In today lesson, we will look at the application of CSMA with collision detection in Local Area Networks. Local area, means private ownership. The short distance between computers usually below one kilometers imply, low cost, high speed and relatively error free communication. Therefore, complex error control is usually unnecessary as machines are constantly moved in a LAN, each machine is giving a unique MAC address. Within a local area network, routing is just broadcasting all messages to all machines in the LAN. So a medium access control protocol is required. The computers and network devices are connected to the cable systems through a Network Interface Card, NIC. The NIC or coordinates the transfer of information between the computers under the network. In IEEE 802.1 standard, the data link layer is divided into two sublayers. The Medium Access Control, MAC sublayer and the Logical Link Control, LLC sublayer. The IEEE defines several MAC standards including IEEE 802.3 ethernet and IEEE 802.5 token Ring. Each MAC standard has an associated set of physical layers over which it can operate. The MAC sublayer provides for the connectionless transfer of datagrams. MAC protocols usually don't include a procedure for error control. The LLC sublayer enhances service provided by MAC sublayer for reliable frame service. The LLC sublayer builds on the MACs datagram service to provide a three HDLC services: unacknowledged connectionless service, reliable connection-oriented service and acknowledged connectionless service. The LLC protocol data unit is encapsulated in MAC frames. The MAC layer add both a header and a trailer. Please note the accumulation of header overhead, of the TCP and IP have added their minimum of 20 bytes of headers. The LLC adds three or four bytes and the MAC sublayer then adds its header and the trailer. The ethernet, Local Area Network protocol, was developed in the 1970s. It has been the dominant Local Area Network standard. The original IEEE 802.3 standard was defined for a bus based coaxial cable not Local Area Network, in which terminal transmissions are broadcast over the bus medium using CSMA with collision detection. The critical system parameter in the CSMA was collision detection, is a mini slot that forms the basis for the contention resolution, which is required of a station to seize control of the channel. It is at least as big as two partition delays, or launchable partition. The IEEE standard also specifies that the rescheduling of retransmission attempts of a collision use a truncated binary exponential backoff algorithm. If a frame is about to undergo it's n_th retransmission attempt, it's retransmission time is determined by selecting an integer number equally likely in the range between zero and 2 powers k minus one. Where k is the minimum of n and 10. The increase of retransmission range after each equation is intended to increase the likelihood that retransmission will succeed. Note that up to 16 retransmissions will be attempt, after that, the system gives up. Another key parameter in the original IEEE 802.3 standard, is a minimum frame size. For collision detection, all frames must take more than one wrong chip of propagation to send a field that has a transmission is still taking place when the noise burst gets back to the sender. And ethernet has a maximum length, about a twenty five hundred meters with four repeaters. So launching a propagation delay, is about 50 microseconds. In a 10 megabits per second of ethernet, that means the frame has to be at least 500 bits long. Therefore, the minimal frame size is defined as 64 bytes, that is a 512 bits. Noted that to accommodate a 10 times increase in narrower band width, it a must accompanied by either 10 times decrease in distance or 10 times increase in the minimum frame size. Now, let's look at the frame structure. Each frame transmission begins from scratch. Preamble helps receivers synchronize their clocks to transmitter clock. The destination and source addresses follow. What if a service data unit is smaller than 64 bytes? Padding is used. There is also maximum frame length limit, which is a 1500 bytes for information field. CSMA with collision detection maximum throughput depends on the normalised delay bandwidth product alpha. That is a ratio of the propagation delay over the frame transmission time. To keep alpha constant, we needed to ease or decrease the propagation delay or increase frame length size. Fast ethernet, operates at 100 megabits per second. To preserve compatibility with 10 megabits per second of ethernet, the frame format, interface and procedures have been kept the same. In gigabit ethernet, the minimum frame size is increased to 512 bytes as the minute slot time increases. Note that a frame structure preserved, but a CSMA with a collision detection, essentially abandoned and operated primarily in our switch model. Today, 10 gigabit ethernet are extensively deployed in metro networks and data centers.
