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The basic component of any network communication is data, which is in the form of packets, frames, or bits. A packet is a unit of data that is transmitted over the network, and contains routing information that helps it to reach its destination. Frames contain information related to the flow control, frame synchronization, and error checking. A bit is a binary digit, 0 or 1, that is a basic unit of communication in the Physical layer of the OSI model.
Data is processed and transmitted from one device to another on a network. Internally, the data transmission starts from the Application layer of one device and travels to the Physical layer while adding some information to the data packet. When the data packet reaches the Physical layer, it is transferred as modulated bits on the medium, which may be wired or wireless. As you may know, OSI model has seven layers, each layer appends some information to the data that is transmitted from the Application layer Layer 7 to the Physical layer Layer 1. At the Network layer Layer 3 , an IP header is prepended to the data that is received from the upper layers.
Once the frame moves to the Physical layer, PHY information is prepended to the frame. Finally, data is transferred as bits, which are binary digits: Now, the MPDU data frame is sent to the Physical layer from where the data frame is forwarded further to reach at the destination device. The Frame aggregation was introduced after the An analogy for frame aggregation is carpooling that is implemented to reduce traffic and subsequently reduce traffic jams.
When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data.
- MAC Series – Basics of MAC Architecture – Part 1 of 3.
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It is possible that the CSMA process will fail and two devices will transmit at the same time. This is called a data collision. If this occurs, the data sent by both devices will be corrupted and will need to be resent. Contention-based media access control methods do not require mechanisms for tracking whose turn it is to access the media; therefore, they do not have the overhead of controlled access methods.
However, the contention-based systems do not scale well under heavy media use. As use and the number of nodes increases, the probability of successful media access without a collision decreases. Additionally, the recovery mechanisms required to correct errors due to these collisions further diminishes the throughput. CSMA is usually implemented in conjunction with a method for resolving media contention. The two commonly used methods are:.
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- 802.11 MAC Series – Basics of MAC Architecture – Part 1 of 3.
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If a data signal is absent, indicating that the media is free, the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later.http://esportsify.net/finding-justice-a-rescue-alaska-mystery-book.php
802.11 MAC Series – Basics of MAC Architecture – Part 1 of 3
Traditional forms of Ethernet were developed to use this method. Almost all wired connections between devices in a LAN today are full-duplex connections - a device is able to send and receive simultaneously. However, wireless connections in a LAN environment still have to take collisions into account. If the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data.
This method is used by As previously stated, the underlying logical topology of Ethernet is a multi-access bus. Every network device is connected to the same, shared media, and all the nodes receive all frames transmitted. The issue is if all devices are receiving every frame, how can each individual device identify if it is the intended receiver without the overhead of having to process and de-encapsulate the frame to get to the IP address?
The issue becomes even more problematic in large, high traffic volume networks where lots of frames are forwarded. To prevent the excessive overhead involved in the processing of every frame, a unique identifier called a MAC address was created to identify the actual source and destination nodes within an Ethernet network. Regardless of which variety of Ethernet is used, MAC addressing provided a method for device identification at the lower level of the OSI model.
An Ethernet MAC address is a bit binary value expressed as 12 hexadecimal digits 4 bits per hexadecimal digit. MAC addresses must be globally unique. This means that the address is encoded into the ROM chip permanently - it cannot be changed by software. This is useful when attempting to gain access to a network that filters based on BIA - consequently, filtering, or controlling, traffic based on the MAC address is no longer as secure.
Different hardware and software manufacturers might represent the MAC address in different hexadecimal formats. The address formats might be similar to:. When a device is forwarding a message to an Ethernet network, it attaches header information to the packet. The header information contains the source and destination MAC address. The source device sends the data through the network. If there is no match, the device discards the frame. Since the creation of Ethernet in , standards have evolved for specifying faster and more flexible versions of the technology.
This ability for Ethernet to improve over time is one of the main reasons that it has become so popular. Early versions of Ethernet were relatively slow at 10 Mbps. The latest versions of Ethernet operate at 10 Gigabits per second and faster. Figure 1 highlights changes in the various versions of Ethernet.
At the data link layer, the frame structure is nearly identical for all speeds of Ethernet. The Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent. Both the Ethernet header and trailer have several sections of information that are used by the Ethernet protocol.
Each section of the frame is called a field. As shown in Figure 2, there are two styles of Ethernet framing:. The differences between framing styles are minimal. The most significant difference between the two standards is the addition of a Start Frame Delimiter SFD and the change of the Type field to a Length field in the Ethernet II Frame Fields: IEEE The Preamble and Start Frame Delimiter fields are not included when describing the size of a frame. Any frame less than 64 bytes in length is considered a "collision fragment" or "runt frame" and is automatically discarded by receiving stations.
The IEEE VLANs are created within a switched network and will be presented in a later course. Also, many quality of service QoS technologies leverage the User Priority field to implement various levels of service, such as priority service for voice traffic. If the size of a transmitted frame is less than the minimum or greater than the maximum, the receiving device drops the frame. Dropped frames are likely to be the result of collisions or other unwanted signals and are therefore considered invalid. At the data link layer the frame structure is nearly identical. At the physical layer different versions of Ethernet vary in their method for detecting and placing data on the media.
Data link layer - Wikipedia
MAC addresses use hexadecimal numbering. Hexadecimal is a word that is used both as a noun and as an adjective. When used by itself as a noun it means the hexadecimal number system. Hexadecimal provides a convenient way to represent binary values. Just as decimal is a base ten number system and binary is a base two number system, hexadecimal is a base sixteen system. The base sixteen number system uses the numbers 0 to 9 and the letters A to F. Figure 1 shows the equivalent decimal and hexadecimal values for binary to It is easier for us to express a value as a single hexadecimal digit than as four binary bits.
Given that 8 bits a byte is a common binary grouping, binary to can be represented in hexadecimal as the range 00 to FF. Leading zeroes are always displayed to complete the 8-bit representation. For example, the binary value is shown in hexadecimal as 0A. It is important to distinguish hexadecimal values from decimal values regarding the characters 0 to 9.
Hexadecimal is usually represented in text by the value preceded by 0x for example 0x73 or a subscript Less commonly, it may be followed by an H, for example 73H. However, because subscript text is not recognized in command line or programming environments, the technical representation of hexadecimal is preceded with "0x" zero X. Therefore, the examples above would be shown as 0x0A and 0x73 respectively. Number conversions between decimal and hexadecimal values are straightforward, but quickly dividing or multiplying by 16 is not always convenient.