Auto-negotiation

MAC Address Format

OUI is the number assigned by the IEEE to vendors such as Alcatel- Lucent

OUI examples: Alcatel-Lucent Canada 00-80-21 and 00-D0-F6, Alcatel-Lucent USA 00-17-CC, Alcatel-Lucent Italia 00-20-60

Unicast Addressing

Unique source and destination MAC addresses

Frame is meant for one particular destination or host

Ethernet II, Src: 138.120.100.2 (00:e0:b1:88:0d:c0), Dst: Dell_c5:79:87 (00:14:22:c5:79:87)‏

Type: IP (0x0800)‏ Trailer: 000000000000

Internet Protocol, Src: 138.120.252.84 (138.120.252.84), Dst: 138.120.132.135 (138.120.132.135)‏

Transmission Control Protocol, Src Port: 8080 (8080), Dst Port: 2730 (2730), Seq: 0, Ack: 3811441139, Len: 0

00:e0:b1:88:0d:c0 00:14:22:c5:79:87

Output

In this slide, an Ethernet frame is composed by the source with the following source and destination addresses:

Src : 00:e0:b1:88:0d:c0

Dest : Dell_c5:79:87 (00:14:22:c5:79:87)

The frame is sent to a hub that connects all devices on a 4-node LAN. The hub being a simple replicator, sends the frame out on all its ports except the port where the frame was received (the port attached to the source). Although all devices receive the frame, only the device whose MAC address matches the destination device accepts the frame.

Broadcast Addressing

Unique source MAC address only, destination address is broadcast

(ff-ff-ff-ff-ff-ff)

Frame is meant for all devices on the LAN in a broadcast domain

Frame 1 (42 bytes on wire, 42 bytes captured)‏

Ethernet II, Src: 192.168.0.101 (00:13:ce:2b:6b:28), Dst: Broadcast (ff:ff:ff:ff:ff:ff)‏

Destination: Broadcast (ff:ff:ff:ff:ff:ff)‏ Source: 192.168.0.101 (00:13:ce:2b:6b:28)‏ Type: ARP (0x0806)‏

Address Resolution Protocol (request)‏ Output

00:13:ce:2b:6b:28

In this slide, an Ethernet frame is composed by the source with the following source and destination addresses:

Src : 00:13:ce:2b:6b:28

Dest : ff:ff:ff:ff:ff:ff

The frame is sent to a hub that connects all devices on a 4-node LAN. The hub being a simple replicator, sends the frame out on all its ports except the port where the frame was received (the port attached to the source). All devices recognize that the destination address (ff-ff-ff-ff-ff-ff) is a special address and process the frame. The output sample shows the use of an Ethernet frame destined for a broadcast address.

Multicast Addressing

Unique source MAC address only, destination address is multicast group

(01-00-5e-01-01-01)

Frame is meant for only devices who are members of that group

Ethernet II, Src: 192.168.0.101 (00:13:ce:2b:6b:28), Dst: 01:00:5e:01:01:01 (01:00:5e:01:01:01)‏ Destination: 01:00:5e:01:01:01 (01:00:5e:01:01:01)‏

Source: 192.168.0.101 (00:13:ce:2b:6b:28)‏ Type: IP (0x0800)‏

Internet Protocol, Src: 192.168.0.101 (192.168.0.101), Dst: 239.1.1.1 (239.1.1.1)‏ Internet Control Message Protocol

00:13:ce:2b:6b:28 01:00:5e:01:01:01 01:00:5e:01:01:01

Output

In this slide, an Ethernet frame is composed by the source with the following source and destination addresses:

Src : 00:13:ce:2b:6b:28

Dest : 01-00-5e-01-01-01

The frame is sent to a hub that connects all devices on a 4-node LAN. The hub being a simple replicator, sends the frame out on all its ports except the port where the frame was received (the port attached to the source). All devices that are members of the particular group (239.1.1.1) process that message.

Ethernet Transmission

Half-duplex transmission

Data sent in one direction at a time

Results in collisions

Uses CSMA/CD to resolve collisions

Hubs are the most common half- duplex devices

Full-duplex transmission

Data sent in both directions at the same time

Requires point-to-point connections

No collisions

An approach to higher network efficiency

Switches are the most common full- duplex devices

Half-duplex transmission is the traditional means of transporting Ethernet frames. Because data is transmitted in one direction at a time over a shared medium, such as a hub, collisions are possible. The CSMA/CD algorithm is used to handle collisions. A hub uses shared media and supports half-duplex only. 10Base-T, which works on half-duplex, is efficient 30 to 40% of the time because of collisions, and as such the effective throughput is only 3 to 4 Mb/s.

Full-duplex transmission has data forwarding in both directions simultaneously. Full-duplex implementations require a point-to-point connection between the sender and the receiver port. Therefore, a switch with 8 ports would have each of the 8 ports connected to the rest of the ports through a dedicated set of wires. This ensures that there is no shared medium and collision is not possible. Because data can be transmitted bi- directionally, the effective rate of a 10-Mb/s full-duplex transmission is 20 Mb/s (that is, 10 Mb/s each way). Therefore, full-duplex transmissions are more efficient than half-duplex. Switches and routers usually support full-duplex transmissions.

When devices such as switches and hubs are interconnected, care must be taken to ensure that the proper transmission parameters are set on the ports. For switch-to-hub connections, the switch port must be set to half-duplex because the hub only supports half-duplex. For switch-to-switch, switch-to-host, or switch-to- router connections, full-duplex can be used.

Half-Duplex Operation (CSMA/CD)

All hosts constantly listen to the line

Host A transmits

Hosts B, C, and D listen to Host A and do not transmit

All hosts receive Host A’s message

Hub

Host A Host B Host C Host D

The CSMA/CD access rules are summarized by the protocol acronym.

Carrier sense (CS)— Each Ethernet LAN-attached host continuously listens for traffic on the medium to determine when gaps between frame transmissions occur.

Multiple access (MA)— LAN-attached hosts can begin transmitting any time that they detect that the network is quiet, meaning that no traffic is travelling across the wire.

Collision detect (CD)— If two or more LAN-attached hosts in the same CSMA/CD network or collision domain begin transmitting at approximately the same time, the bit streams from the transmitting hosts will interfere (collide) with each other, and both transmissions will be unreadable. If that happens, each transmitting host must be capable of detecting that a collision has occurred before it has finished sending its respective frame. Each host must stop transmitting as soon as it has detected the collision and must wait a random length of time as determined by a back-off algorithm before attempting to retransmit the frame. In this event, each transmitting host transmits a 32-bit jam signal alerting all LAN-attached hosts of a collision before running the back-off algorithm.

The CSMA/CD reduces the chance of collisions but does not prevent them. Both hosts A and B could decide to transmit at once because no other hosts are transmitting a message on the line (idle line).

Half-Duplex Operation (CSMA/CD) (continued)

All hosts constantly listen to the line

Host A and Host B transmit simultaneously

Messages collide

Both hosts back off for a random time interval

Hub

Host A Host B Host C Host D

When host A and host B transmit frames at the same time, they both detect collisions and corruption of the data.

Both host A and host B generate a jam signal, which is received by other hosts so that they discard the data that was just corrupted by the collision.

A random back-off timer is then started on the transmitting hosts. Depending on whose timer expires first, either host A or host B transmits if they detect no other transmission on the line.

Full-duplex Operation

Point-to-point only

Attached to a dedicated switched port

Requires full-duplex support on both ends

Collision-free

Full-duplex operation is an optional MAC layer capability that allows simultaneous two-way transmission over point-to-point links.

Full-duplex transmission involves no media contention, no collisions, and no need to schedule retransmissions. There are exactly two hosts connected on a full-duplex point-to-point link.

The link bandwidth is effectively doubled because each link can now support full-rate, simultaneous, two-way transmission.

Ethernet auto-negotiable operation

In document Alcatel-Lucent Scalable IP Networks Student Guide v2.01(1) (Page 151-160)