Network Sales and Services Handbook (Cisco Press Networking Technology)

Ethernet LANs are covered by the IEEE 802.3 standard and are deployed in approximately 85 percent of the world's LANs. The IEEE 802.3 standard also defines the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) protocol.

Ethernet can operate in either half-duplex or full-duplex mode. Fast Ethernet and Gigabit Ethernet (discussed later in this chapter) operate in full-duplex mode. Half-duplex communication is when the host sends and receives data over the same wire, but not at the same time; for example, the host can send or receive, but not simultaneously. Full-duplex communication is when the host can send and receive data over the same wire at the same time.

Technical Note: CSMA/CD The CSMA/CD protocol was developed as a means by which two or more hosts could share a common media in a switch-less environment. In this shared environment, the CSMA/CD protocol does not require central arbitration, access tokens, or assigned time slots to indicate when a host will be allowed to transmit. Each Ethernet NIC determines when it will be allowed to send a frame based on sensing a data carrier on the network medium. The CSMA/CD access rules are as follows: Carrier sense (CS) Each Ethernet LAN host listens for traffic on the medium to determine when gaps between frame transmissions occur. Multiple access (MA) LAN hosts can begin transmitting any time they detect that the network is quiet, meaning that there is no traffic across the wire. Collision detect (CD) If two or more LAN hosts in the same CSMA/CD network, or collision domain, begin transmitting at the same time, the bit streams from the transmitting hosts will interfere (collide) with each other, and both transmissions will be unreadable. Each host stops transmitting as soon as it has detected this collision and then waits a random length of time (as determined by a back-off algorithm) before attempting to retransmit the frame. In this event, each transmitting host will transmit a 32-bit jam signal alerting all LAN hosts of a collision before running the backoff algorithm. In other words, in a room full of people, if everyone tried to talk at the same time, no one would be able to communicate. With CSMA/CD, each person would wait to ensure that the medium (in this case, air) was clear (meaning that no one else was talking) before speaking. If two people started to talk at the same time, each would stop talking, wait a random period of time, and start talking again.

In a half-duplex (traditional) Ethernet LAN environment, collisions are part of normal operation. Collisions occur more frequently in a heavily used network, resulting in network performance decreases (as traffic load increases). If the Ethernet LAN experiences traffic load in excess of 30 percent, it is considered to be saturated and it is then worthwhile to consider redesign and/or LAN segmentation initiatives.

Fast Ethernet

Fast Ethernet (100 Mbps) is a high-speed LAN technology that provides increased bandwidth to desktop users, as well as to servers and server clusters (server farms) in data centers.

Fast Ethernet networks support autonegotiation (as an optional feature). Autonegotiation enables a network device and a hub to exchange information about their capabilities.

Autonegotiation functions by working through a list of supported configurations until a matching configuration is found. Autonegotiation supports the following capabilities:

• Speed matching for network devices that support both 10- and 100-Mbps operation

• Full-duplex operation mode

• Automatic signaling configuration differing Ethernet network hosts

There are several implementations of Fast Ethernet found today:

• Deployed to the desktop in large organizations with multimedia, Computer Assisted Design/Computer Assisted Modeling (CAD/CAM) or large data requirements, such as database updates

• Small data center implementations, providing faster access to server farms and other storage devices

• Inter-Switch Link (ISL) implementations, where single (100 Mbps) or multiple (up to 800 Mbps) Fast Ethernet channels are bundled together, providing backbone trunks between 10/100 Mbps Ethernet switches, as illustrated in Figure 5-6.

  Figure 5-6. Ethernet ISL

Gigabit Ethernet

Gigabit Ethernet (GigE) is Fast Ethernet with an extra zero added, and like Fast Ethernet, Gigabit Ethernet often is deployed to the desktop in larger organizations with multimedia, CAD/CAM, or large data requirements, such as large database updates or medical imagery storage, where Fast Ethernet does not provide enough bandwidth for efficient operation. Gigabit Ethernet also is found in larger data center implementations to provide faster access to server farms and other storage devices.

Like Fast Ethernet, Gigabit Ethernet can be deployed in ISL implementations where Gigabit Ethernet channels of 2 Gbps to 8 Gbps can be bundled to provide backbone trunks between 10/100 Mbps Ethernet switches.

10 Gigabit Ethernet

10 Gigabit Ethernet was recently standardized in the IEEE 802.3ae specifications. 10 Gigabit Ethernet is full duplex, operating like Fast Ethernet and Gigabit Ethernet with no inherent distance limitations.

10 Gigabit Ethernet matches the speed of the fastest WAN backbone technology today, OC-192. (OC-192 runs at approximately 9.5 Gbps.) 10 Gigabit Ethernet has applications across LANs, metropolitan area networks (MANs), and wide area networks (WANs).

10 Gigabit Ethernet fits well in these network environments:

• Server interconnection for server clusters

• Aggregation of multiple Gigabit Ethernet segments into 10 Gigabit Ethernet downlinks

• Switch-to-switch links for very high-speed connections between switches in the same data center, in an enterprise backbone, or in different buildings