mode (Asynchronous Transfer Mode) is an advanced packet switching system that can transmit data, audio and video simultaneously on both LAN and WAN networks.

Figure 13: Communication via ATM

This is one of the fastest WAN connection methods available today, reaching speeds of 155 Mbit / s to 622 Mbit / s. In fact, it could theoretically support the higher speed of current transmission media. However, high speed means higher costs, ATM is much more expensive than ISDN, X25 or FrameRelay. Features of ATM include:

Use small (fixed) data packets (cell) with fixed size (53 bytes), easier to handle than data packets with variable size in X.25 and Frame Relay.

1. High data transfer rate, theoretically can reach 1.2 Gbit / s
2. High quality, low noise, almost no need for error checking
3. Can be used with many different physical transmission media (coaxial cable, flat wire cable, fiber optic cable)
4. Can transmit multiple data types simultaneously