Loopback, or loop-back, refers to the routing of electronic signals, digital data streams, or flows of items back to their source without intentional processing or modification. This is primarily a means of testing the communications infrastructure.
Many example applications exist. It may be a communication channel with only one communication endpoint. Any message transmitted by such a channel is immediately and only received by that same channel. In telecommunications, loopback devices perform transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal. Loop around is a method of testing between stations that are not necessarily adjacent, wherein two lines are used, with the test being done at one station and the two lines are interconnected at the distant station. A patch cable may also function as loopback, when applied manually or automatically, remotely or locally, facilitating a loop-back test.
Where a system (such as a modem) involves round-trip analog-to-digital processing, a distinction is made between analog loopback, where the analog signal is looped back directly, and digital loopback, where the signal is processed in the digital domain before being re-converted to an analog signal and returned to the source.
- DB9 null modem male to female slimline data transfer serial port adapter 2 Pack. B&B Electronics 232CAMS DB9 to DB25 Adapter Cable, Serial RS-232 Loopback Conn.
- Jan 23, 2015 Instead of using uCon, you can use Putty to run the loopback test on your USB-to-TTL adapter. Putty is open source, offers more functions and you can set any desired baud rate you want to. Here is how you can test your USB-to-TTL adapter using Putty: Use any jumper wire and connect the RXD pin to the TXD pin of your USB-to-TTL adapter.
Jul 29, 2017 I used this to test a Serial Port, I did receive the echo, but I was curious if the manufacturer used the reference Serial Loopback Design (such as the one from Oracle, see attached). I tested it out and found that pin 1-4-6 is connected to pin 9. I'm not sure why it was done, and I couldn't find any explanation for it. If the loopback test fails, make sure that the serial cable or gender changer that you are using works and that the adapter is in the correct port. You can check multiple ports at the same time by opening multiple sessions, putting the loopback adapter on one port, and trying to type into each session. The loopback adapter is quite a useful tool that was designed for testing out the network configurations. Over many years, some resourceful geeks and administrators have founded that the Loopback adapter has some handy features as well like connecting two computers without a crossover cable or connecting a virtual machine to the Internet.
- 3Virtual loopback interface
Telecommunications[edit]
In telecommunications, loopback, or a loop, is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.
A hardware loop is a simple device that physically connects the receiver channel to the transmitter channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
A modem can be configured to loop incoming signals from either the remote modem or the local terminal. This is referred to as loopback or software loop.
Serial interfaces[edit]
A serial communications transceiver can use loopback for testing its functionality. For example, a device's transmit pin connected to its receive pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Test). Some devices include built-in loopback capability.
A simple serial interface loopback test, called paperclip test, is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard RS-232 interface using D-subminiature DE-9 or DB-25 connectors.
Virtual loopback interface[edit]
Implementations of the Internet Protocol Suite include a virtual network interface through which network applications can communicate when executing on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any network interface controller. Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device.
![Loopback Loopback](/uploads/1/2/4/8/124870404/781517885.jpg)
Unix-like systems usually name this loopback interface
lo
or lo0
.Various Internet Engineering Task Force (IETF) standards reserve the IPv4 address block 127.0.0.0/8, in CIDR notation and the IPv6 address ::1/128 for this purpose. The most common IPv4 address used is 127.0.0.1. Commonly these loopback addresses are mapped to the hostnames, localhost or loopback.
MPLS[edit]
One notable exception to the use of the 127.0.0.0/8 network addresses is their use in Multiprotocol Label Switching (MPLS) traceroute error detection, in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.
Martian packets[edit]
Any IP datagram with a source or destination address set to a loopback address must not appear outside of a computing system, or be routed by any routing device. Packets received on an interface with a loopback destination address must be dropped. Such packets are sometimes referred to as Martian packets.[1] As with other bogus packets, they may be malicious and any problems they might cause can be avoided by applying bogon filtering.
Serial Loopback Adapter Pinout
Management interface[edit]
Some computer network equipment use the term 'loopback' for a virtual interface used for management purposes. Unlike a proper loopback interface, this type of loopback device is not used to talk with itself.
Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the physical interfaces on the device. Such a loopback device is also used for management datagrams, such as alarms, originating from the equipment. The property that makes this virtual interface special is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes.
Loopback interfaces of this sort are often used in the operation of routing protocols, because they have the useful property that, unlike real physical interfaces, they will not go down when a physical port fails.
Other applications[edit]
Serial Loopback Adapter
The audio systems Open Sound System (OSS), Advanced Linux Sound Architecture (ALSA) and PulseAudio have loopback modules for recording the audio output of applications for testing purposes. Unlike physical loopbacks, this does not involve double analog/digital conversion and no disruption is caused by hardware malfunctions.
See also[edit]
References[edit]
This article incorporates public domain material from the General Services Administration document 'Federal Standard 1037C' (in support of MIL-STD-188).
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Loopback&oldid=884575344'
![Serial Loopback Adapter Serial Loopback Adapter](/uploads/1/2/4/8/124870404/816846828.jpg)
When you install a loopback adapter, the loopback adapter assigns a local IP address for your computer.
After the loopback adapter is installed, there are at least two network adapters on your computer: your own network adapter and the loopback adapter. To run Oracle Database on Windows, set the loopback adapter as the primary adapter.
You can change the bind order for the adapters without reinstalling the loopback adapter. The bind order of the adapters to the protocol indicates the order in which the adapters are used. When the loopback adapter is used first for the TCP/IP protocol, all programs that access TCP/IP first probe the loopback adapter. The local address is used for tools, such as Oracle Enterprise Manager. Any other applications that use a different Ethernet segment are routed to the network card.
A loopback adapter is required if:
- You are installing on a non-networked computer and plan to connect the computer to a network after installation.
- Checking if a Loopback Adapter is Installed on Your Computer
To check if a loopback adapter is installed on your computer, run theipconfig /all
command: - Installing a Loopback Adapter on Windows 7
Use this procedure to install a loopback adapter on Windows 7. - Installing Microsoft KM-TEST Loopback Adapter on Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2
Use this procedure to install Micrsoft KM-TEST loopback adapter on different Windows versions. - Removing a Loopback Adapter
Use this procedure to remove a loopback adapter.
Related Topics
Checking if a Loopback Adapter is Installed on Your Computer
To check if a loopback adapter is installed on your computer, run the
ipconfig /all
command: Note:
Loopback Adapter installed on the computer must be made the Primary Network Adapter.
If there is a loopback adapter installed, then see a section that lists the values for the loopback adapter. For example:
Npcap Loopback Adapter
Installing a Loopback Adapter on Windows 7
Use this procedure to install a loopback adapter on Windows 7.
To install a loopback adapter on Windows 7:
- Click Start and enter
hdwwiz
in the Search box. - Click hdwwiz to start the Add Hardware wizard.
- In the Welcome window, click Next.
- In the The wizard can help you install other hardware window, select Install the hardware that I manually select from a list, and click Next.
- From the list of hardware types, select the type of hardware you are installing, select Network adapters, and click Next.
- In the Select Network Adapter window, make the following selections:
- Manufacturer: Select Microsoft.
- Network Adapter: Select Microsoft Loopback Adapter.
- Click Next.
- In the The wizard is ready to install your hardware window, click Next.
- In the Completing the Add Hardware Wizard window, click Finish.
- Click Manage Network Connections. This displays the Network Connections Control Panel item.
- Right-click the connection that was just created. This is usually named 'Local Area Connection 2'. Choose Properties.
- On the General tab, select Internet Protocol (TCP/IP), and click Properties.
- In the Properties dialog box, click Use the following IP address and do the following:
- IP Address: Enter a non-routable IP for the loopback adapter. Oracle recommends the following non-routable addresses:
192.168.
x
.x
(x
is any value between 0 and 255)10.10.10.10
- Subnet mask: Enter
255.255.255.0
. - Record the values you entered, which you need later in this procedure.
- Leave all other fields empty.
- Click OK.
- Click Close.
- Close Network Connections.
- Restart the computer.
- Add a line to the
DRIVE_LETTER
: WINDOWSsystem32driversetchosts
file with the following format, after thelocalhost
line:where:IP_address
is the non-routable IP address you entered in step 13.hostname
is the name of the computer.domainname
is the name of the domain.
For example: - Check the network configuration:
- Open System in the Control Panel, and verify that Full computer name displays the host name and the domain name, for example,
sales.us.example.com
. - Click Change. In Computer name, you must see the host name, and in Full computer name, you must see the host name and domain name. Using the previous example, the host name is now
sales
and the domain nameus.example.com
. - Click More. In Primary DNS suffix of this computer, you must see the domain name, for example,
us.example.com
.
Installing Microsoft KM-TEST Loopback Adapter on Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2
Use this procedure to install Micrsoft KM-TEST loopback adapter on different Windows versions.
To install a loopback adapter on Windows 8, Windows 8.1, Windows Server 2012, or Windows Server 2012 R2, perform the following steps:
- Open the Windows Control Panel.
- Double-click Add Hardware to start the Add Hardware wizard.
- In the Welcome window, click Next.
- In the The wizard can help you install other hardware window, select Install the hardware that I manually select from a list, and click Next.
- From the list of hardware types, select the type of hardware you are installing, select Network adapters, and click Next.
- In the Select Network Adapter window, make the following selections:
- Manufacturer: Select Microsoft.
- Network Adapter: Select Microsoft KM-TEST Loopback Adapter.
Then continue with the same steps as given for Windows Server 2008.
Removing a Loopback Adapter
Use this procedure to remove a loopback adapter.
To remove a loopback adapter, perform the following steps:
- Display System in the Windows Control Panel.
- In the Hardware tab, click Device Manager. This tab is not available with Windows Server 2008. Click Device Manager instead.
- In the Device Manager window, expand Network adapters. You must see Microsoft Loopback Adapter.
- Right-click Microsoft Loopback Adapter and select Uninstall. In Windows 8, Windows 8.1, Windows Server 2012, or Windows Server 2012 R2, right-click Microsoft KM-TEST Loopback Adapter and select Uninstall.
- Click OK.
- Restart the computer.
- Remove the line from the
DRIVE_LETTER:
WINDOWSsystem32driversetchosts
file, added after the localhost line while installing the loopback adapter on other Windows operating systems.