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Chapter: Computer Networks : Data Link Layer

Standard Ethernet

The original Ethernet was created in 1976 at Xerox's Palo Alto Research Center (PARC). Since then, it has gone through four generations: a. Standard Ethernet (l0 Mbps), b. Fast Ethernet (100 Mbps), c. Gigabit Ethernet (l Gbps) d. Ten-Gigabit Ethernet (l0 Gbps)

Standard Ethernet


The original Ethernet was created in 1976 at Xerox's Palo Alto Research Center (PARC). Since then, it has gone through four generations:

a. Standard Ethernet (l0 Mbps),

b. Fast Ethernet (100 Mbps),

c. Gigabit Ethernet (l Gbps)

d. Ten-Gigabit Ethernet (l0 Gbps)


MAC Sublayer


In Standard Ethernet, the MAC sublayer governs the operation of the access method. It also frames data received from the upper layer and passes them to the physical layer.


1. Frame Format


The Ethernet frame contains seven fields: preamble, SFD, DA, SA, length or type of protocol data unit (PDU), upper-layer data, and the CRC. Ethernet does not provide any mechanism for acknowledging received frames, making it what is known as an unreliable medium. Acknowledgments must be implemented at the higher layers.



Preamble: The first field of the 802.3 frame contains 7 bytes (56 bits) of alternating 0sand 1s that alerts the receiving system to the coming frame and enables it to synchronize its input timing. The pattern provides only an alert and a timing pulse.


Start frame delimiter (SFD). The second field (l byte: 10101011) signals the beginningof the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address.


Destination address (DA). The DA field is 6 bytes and contains the physical address ofthe destination station or stations to receive the packet.


Source address (SA). The SA field is also 6 bytes and contains the physical address ofthe sender of the packet.


Length or type. This field is defined as a type field or length field. The original Ethernetused this field as the type field to define the upper-layer protocol using the MAC frame.


Data. This field carries data encapsulated from the upper-layer protocols. It is aminimum of 46 and a maximum of 1500 bytes.


CRC. The last field contains error detection information, in this case a CRC-32.


2. Frame Length


Ethernet has imposed restrictions on both the minimum and maximum lengths of a frame. An Ethernet frame needs to have a minimum length of 512 bits or 64 bytes. Part of this length is the header and the trailer. If we count 18 bytes of header and trailer, then the minimum length of data from the upper layer is 64 - 18 = 46 bytes. If the upper-layer packet is less than 46 bytes, padding is added to make up the difference.


The standard defines the maximum length of a frame as 1518 bytes. If we subtract the 18 bytes of header and trailer, the maximum length of the payload is 1500 bytes. The maximum length restriction has two historical reasons.



3. Addressing


Each station on an Ethernet network has its own network interface card (NIC). The NIC fits inside the station and provides the station with a 6-byte physical address. The Ethernet address is 6 bytes (48 bits), nominally written in hexadecimal notation, with a colon between the bytes.



Unicast, Multicast, and Broadcast Addresses:


A source address is always a unicast address-the frame comes from only one station. The destination address, however, can be unicast, multicas

t, or broadcast. If the least significant bit of the first byte in a destination address is 0, the address is unicast; otherwise, it is multicast. The broadcast address is a special case of the multicast address; the recipients are all the stations on the LAN. A broadcast destination address is forty-eight 1s.


Example 2.13

Define the type of the following destination addresses:

a. 4A:30:10:21:1O:1A

b. 47:20:1B:2E:08:EE




To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F's, the address is broadcast. Therefore, we have the following:

·   This is a unicast address because A in binary is 1010 (even).

·    This is a multicast address because 7 in binary is 0111 (odd).

·           This is a broadcast address because all digits are F's

Example 13.2

Show how the address 47:20:1B:2E:08:EE is sent out on line.




The address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below:


4. Physical layer

The Standard Ethernet defines several physical layer implementations; four of the most Common.

Encoding and Decoding


All standard implementations use digital signaling (baseband) at 10 Mbps. At the sender, data are converted to a digital signal using the Manchester scheme; at the receiver, the received signal is interpreted as Manchester and decoded into data. Manchester encoding is self-synchronous, providing a transition at each bit interval. Figure 2.60 shows the encoding scheme for Standard Ethernet

a. 10Base5: Thick Ethernet

The first implementation is called 10Base5, thick Ethernet, or Thicknet. The nickname derives from the size of the cable, which is roughly the size of a garden hose and too stiff to bend with your hands. 10Base5 was the first Ethernet specification to use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable

b. 10Base2: Thin Ethernet


The second implementation is called 10Base2, thin Ethernet, or Cheapernet. 10Base2 also uses a bus topology, but the cable is much thinner and more flexible. The cable can be bent to pass very close to the stations. In this case, the transceiver is normally part of the network interface card (NIC), which is installed inside the station.


c. 10Base-T: Twisted-Pair Ethernet


The third implementation is called 10Base-T or twisted-pair Ethernet. 10Base-T uses a physical star topology. The stations are connected to a hub via two pairs of twisted cable.


Note that two pairs of twisted cable create two paths between the station and the hub. Any collision here happens in the hub. Compared to 10Base5 or 10Base2, we can see that the hub actually replaces the coaxial cable as far as a collision is concerned. The maximum length of the twisted cable here is defined as 100 m, to minimize the effect of attenuation in the twisted cable.

d. 10Base-F: Fiber Ethernet


Although there are several types of optical fiber 10-Mbps Ethernet, the most common is called 10Base-F. 10Base-F uses a star topology to connect stations to a hub. The stations are connected to the hub using two fiber-optic cables


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