Category Archives: Data Link Layer

Ethernet

Ethernet is the most widely installed set of frame-based computer networking technologies for Local Area Networks (LAN). It defines numerous signaling and wiring standards for the physical layer (Layer 1) of the open systems interconnection (OSI) model. This is done through a common addressing format, and a network access at the Media Access Control (MAC).

This technology is specified in the IEEE 802.3 standard. The most common wired LAN technology is comprised of a combination of the fiber optic versions for backbones sites. It is also comprised of Ethernet’s twisted pair versions for hooking end systems to the network.

An Ethernet LAN characteristically utilizes special grades of twisted pair wires or coaxial cable. 10BASE-T is a common Ethernet technology that gives a transmission speed of up to 10 Mbps. Devices are connected to the cable. These multiple devices gain access and are regulated through a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA-CD).

Ethernet was initially based on the scheme of computers communicating with a shared coaxial cable to serve as the medium of transmission. This method displayed some resemblance to radio systems. However, the two differ greatly because collisions are more easily detected in a cable broadcast system than in a radio.

From this concept, Ethernet developed into a set of networking technologies that are applied for most LAN’s nowadays. Today, point-to-point links connected by Ethernet hubs or switches are used instead of a coaxial cable. This development resulted to increased reliability, the ability to troubleshoot, and cost reduction.

Despite the major changes in the Ethernet technology, all Ethernet versions share the same frame formats so these can be readily interconnected.

One Ethernet technology, the 100 BASE-T, is characteristically used for LAN backbone systems. Data can be transmitted up to 100 megabits per second. Gigabit Ethernet gives a greater backbone support at 1000 megabits per second. 10 Gigabits supports data at 10 billion bits per second.

Robert Metcalfe, one of Ethernet’s developers, coined the term “Ethernet.” The name came from the passive substance called “luminiferous ether.” This substance was once thought to encompass the universe, transporting light all over. Ethernet was named because the luminiferous ether was compared to the way cabling, also a passive medium, could bring data everywhere throughout the network.

Robert Metcalfe, David Boggs, Butler Lampson, and Chuck Thacker originally developed Ethernet at Xerox in 1973.

Related Articles on Ethernet

ARP Cache

The Address Resolution Protocol (ARP) cache is a table that saves mappings between Network Layer (Layer 3) and Data Link Layer (Layer 2) addresses. The Network Layer contains the Internet Protocol (IP) addresses, while the Data Link Layer contains the Media Access Control (MAC) addresses.

Each system employing the ARP protocol uses this cache of recent mappings to minimize the number of ARP requests. The Operating System stores the cache in Random Access Memory (RAM).

Sometimes, the Microsoft Windows ARP cache becomes corrupted and has to be cleared. The first sign that may be noticed is that connections to the Internet will time out and fail.

Viewing the ARP Cache

To take a look at the ARP cache, go to the start menu and click on Run. In the Command line interface, type command: C:\>arp -a

Clearing the ARP Cache

To solve this issue, the ARP cache has to be cleared. This is performed with the command: C:\>netsh interface ip delete arpcache. Then click OK.

If Clearing Fails

There are times when the ARP cache cannot work properly because of a bug in Microsoft Windows. At these instances, a system error message appears. The bug happens when Routing and Remote Services are allowed. These services have almost no use and can be disabled.

To disable Routing and Remote Services:

  1. Click the Start button of the Windows Explorer. Go to the Control Panel.
  2. If it is in Category view, switch to Classic View. Then, click the Administrative Tools link.
  3. Double-click the Computer Management link.
  4. Double-click Services and Applications and give another double-click to Services.
  5. Double-click Routing and Remote Services.
  6. Set the Startup Type to Disabled (options are Manual, Automatic, and Disabled)
  7. Click the Stop button for the Service Status.
  8. Hit the OK button to save the changes.

Then, clear the ARP cache once more. The ARP should be full without any error messages.

How Can the ARP Cache Help?

Whatever IP device is currently being used, it will have an ARP cache. This will be utilized in the troubleshooting of network connectivity. If all things are going smooth for ARP, there will be a dynamic ARP entry, with IP and MAC values. If the ARP entry is not complete, the ARP cache can be cleared, then another communication can be attempted.

Additional Reading on ARP Cache

Change MAC Address

Every Ethernet card has a unique MAC address written into it at the time it was manufactured. However, there are a number of reasons why the MAC address should be changed. It is entirely up to the user.

Changing to Work With ISP

Most Internet Service Providers (ISPs) assign one permanent IP address per client. However, this method is an ineffective approach, because IP addresses are presently in short supply. As an alternative, some ISPs make sure that each client has one dynamic address that may change each time the client hooks up to the Web. They then register and track the MAC addresses of the device connected to the ISP.

ISPs monitor the MAC address of clients with a broadband router, a cable modem, or the computer hosting the Internet connection. ISPs ensure that the MAC address of equipments used always matches the registered value. If a client replaces that device or its network adapter, the MAC address will no longer match the registered value.

Changing a MAC Address Through Cloning

Requesting for ISPs to update the MAC Address will take time, and while actions are not yet taken, the user cannot surf the Internet.

A better means to promptly work around this trouble is to change the MAC address on the new device so that it corresponds to the original device. Cloning is the process wherein the MAC address is emulated through the software. ISPs see that the emulated MAC address is identical to the original hardware address. Many broadband routers support cloning as an advanced configuration choice. The specific cloning procedure will depend on the type of router to be used.

Changing Through the Operating System

The specific procedure to change the MAC address depends on each Operating System.

In Microsoft Windows, the MAC address is saved in a registry key. Find the registry key with ‘regedit’, and then change it. With Windows XP, the MAC address on some network cards can be changed through the Advance tab in the Properties menu of the network adapter. A software utility program is designed to be more dependable and is a much easier means to change the MAC address. Macshift is an example.

In Linux, the MAC address can be changed by using the GNU MAC Changer or by using ‘ifconfig <interface>hw<class><address>’.

In HP-UX, Networking and Communications should be used. Click on the ‘Interface’, and then go to ‘Action.’ Click on ‘Modify,’ and then ‘Advanced options.’