Реферат: How Ethernet Works Essay Research Paper What
To translate this into Ethernet terms, each interface must wait until there is no signal on the channel, then it can begin transmitting. If some other interface is transmitting there will be a signal on the channel, which is called carrier. All other interfaces must wait until carrier ceases before trying to transmit, and this process is called Carrier Sense.
All Ethernet interfaces are equal in their ability to send frames onto the network. No one gets a higher priority than anyone else, and democracy reigns. This is what is meant by Multiple Access. Since signals take a finite time to travel from one end of an Ethernet system to the other, the first bits of a transmitted frame do not reach all parts of the network simultaneously. Therefore, it’s possible for two interfaces to sense that the network is idle and to start transmitting their frames simultaneously. When this happens, the Ethernet system has a way to sense the “collision” of signals and to stop the transmission and resend the frames. This is called Collision Detect.
The CSMA/CD protocol is designed to provide fair access to the shared channel so that all stations get a chance to use the network. After every packet transmission all stations use the CSMA/CD protocol to determine which station gets to use the Ethernet channel next.
II.Collisions
If more than one station happens to transmit on the Ethernet channel at the same moment, then the signals are said to collide. The stations are notified of this event, and instantly reschedule their transmission using a specially designed backoff algorithm. As part of this algorithm the stations involved each choose a random time interval to schedule the retransmission of the frame, which keeps the stations from making transmission attempts in lock step.
It’s unfortunate that the original Ethernet design used the word “collision” for this aspect of the Ethernet medium access control mechanism. If it had been called something else, such as “stochastic arbitration event (SAE),” then no one would worry about the occurrence of SAEs on an Ethernet. However, “collision” sounds like something bad has happened, leading many people to think that collisions are an indication of network failure.
The truth of the matter is that collisions are absolutely normal and expected events on an Ethernet, and simply indicate that the CSMA/CD protocol is functioning as designed. As more computers are added to a given Ethernet, and as the traffic level increases, more collisions will occur as part of the normal operation of an Ethernet.
The design of the system ensures that the majority of collisions on an Ethernet that is not overloaded will be resolved in microseconds, or millionths of a second. A normal collision does not result in lost data. In the event of a collision the Ethernet interface backs off (waits) for some number of microseconds, and then automatically retransmits the data.
On a network with heavy traffic loads it may happen that there are multiple collisions for a given frame transmission attempt. This is also normal behavior. If repeated collisions occur for a given transmission attempt, then the stations involved begin expanding the set of potential backoff times from which they chose their random retransmission time.
Repeated collisions for a given packet transmission attempt indicate a busy network. The expanding backoff process, formally known as “truncated binary exponential backoff,” is a clever feature of the Ethernet MAC that provides an automatic method for stations to adjust to traffic conditions on the network. Only after 16 consecutive collisions for a given transmission attempt will the interface finally discard the Ethernet packet. This can happen only if the Ethernet channel is overloaded for a fairly long period of time, or is broken in some way.
III.Best Effort Data Delivery
This brings up an interesting point, which is that the Ethernet system, in common with other LAN technologies, operates as a best effort” data delivery system. To keep the complexity and cost of a LAN to a reasonable level, no guarantee of reliable data delivery is made. While the bit error rate of a LAN channel is carefully engineered to produce a system that normally delivers data extremely well, errors can still occur.
A burst of electrical noise may occur somewhere in a cabling system, for example, corrupting the data in a frame and causing it to be dropped. Or a LAN channel may become overloaded for some period of time, which in the case of Ethernet can cause 16 collisions to occur on a transmission attempt, leading to a dropped frame. No matter what technology is used, no LAN system is perfect, which is why higher protocol layers of network software are designed to recover from errors.
It is up to the high-level protocol that is sending data over the network to make sure that the data is correctly received at the destination computer. High-level network protocols can do this by establishing a reliable data transport service using sequence numbers and acknowledgment mechanisms in the packets that they send over the LAN.
IV.Ethernet Frame and Ethernet Addresses
The heart of the Ethernet system is the Ethernet frame, which is used to deliver data between computers. The frame consists of a set of bits organized into several fields. These fields include address fields, a variable size data field that carries from 46 to 1,500 bytes of data, and an error checking field that checks the integrity of the bits in the frame to make sure that the frame has arrived intact.
The first two fields in the frame carry 48-bit addresses, called the destination and source addresses. The IEEE controls the assignment of these addresses by administering a portion of the address field. The IEEE does this by providing 24-bit identifiers called “Organizationally Unique Identifiers” (OUIs), since a unique 24-bit identifier is assigned to each organization that wishes to build Ethernet interfaces. The organization, in turn, creates 48-bit addresses using the assigned OUI as the first 24 bits of the address. This 48-bit address is also known as the physical address, hardware address, or MAC address.
A unique 48-bit address is commonly pre-assigned to each Ethernet interface when it is manufactured, which vastly simplifies the setup and operation of the network. For one thing, pre-assigned addresses keep you from getting involved in administering the addresses for different groups using the network. And if you’ve ever tried to get different work groups at a large site to cooperate and voluntarily obey the same set of rules, you can appreciate what an advantage this can be.
As each Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces look at the first 48-bit field of the frame, which contains the destination address. The interfaces compare the destination address of the frame with their own address. The Ethernet interface with the same address as the destination address in the frame will read in the entire frame and deliver it to the networking software running on that computer. All other network interfaces will stop reading the frame when they discover that the destination address does not match their own address.
V.Multicast and Broadcast Addresses
A multicast address allows a single Ethernet frame to be received by a group of stations. Network software can set a station’s Ethernet interface to listen for specific multicast addresses. This makes it possible for a set of stations to be assigned to a multicast group which has been given a specific multicast address. A single packet sent to the multicast address assigned to that group will then be received by all stations in that group.
There is also the special case of the multicast address known as the broadcast address, which is the 48-bit address of all ones. All Ethernet interfaces that see a frame with this destination address will read the frame in and deliver it to the networking software on the computer.
VI.High-Level Protocols and EthernetAddresses
Computers attached to an Ethernet can send application data to one another using high-level protocol software, such as the TCP/IP protocol suite used on the worldwide Internet. The high-level protocol packets are carried between computers in the data field of Ethernet frames. The system of high-level protocols carrying application data and the Ethernet system are independent entities that cooperate to deliver data between computers.
High-level protocols have their own system of addresses, such as the 32-bit address used in the current version of IP. The high-level IP-based networking software in a given station is aware of its own 32-bit IP address and can read the 48-bit Ethernet address of its network interface, but it doesn’t know what the Ethernet addresses of other stations on the network may be.
To make things work, there needs to be some way to discover the Ethernet addresses of other IP-based stations on the network. For several high-level protocols, including TCP/IP, this is done using yet another high-level protocol called the Address Resolution Protocol (ARP). As an example of how Ethernet and one family of high-level protocols interact, let’s take a quick look at how the ARP protocol functions.
VII.Operation of the ARP Protocol
The operation of ARP is straightforward. Let’s say an IP-based station (station “A”) with IP address 192.0.2.1 wishes to send data over the Ethernet channel to another IP-based station (station “B”) with IP address 192.0.2.2. Station “A” sends a packet to the broadcast address containing an ARP request. The ARP request basically says “Will the station on this Ethernet channel that has the IP address of 192.0.2.2 please tell me what the address of its Ethernet interface is?”
Since the ARP request is sent in a broadcast frame, every Ethernet interface on the network reads it in and hands the ARP request to the networking software running on the station. Only station “B” with IP address 192.0.2.2 will respond, by sending a packet containing the Ethernet address of station “B” back to the requesting station. Now station “A” has an Ethernet address to which it can send data destined for station “B,” and the high-level protocol communication can proceed.