# 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|COSC264

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Carrier Sense Multiple Access

The MAC protocol that non-switched variants of Ethernet use is carrier sense multiple access/collision detection (CSMA/CD). In the original form of Ethernet, all the computers were attached to a bus (a piece of coaxial cable which acted as a common highway for data transmission). Only one conversation between two network stations at a time was possible and a protocol such as CSMA/CD was needed to allow a computer access to the bus. When CSMA/CD is in use, a station that wishes to transmit listens to the bus. If there seems to be no activity, the station transmits (carrier sense). Multiple access means that all stations have access to the network medium (the cable). Once a station starts transmitting. all other stations will almost immediately detect the transmission and will wait until it has finished before trying to send anything themselves. IIowever, it is still possible that two stations will both detect that the bus is idle, and that both start to transmit at about the same time. Then there will be a collision of data. The reason why this can happen is that any signal takes some time to propagate along the bus. Collision detection is needed to deal with this problem.

If a station detects a collision while it is transmitting, it sends a brief jamming signal. This signal lets the other stations know that there has been a collision. After sending the jamming signal, the station ceases transmission and then waits for a random time period. When this period is up, the station attempts to transmit again. If there are repeated collisions, this indicates a busy medium. To adjust for this, the time delay between repeated retransmission attempts is progressively increased. This is called the binary exponential backoff algorithm. If there are sixteen unsuccessful attempts to transmit (a very rare occurrence), the frame transmission is abandoned and the upper layer is informed of this.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Ethernet Frame Format

In both types of frame, the next two fields are for the destination (receiving station) and source (sending station) addresses. Both these addresses are 48 bits long and are usually shown as 12 hexadecimal digits. Every Ethernet card in the world has a unique MAC address. The first six hex digits indicate the manufacturer of the card; the second six are a unique identifier. For example, a certain Ethernet $\mathrm{NIC}$ has the following MAC address: 00-02-44-37-60-FA. The 00-02-44 part of the number identifies the manufacturer; 37-60-FA is the unique identifier.

The purpose of the next (two-byte) field differs in the two types of frame. In Ethernet II, the receiving station has to find out which higher-layer protocol is being carried in an incoming frame. It needs to know this in order to know to which upper-layer protocol it must give the data. It finds this out by looking inside the Type field. In IEEE 802.3, this field can be used as a Type field, but alternatively can be used to carry the length of the data in bytes. There is no need to use this field to identify the protocol if the LLC field (missing from Ethernet II) is being used to do this. If the number is equal to or greater than 600 hexadecimal (1536 decimal), then it is taken to indicate the length.

The whole point of sending an Ethernet frame is to carry some data. The Data field is the place where the data is put. The greatest size of a frame that is allowed in low-speed versions of Ethernet is 1518 bytes; the minimum size is 46 bytes. If the frame would otherwise be below the minimum size, it is padded out with extra bytes to make it legal. The IEEE $802.3$ frame also carries the LLC information within the Data field. The $1000 \mathrm{Base}-\mathrm{T}$ Gigabit Ethernet standard permits frames larger than 1518 bytes. Up to 9 kbytes can be carried in one frame-a so-called jumbo frame.

Finally, in the Frame Check Sequence (FCS) field, there is a 32 bits CRC code to check for errors. This checks the integrity of the whole frame except the Preamble/Start Frame Delimiter and of course the CRC field itself. (CRCs were explained in Section 2.6.1.) Any frame with an invalid CRC is simply thrown away without being processed any further because it is useless.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Ethernet Frame Format

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