The Ethernet Mac Address

The Ethernet MAC Address
Initially, Ethernet was implemented as part of a bus topology. Every network device was connected to the same, shared media. In low traffic or small networks, this was an acceptable deployment. The main problem to solve was how to identify each device. The signal could be sent to every device, but how would each device identify if it were the intended receiver of the message?
A unique identifier called a Media Access Control (MAC) address was created to assist in determining the source and destination address within an Ethernet network. Regardless of which variety of Ethernet was used, the naming convention provided a method for device identification at a lower level of the OSI model.
As you will recall, MAC addressing is added as part of a Layer 2 PDU. An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits.
















MAC Address Structure
The MAC address value is a direct result of IEEE-enforced rules for vendors to ensure globally unique addresses for each Ethernet device. The rules established by IEEE require any vendor that sells Ethernet devices to register with IEEE. The IEEE assigns the vendor a 3-byte code, called the Organizationally Unique Identifier (OUI).

IEEE requires a vendor to follow two simple rules:
• All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes.
• All MAC addresses with the same OUI must be assigned a unique value (vendor code or serial number) in the last 3 bytes.

The MAC address is often referred to as a burned-in address (BIA) because it is burned into ROM (Read-Only Memory) on the NIC. This means that the address is encoded into the ROM chip permanently - it cannot be changed by software.
However, when the computer starts up, the NIC copies the address into RAM. When examining frames, it is the address in RAM that is used as the source address to compare with the destination address. The MAC address is used by the NIC to determine if a message should be passed to the upper layers for processing.

Network Devices
When the source device is forwarding the message to an Ethernet network, the header information within the destination MAC address is attached. The source device sends the data through the network. Each NIC in the network views the information to see if the MAC address matches its physical address. If there is no match, the device discards the frame. When the frame reaches the destination where the MAC of the NIC matches the destination MAC of the frame, the NIC passes the frame up the OSI layers, where the decapsulation process take place.
All devices connected to an Ethernet LAN have MAC-addressed interfaces. Different hardware and software manufacturers might represent the MAC address in different hexadecimal formats. The address formats might be similar to 00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or 0005.9A3C.7800. MAC addresses are assigned to workstations, servers, printers, switches, and routers - any device that must originate and/or receive data on the network.



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Ethernet Future Options

Ethernet Future Options

The IEEE 802.3ae standard was adapted to include 10 Gbps, full-duplex transmission over fiber-optic cable. The 802.3ae standard and the 802.3 standards for the original Ethernet are very similar. 10-Gigabit Ethernet (10GbE) is evolving for use not only in LANs, but also for use in WANs and MANs.
Because the frame format and other Ethernet Layer 2 specifications are compatible with previous standards, 10GbE can provide increased bandwidth to individual networks that is interoperable with the existing network infrastructure.


10Gbps can be compared to other varieties of Ethernet in these ways:
• Frame format is the same, allowing interoperability between all varieties of legacy, fast, gigabit, and 10 gigabit Ethernet, with no reframing or protocol conversions necessary.
• Bit time is now 0.1 ns. All other time variables scale accordingly.
• Because only full-duplex fiber connections are used, there is no media contention and CSMA/CD is not necessary.
• The IEEE 802.3 sublayers within OSI Layers 1 and 2 are mostly preserved, with a few additions to accommodate 40 km fiber links and interoperability with other fiber technologies.
With 10Gbps Ethernet, flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible.

Future Ethernet Speeds
Although 1-Gigabit Ethernet is now widely available and 10-Gigabit products are becoming more available, the IEEE and the 10-Gigabit Ethernet Alliance are working on 40-, 100-, or even 160-Gbps standards. The technologies that are adopted will depend on a number of factors, including the rate of maturation of the technologies and standards, the rate of adoption in the market, and the cost of emerging products.



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Ethernet

Historic Ethernet

The foundation for Ethernet technology was first established in 1970 with a program called Alohanet. Alohanet was a digital radio network designed to transmit information over a shared radio frequency between the Hawaiian Islands.
Alohanet required all stations to follow a protocol in which an unacknowledged transmission required re-transmitting after a short period of waiting. The techniques for using a shared medium in this way were later applied to wired technology in the form of Ethernet.
Ethernet was designed to accommodate multiple computers that were Interconnected on a shared bus topology.
The first version of Ethernet incorporated a media access method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD managed the problems that result when multiple devices attempt to communicate over a shared physical medium.


















Early Ethernet Media
The first versions of Ethernet used coaxial cable to connect computers in a bus topology. Each computer was directly connected to the backbone. These early versions of Ethernet were known as Thicknet, (10BASE5) and Thinnet (10BASE2).
10BASE5, or Thicknet, used a thick coaxial that allowed for cabling distances of up to 500 meters before the signal required a repeater. 10BASE2, or Thinnet, used a thin coaxial cable that was smaller in diameter and more flexible than Thicknet and allowed for cabling distances of 185 meters.
The ability to migrate the original implementation of Ethernet to current and future Ethernet implementations is based on the practically unchanged structure of the Layer 2 frame. Physical media, media access, and media control have all evolved and continue to do so. But the Ethernet frame header and trailer have essentially remained constant.
The early implementations of Ethernet were deployed in a low-bandwidth LAN environment where access to the shared media was managed by CSMA, and later CSMA/CD. In additional to being a logical bus topology at the Data Link layer, Ethernet also used a physical bus topology. This topology became more problematic as LANs grew larger and LAN services made increasing demands on the infrastructure.
The original thick coaxial and thin coaxial physical media were replaced by early categories of UTP cables. Compared to the coaxial cables, the UTP cables were easier to work with, lightweight, and less expensive.
The physical topology was also changed to a star topology using hubs. Hubs concentrate connections. In other words, they take a group of nodes and allow the network to see them as a single unit. When a frame arrives at one port, it is copied to the other ports so that all the segments on the LAN receive the frame. Using the hub in this bus topology increased network reliability by allowing any single cable to fail without disrupting the entire network. However, repeating the frame to all other ports did not solve the issue of collisions. Later in this chapter, you will see how issues with collisions in Ethernet networks are managed with the introduction of switches into the network.

















Ethernet Collision Management

Legacy Ethernet
In 10BASE-T networks, typically the central point of the network segment was a hub. This created a shared media. Because the media is shared, only one station could successfully transmit at a time. This type of connection is described as a half-duplex communication.
As more devices were added to an Ethernet network, the amount of frame collisions increased significantly. During periods of low communications activity, the few collisions that occur are managed by CSMA/CD, with little or no impact on performance. As the number of devices and subsequent data traffic increase, however, the rise in collisions can have a significant impact on the user's experience.
A good analogy is when we leave for work or school early in the morning, the roads are relatively clear and not congested. Later when more cars are on the roads, there can be collisions and traffic slows down.

Current Ethernet
A significant development that enhanced LAN performance was the introduction of switches to replace hubs in Ethernet-based networks. This development closely corresponded with the development of 100BASE-TX Ethernet. Switches can control the flow of data by isolating each port and sending a frame only to its proper destination (if the destination is known), rather than send every frame to every device.
The switch reduces the number of devices receiving each frame, which in turn reduces or minimizes the possibility of collisions. This, and the later introduction of full-duplex communications (having a connection that can carry both transmitted and received signals at the same time), has enabled the development of 1Gbps Ethernet and beyond.




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