Wired LANs: Ethernet

Shashank SrivastavaMotilal Nehru national Institute Of Information Technology, Allahabad

4 Sep 2013

IEEE STANDARDS

In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipments from a variety of manufacturers.

Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.

IEEE has subdivided the data link layer into two sublayers: logical link control(LLC) and media access control(MAC).

IEEE has also created several physical layer standards for different LAN protocols.

IEEE standard for LANs

Logical Link Control (LLC)

LLC provides one single data link control protocol for all IEEE LANs.

LLC provides flow and error control for the upper layer protocols.

Medium Access Control (MAC)

Specifies access method and framing format specific to corresponding LAN protocol.

Example: CSMA/CD method for Ethernet LANs.and token passing method for Token Ring and Token Bus LANs.

STANDARD ETHERNET

It has gone through four generations.

802.3 MAC frame

MAC sublayer

Preamble – It is 7 bytes field of alternating 0s and 1s that alert 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. •The 56 bit pattern allows the stations to miss some bits at the beginning of the frame. •The preamble is added at the physical layer and is not a part of frame.

Start Frame Delimiter (SFD) – The second field (10101011) signals the beginning of the frame. •SFD warns the station that this is last chance for synchronization. •The last two bits is 11 and alerts the receiver that the next field is the destination address.

DA- Destination physical address. 6 bytes.

SA- Source physical address. 6 bytes.

Length or type – The original Ethernet used this field as the type field to define the upper layer protocol using MAC frame. •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 – This field contains error detection information, in this case a CRC-32.

Minimum and maximum lengths

Frame length:Minimum: 64 bytes (512 bits)

Maximum: 1518 bytes (12,144 bits)

The minimum length restriction is required for correct operation of CSMA/CD.

Removing header and trailer, the minimum length of data from upper layer is 64-18=46 bytes.

If upper layer data is less than 46 bytes, padding is added to make up the difference.

Maximum length restriction prevents one station from monopolizing the shared medium, blocking other station that have data to send.

Addressing:

Ethernet address is 6 bytes(48 bits), normally 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.

Destination address can be unicast, multicast or broadcast.

Unicast and multicast addresses

The least significant bit of the first byte defines the type of address.

If the bit is 0, the address is unicast;otherwise, it is multicast.

The broadcast destination address is a special case of the multicast address in

which all bits are 1s.

Define the type of the following destination addresses:a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EEc. FF:FF:FF:FF:FF:FF

SolutionTo 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:a. This is a unicast address because A in binary is 1010.b. This is a multicast address because 7 in binary is 0111.c. This is a broadcast address because all digits are F’s.

Example

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

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

Example

Access method : CSMA/CDStandard Ethernet uses 1-persistent CSMA/CD.

Slot Time:slot time = round trip time + time required to send the jam sequence

Slot time in Ethernet is defined in bits. It is the time required for a station to send 512 bits. Actual slot time depends on data rate; for traditional 10-Mbps Ethernet, it is 51.2 us.

Slot time and maximum network length:

For traditional Ethernet, we calculateMax Length = Propagation speed x (slot time/2) = 2x108 x (51.2 x10-6 /2) = 5120 m

The delay times in repeaters and interfaces and the time required to send the jam sequence is also considered. These reduces the maximum length of traditional Ethernet network to 2500 m (just 48 percent of theoretical calculation).

Physical Layer – The standard Ethernet defines several physical layer implementations.

Categories of Standard Ethernet

Encoding in a Standard Ethernet implementation

10Base5 Thick Ethernet-

•It is called 10Base5, thick Ethernet, or Thicknet.

•First Ethernet specification to use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable.

•The transceiver is responsible for transmitting, receiving and detecting collisions.

•Transceiver is connected to the station via a transceiver cable that provides separate paths for sending and receiving. This means that collision can only happen in the coaxial cable.

10Base2: Thin Ethernet or Cheapernet

•Cable is much thinner and more flexible.

•Transceiver is normally part of the network interface card (NIC), which is installed inside the station.

•More cost effective because thin coaxial cable is less expensive than thick coaxial, and tee connections are much cheaper than taps.

10Base-T: Twisted pair Ethernet

It uses star topology.

Stations are connected to a hub via two pairs of twisted cable.Two pairs of twisted cable create two paths (one for sending and one for receiving).

Any collision here happens in the hub

The maximum length of cable is defined as 100 m, to minimize the effect of attenuation in the twisted cable.

10Base-F: Fiber Ethernet

It uses star topology.

Stations are connected to a hub via two fiber optics cable.

Collision happens in the hub

Summary of Standard Ethernet implementations

CHANGES IN THE STANDARD

The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data rates.

These changes actually opened the road to the evolution of the Ethernet to become compatible with other high-data-rate LANs.

Bridged Ethernet

Sharing bandwidth

Raising the bandwidth:

A bridge divides the network into two or more networks.

Bandwidth-wise, each network is independent.

Suppose there are 12 stations. And bandwidth is 10 Mbps.

If we divide the network into 2 networks using bridge, each network has a capacity of 10 Mbps.

The 10 Mbps capacity is shared between 7 stations, 6+1(bridge acts as a station in each segment), not 12 stations.

A network with and without a bridge

Separating collision domains:

Collisions domains become much smaller and possibility of collision is reduced.

With bridging, lesser number of channels compete for access to the medium.

Collision domains in an unbridged network and a bridged network

Switched Ethernet:

A layer 2 switch is an N-port bridge with additional sophistication that allows faster handling of packets.

Collision domain is divided into N domains.

Switched Ethernet

Full-duplex switched Ethernet

No need of CSMA/CD in a full-duplex switched Ethernet:

In full duplex switched network, there is no need for CSMA/CD method.

In full duplex switching network, each station is connected to the switch via two separate links.

Each link is point to point dedicated path between the station and the switch.

So there is no longer a need for carrier sensing; there is no need for collision detection

So, the carrier sensing and collision detection functionalities of the MAC sublayer can be turned off.

FAST ETHERNET

Fast Ethernet was designed to compete with LAN protocols such as FDDI (fiber distributed data interface) or Fiber Channel.

IEEE created Fast Ethernet under the name 802.3u.

Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps.

Goals of Fast Ethernet –

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.

MAC sublayer –

1. MAC sublayer was untouched. But the topology is star only.

2. There are two options for star topology- half duplex and full duplex.

3. Hub is used for half duplex and switch is used for full duplex.

4. CSMCA/CD is access method.

Auto negotiation:

New feature added to Fast Ethernet.

Allows two devices to negotiate the mode or data rate of operation.

Fast Ethernet topology

Fast Ethernet implementations

GIGABIT ETHERNET

The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps).

The IEEE committee calls the standard 802.3z.

Gigabit Ethernet access methods include half-duplex mode using traditional CSMA/CD (not common) and full-duplex mode (most popular method).

Goals of Gigabit ethernet –

1. Upgrade the data rate to 1 Gbps.

2. Make it compatible with standard or Fast Ethernet

3. Keep the same 48 bit address

4. Keep the same frame format

5. Keep the same minimum and maximum frame lengths.

6. To support autonegotiation as defined in Fast Ethernet

In the full-duplex mode of Gigabit Ethernet, there is no collision;

the maximum length of the cable is determined by the signal attenuation

in the cable.

Topologies of Gigabit Ethernet

Gigabit Ethernet implementations

Summary of Gigabit Ethernet implementations

Ten-GIGABIT ETHERNET

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

MAC Sublayer:Ten-Gigabit Ethernet operates only in full duplex mode, means no need for CSMA/CD.

Physical Layer:Physical layer is designed for using fiber-optic cable over long distances. Three implementations are used: 10GBase-S, 10GBase-L, and 10GBase-E.

Goals of Ten Gigabit Ethernet –

1. Upgrade the data rate to 10 Gbps.

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

3. Keep the same 48 bit address

4. Keep 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 or a wide area network.

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

Summary of Ten-Gigabit Ethernet implementations