Ethernet

Introduction

Ethernet History

Broadcast Network Operation

Ethernet Protocol

Ethernet at 10Mbps

Ethernet at 100Mbps

Ethernet at 1000Mbps and above

Ethernet at 100 Mbps

10 Mbps was soon not enough bandwidth for many networks and so in 1992 the IEEE started defining a standard for a faster LAN.

Challenges

Instead of creating a completely new protocol, the IEEE decided to keep all the old packet formats, interfaces and procedural rules and simply reduce the bit time from 100 nsec to 10 nsec. This effectively increased the bandwidth to 100 Mbps. The protocol was officially known as 802.3u, but was more commonly called Fast Ethernet.

The change in bit time presented a number of challenges to the designers of Fast Ethernet. The reduction in bit time increases the number of bits sent within a set time period, however these bits still take the same amount of time to travel across a length of wire (i.e. the propagation delay is the same over a given length). This means that more bits will be send during the 2 * Propagation Delay used as the collision window (see Broadcast Network Operation). Either the minimum frame size needs to be increased, or the propagation delay (i.e. cable length) needs to be reduced. Changing the minimum frame size would have caused problems with the backward compatability of this standard, so the maximum network size was reduced, with typically maximum cable lengths of 100m.

Media Types

• 100Base-T4 - This uses category 3 twisted pairs, which is normally already present in offices for the telephone network. These wires can handle clock rates up to 25 MHz, so to achieve 100 Mhz four twisted pairs are required. Of these four twisted pairs, one is always to the hub, one is always from the hub, and other two are switchable to the current transmission direction.
• 100Base-TX - This uses category 5 twisted pairs, which are more expensive than category 3 cables. In this scheme the design is easier, because these wires can handle clock rates up to 125 MHz and beyond. Only two twisted pairs per station are used, one to the hub and one from it. This scheme allows full duplex communication. Stations can transmit at 100 Mbps and receive at 100 Mbps at the same time.
• 100Base-FX - This uses two strands of multimode fibre, one for each direction, so it, too, is full duplex with 100 Mbps in each direction. In addition, the distance between a station and a hub can be up to 2 km.
  

Changes in The Ethernet Stack

The various layers of the Fast Ethernet protocol architecture are shown in below. The MII is the interface between the MAC layer and the Physical layer. It allows any physical layer to be used with the MAC layer. The MII provides 2 media status signals, one indicates presence of the carrier, and the other indicates absence of collision. The Reconciliation Sublayer maps these signals to Physical Signaling (PLS) primitives understood by the existing MAC.

• Physical Coding Sublayer (PCS) - This sublayer provides a uniform interface to the Reconciliation layer for all physical media. Carrier Sense and Collision Detect indications are generated by this sublayer. It also manages the auto-negotiation process by which the NIC (Network Interface) communicates with the network to determine the network speed (10 or 100 Mbps) and mode of operation (half-duplex or full-duplex).
• Physical Medium Attachment (PMA) - This sublayer provides a medium-independent means for the PCS to support various serial bit-oriented physical media. This layer serializes code groups for transmission and deserialises bits received from the medium into code groups.
• Physical Medium Dependent (PMD) - This maps the physical medium to the PMA. It defines the physical layer signaling used for various media.

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