Consideraciones inherentes al moldeo por inyección

By default, each Cisco switch port uses Ethernet auto-negotiation to determine the speed and duplex setting (half or full). The switches can also set their duplex setting with the duplex interface subcommand, and their speed with—you guessed it—the speed interface subcommand. Switches can dynamically detect the speed setting on a particular Ethernet segment by using a few different methods. Cisco switches (and many other devices) can sense the speed using the Fast

Link Pulses (FLP) of the auto-negotiation process. However, if auto-negotiation is disabled on

either end of the cable, the switch detects the speed anyway based on the incoming electrical signal. You can force a speed mismatch by statically configuring different speeds on either end of the cable, causing the link to no longer function.

Switches detect duplex settings through auto-negotiation only. If both ends have auto- negotiation enabled, the duplex is negotiated. However, if either device on the cable disables auto-negotiation, the devices without a configured duplex setting must assume a default. Cisco switches use a default duplex setting of half duplex (HDX) (for 10-Mbps and 100-Mbps interfaces) or full duplex (FDX) (for 1000-Mbps interfaces). To disable auto-negotiation on a Cisco switch port, you simply need to statically configure the speed and the duplex settings. Ethernet devices can use FDX only when collisions cannot occur on the attached cable; a collision-free link can be guaranteed only when a shared hub is not in use. The next few topics review how Ethernet deals with collisions when they do occur, as well as what is different with Ethernet logic in cases where collisions cannot occur and FDX is allowed.

NOTE Cross-over cables can also be used between a pair of PCs, swapping the transmit pair on one end (1,2) with the receive pins at the other end (3,6).

CSMA/CD

The original Ethernet specifications expected collisions to occur on the LAN. The media was shared, creating a literal electrical bus. Any electrical signal induced onto the wire could collide with a signal induced by another device. When two or more Ethernet frames overlap on the transmission medium at the same instant in time, a collision occurs; the collision results in bit errors and lost frames.

The original Ethernet specifications defined the Carrier Sense Multiple Access with Collision

Detection (CSMA/CD) algorithm to deal with the inevitable collisions. CSMA/CD minimizes the

number of collisions, but when they occur, CSMA/CD defines how the sending stations can recognize the collisions and retransmit the frame. The following list outlines the steps in the CSMA/CD process:

1. A device with a frame to send listens until the Ethernet is not busy (in other words, the device cannot sense a carrier signal on the Ethernet segment).

2. When the Ethernet is not busy, the sender begins sending the frame. 3. The sender listens to make sure that no collision occurred.

4. If there was a collision, all stations that sent a frame send a jamming signal to ensure that all stations recognize the collision.

5. After the jamming is complete, each sender of one of the original collided frames randomizes a timer and waits that long before resending. (Other stations that did not create the collision do not have to wait to send.)

6. After all timers expire, the original senders can begin again with Step 1.

Collision Domains and Switch Buffering

A collision domain is a set of devices that can send frames that collide with frames sent by another device in that same set of devices. Before the advent of LAN switches, Ethernets were either physically shared (10BASE2 and 10BASE5) or shared by virtue of shared hubs and their Layer 1 “repeat out all other ports” logic. Ethernet switches greatly reduce the number of possible collisions, both through frame buffering and through their more complete Layer 2 logic. By definition of the term, Ethernet hubs:

■ Operate solely at Ethernet Layer 1

■ Repeat (regenerate) electrical signals to improve cabling distances ■ Forward signals received on a port out all other ports (no buffering)

As a result of a hub’s logic, a hub creates a single collision domain. Switches, however, create a different collision domain per switch port, as shown in Figure 1-2.

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Ethernet Layer 1: Wiring, Speed, and Duplex 11

Figure 1-2 Collision Domains with Hubs and Switches

Switches have the same cabling and signal regeneration benefits as hubs, but switches do a lot more—including sometimes reducing or even eliminating collisions by buffering frames. When switches receive multiple frames on different switch ports, they store the frames in memory buffers to prevent collisions.

For instance, imagine that a switch receives three frames at the same time, entering three different ports, and they all must exit the same switch port. The switch simply stores two of the frames in memory, forwarding the frames sequentially. As a result, in Figure 1-2, the switch prevents any frame sent by Larry from colliding with a frame sent by Archie or Bob—which by definition puts each of the PCs attached to the switch in Figure 1-2 in different collision domains.

When a switch port connects via cable to a single other non-hub device—for instance, like the three PCs in Figure 1-2—no collisions can possibly occur. The only devices that could create a collision are the switch port and the one connected device—and they each have a separate twisted pair on which to transmit. Because collisions cannot occur, such segments can use full-duplex logic.

When a switch port connects to a hub, it needs to operate in HDX mode, because collisions might occur due to the logic used by the hub.

NOTE NICs operating in HDX mode use loopback circuitry when transmitting a frame. This circuitry loops the transmitted frame back to the receive side of the NIC, so that when the NIC receives a frame over the cable, the combined looped-back signal and received signal allows the NIC to notice that a collision has occurred.

Larry Archie Bob SW1 Larry 1 Collision Domain 10BASE-T, using Shared hub

Multiple Collision Domain 10BASE-T, using Switch

Solid Lines Represent Twisted Pair Cabling Archie

Bob

Hub1

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