Ethernet

  1. Bridged Ethernet
  2. Switched Ethernet
  3. Fast Ethernet
  4. Gigabit Ethernet

IEEE Project 802 (ETHERNET) has created a sublayer called Media Access Control (MAC) that defines the specific access method for each LAN.

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.

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,

Preamble. The first field contains 7 bytes (56 bits) of alternating Os and 1 s that alerts the receiving system to the coming frame and enables it to synchronize its input timing.

The preamble is actually added at the physical layer and is not (formally)part of the frame.

Start frame delimiter (SFD). The second field (1 byte: 10101011) signals the

beginning of 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 of the destination station or stations to receive the packet.

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

address of the sender of the packet.

Length or type. This field is defined as a type field or length field. The IEEE standard used it as the length field to define the number of bytes in the data field.

Data. This field carries data encapsulated from the upper-layer protocols. It is a

minimum of 46 and a maximum of 1500 bytes.

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

Bridged Ethernet:

The first step in the Ethernet evolution was the division of a LAN by bridges. Bridges have two effects on an Ethernet LAN:

They raise the bandwidth and they separate collision domains.

Raising the Bandwidth

In an unbridged Ethernet network, the total capacity (10 Mbps) is shared among all sta-

tions. The problem is if all the system want to transmit the data at same time means the speed of the N/W will be decreased.

A bridge divides the network into two or more networks. Bandwidth-wise, each network is independent. For example, a network with 12 stations is divided into two networks, each with 6 stations.

Now each network has a capacity of 10 Mbps. The 10-Mbps capacity in each segment or subnet is now shared between 6 stations (actually 7 because the bridge acts as a station in each segment), not 12 stations.

In a network with a heavy load, each station theoretically is offered 10/6 Mbps instead of 10/12 Mbps, assuming that the traffic is not going through the bridge.

if we further divide the network, we can gain more bandwidth for each segment. For ex, if we use a four-port bridge, each station is now offered 10/3 Mbps, which is 4 times more than an unbridged network.

Separating Collision Domains

Another advantage of a bridge is the separation of the collision domain. Figure shows the collision domains for an unbridged and a bridged network. You can see that the collision domain becomes much smaller and the probability of collision is reduced tremendously. Without bridging, 12 stations contend for access to the medium; with bridging only 3 stations contend for access to the medium.

Switched Ethernet

The idea of a bridged LAN can be extended to a switched LAN. Instead of having two

to four networks, why not have N networks, where N is the number of stations on the

LAN? In other words, if we can have a multiple-port bridge, why not have an N-port

Full-Duplex Ethernet

One of the limitations of 10Base5 and 10Base2 is that communication is half-duplex

a station can either send or receive, but may not do both at the same time. (10Base-T is always full-duplex);

The next step in the evolution was to move from switched Ethernet to full-duplex switched Ethernet. The full-duplex mode increases the capacity of each domain from 10 to 20 Mbps.

Below figure shows a switched Ethernet in full-duplex mode. Note that instead of using one link between the station and the switch, the configuration uses two links: one to transmit and one to receive.

FAST ETHERNET / IEEE 802.3u

Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u.

The goals of Fast Ethernet can be summarized as follows:

1. Upgrade the data rate to 100 Mbps.

2. Make it compatible with Standard Ethernet.

3. Keep the same 48-bit address.

4. Keep the same frame format.

5. Keep the same minimum and maximum frame lengths.

GIGABIT ETHERNET / IEEE 802.3z

The goals of the Gigabit Ethernet design can be summarized as follows:

1. Upgrade the data rate to 1 Gbps.

2. Make it compatible with Standard or Fast Ethernet.

3. Use the same 48-bit address.

4. Use the same frame format.

5. Keep the same minimum and maximum frame lengths.

6. To support autonegotiation as defined in Fast Ethernet.

Ten-Gigabit Ethernet / IEEE 802.3ae

The IEEE committee created Ten-Gigabit Ethernet and called it Standard 802.3ae.

The goals of the Ten-Gigabit Ethernet design can be summarized as follows:

1. Upgrade the data rate to 10 Gbps.

2. Make it compatible with Standard, Fast, and Gigabit Ethernet.

3. Use the same 48-bit address.

4. Use the same frame format.

5. Keep the same minimum and maximum frame lengths.

6. Allow the interconnection of existing LANs into a metropolitan area network (MAN)

or a wide area network (WAN).

7. Make Ethernet compatible with technologies such as Frame Relay and ATM (see

Chapter 18).

Token Ring / IEEE 802.5

A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time.

The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. The data transfer rate is 4 Mbps to 16 Mbps.

  1. Empty information frames are continuously circulated on the ring.
  2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
  3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
  4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
  5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.

The token scheme can also be used with bus topology LANs. FDDI also uses Token ring Protocol

FDDI

FDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km (124 miles).

The FDDI protocol is based on the Token Ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN).

An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).

FDDI is a product of American National Standards Committee X3-T9

As described above FDDI networks implements a recovery mechanism which enable the network to function properly even under a broken ring. FDDI uses two rings to achieve recovery capabilities. As shown a token is passed simultaneously on the network's inner and outer rings which backup each other. / As shown in the following figure in case of broken connection or station malfunction, the closest station closes the network loop by sending the token it received from the outer/inner ring back using the inner/outer ring. This feature is called Self healing.
Token Ring Frame Formats (tokens and data/command)

Tokens consist of:

1. Start delimiter - which alerts the stations of a token arrival (or data/command frame).

2. Access control byte - which contains the priority and reservation fields, a token bit to differentiate token from data/command frame and a monitor bit checking whether a frame is circling the ring endlessly.

3. After the Access control byte a frame control byte arrives and indicates whether it is a data or control information (and which) frame.

4. Then arrives two address fields (source & destination) each 6 bytes long.

5. Data follows these fields. Data/Command Frames carry information for upper-layer

protocols.

6. FCS (frame check sequence) Used to check the sequence/order of frames.

7. End delimiter - which signals the end of a frame, end of a logical sequence and . . . . damaged frames.

Comparison of basic characteristics

Technology
/
Data Rate (Mbps)
/
Maximum Segment Length (m)
/
Media
/
Rings
/
Recovery
IBM Token Ring / 4/16 / 250 Shielded
72 Unshielded / Twisted pair / 1 / Can handle a computer failure but can’t recover from a broken connection.
IEEE 802.5 / 4/16 / 250 / Not specified / 1 / Can handle a computer failure but can’t recover from a broken connection.
CDDI / 4/16 / 250 Shielded
72 Unshielded / Twisted pair / 2 / Can recover from a broken connection (Self healing).
FDDI / 100 / Unlimited / Optical fiber / 2 / Can recover from a broken connection (Self healing).

Since then, it has gone through four generations: Standard Ethernet (10 t Mbps), Fast

Ethernet (100 Mbps), Gigabit Ethernet (1 Gbps), and Ten-Gigabit Ethernet (10 Gbps)

Manchester Encoding

None of the versions of Ethernet uses straight binary encoding with 0 volts for a 0 bit and 5 volts for a 1 bit because it leads to ambiguities.

If one station sends the bit string 0001000, others might falsely interpret it as 10000000 or 01000000 because they cannot tell thedifference between an idle sender (0 volts) and a 0 bit (0 volts). This problem can be solved by using +1 volts for a 1 and -1 volts for a 0,but there is still the problem of a receiver sampling the signal at a slightly different frequency than the sender used to generate it.

Differentclock speeds can cause the receiver and sender to get out of synchronization about where the bit boundaries are, especially after a longrun of consecutive 0s or a long run of consecutive 1s.

What is needed is a way for receivers to unambiguously determine the start, end, or middle of each bit without reference to an externalclock.

Two such approaches are called Manchester encoding and differential Manchester encoding. With Manchester encoding, eachbit period is divided into two equal intervals. A binary 1 bit is sent by having the voltage set high during the first interval and low in thesecond one. A binary 0 is just the reverse: first low and then high. This scheme ensures that every bit period has a transition in the middle,making it easy for the receiver to synchronize with the sender. A disadvantage of Manchester encoding is that it requires twice as muchbandwidth as straight binary encoding because the pulses are half the width. For example, to send data at 10 Mbps, the signal has tochange 20 million times/sec. Manchester encoding is shown in Fig. 4-16(b).

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MC9231 Computer Networks Unit – II

Ethernet, Fast Ethernet, Token Ring, FDDI