Device addresses at this layer are referred to as physical addresses. Data link layer addressing is contained within the frame header and specifies the frame destination node on the local network. The frame header may also contain the source address of the frame.
Unlike Layer 3 logical addresses, which are hierarchical, physical addresses do not indicate on what network the device is located. Rather, the physical address is a unique device specific address. If the device is moved to another network or subnet, it will still function with the same Layer 2 physical address.
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An address that is device-specific and non-hierarchical cannot be used to locate a device across large networks or the Internet. This would be like trying to find a single house within the entire world, with nothing more than a house number and street name.
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The physical address, however, can be used to locate a device within a limited area. For this reason, the data link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host regardless of the number of network hops along the route.
MAC service data unit
If the data must pass onto another network segment, an intermediate device, such as a router, is necessary. The router must accept the frame based on the physical address and de-encapsulate the frame in order to examine the hierarchical address, or IP address. Using the IP address, the router is able to determine the network location of the destination device and the best path to reach it. Once it knows where to forward the packet, the router then creates a new frame for the packet, and the new frame is sent onto the next segment toward its final destination. Data link layer protocols add a trailer to the end of each frame.
What is Logical Link Control?
The trailer is used to determine if the frame arrived without error. This process is called error detection and is accomplished by placing a logical or mathematical summary of the bits that comprise the frame in the trailer. Error detection is added at the data link layer because the signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent.
A transmitting node creates a logical summary of the contents of the frame. This is known as the cyclic redundancy check CRC value. When the frame arrives at the destination node, the receiving node calculates its own logical summary, or CRC, of the frame. The receiving node compares the two CRC values.
If the two values are the same, the frame is considered to have arrived as transmitted. Therefore, the FCS field is used to determine if errors occurred in the transmission and reception of the frame. The error detection mechanism provided by the use of the FCS field discovers most errors caused on the media. There is always the small possibility that a frame with a good CRC result is actually corrupt.
Errors in bits may cancel each other out when the CRC is calculated.
Upper layer protocols would then be required to detect and correct this data loss. However, the actual Layer 2 protocol used depends on the logical topology of the network and the implementation of the physical layer. Given the wide range of physical media used across the range of topologies in networking, there are a correspondingly high number of Layer 2 protocols in use.
Each protocol performs media access control for specified Layer 2 logical topologies. This means that a number of different network devices can act as nodes that operate at the data link layer when implementing these protocols. These devices include the network adapter or network interface cards NICs on computers as well as the interfaces on routers and Layer 2 switches. The Layer 2 protocol used for a particular network topology is determined by the technology used to implement that topology.
The technology is, in turn, determined by the size of the network - in terms of the number of hosts and the geographic scope - and the services to be provided over the network. A LAN typically uses a high bandwidth technology that is capable of supporting large numbers of hosts. A LAN's relatively small geographic area a single building or a multi-building campus and its high density of users make this technology cost effective. However, using a high bandwidth technology is usually not cost-effective for WANs that cover large geographic areas cities or multiple cities, for example.
The cost of the long distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity. Ethernet is the dominant LAN technology. It is a family of networking technologies that are defined in the IEEE Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.
However, the methods for detecting and placing data on the media vary with different implementations. Shared media requires that the Ethernet frame header use a data link layer address to identify the source and destination nodes. An Ethernet MAC address is 48 bits and is generally represented in hexadecimal format.
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The figure shows the many fields of the Ethernet frame. At the data link layer, the frame structure is nearly identical for all speeds of Ethernet. However, at the physical layer, different versions of Ethernet place the bits onto the media differently.
Ethernet is discussed in more detail in the next chapter. PPP is a protocol used to deliver frames between two nodes. Unlike many data link layer protocols that are defined by electrical engineering organizations, the PPP standard is defined by RFCs. PPP can be used on various physical media, including twisted pair, fiber-optic lines, and satellite transmission, as well as for virtual connections.
PPP uses a layered architecture. To accommodate the different types of media, PPP establishes logical connections, called sessions, between two nodes. These sessions also provide PPP with a method for encapsulating multiple protocols over a point-to-point link. Each protocol encapsulated over the link establishes its own PPP session. This includes authentication, compression, and multilink the use of multiple physical connections. The IEEE However, there are many differences at the MAC sublayer and physical layer.
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In a wireless environment, the environment requires special considerations. There is no definable physical connectivity; therefore, external factors may interfere with data transfer and it is difficult to control access. To meet these challenges, wireless standards have additional controls. The most likely opportunity for medium contention is just after the medium becomes available. Making the nodes back off for a random period greatly reduces the likelihood of a collision.
If the sending station does not detect the acknowledgement frame, either because the original data frame or the acknowledgment was not received intact, the frame is retransmitted. This explicit acknowledgement overcomes interference and other radio-related problems.
Other services supported by Frame Start. Error Detection. Frame Stop. Standard Organization. Networking Standards. Destination Address.
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Source Address. Protocol Type. Frame Check Sequence. Frame Control. These acronyms are defined as:. In contrast to the IEEE When the TransmitEnd method is executed, the channel will model a single uniform signal propagation delay in the medium and deliver copes of the packet to each of the devices attached to the packet via the CsmaNetDevice::Receive method. The state of the channel may be sensed by calling CsmaChannel::GetState.
Properly received packets are forwarded up to higher levels from the CsmaNetDevice via a callback mechanism. The class CsmaChannel models the actual transmission medium. There is no fixed limit for the number of devices connected to the channel. The data rate provided to the channel is used to set the data rates used by the transmitter sections of the CSMA devices connected to the channel.