Chapter 5 Book Terms

Any device that runs a link-layer?

Node

Link access

A medium access control (MAC) protocol specifies the rules by which a frame is transmitted onto the link. For point-to-point links that have a single sender at one end of the link and a single receiver at the other end of the link, the MAC protocol is simple (or nonexistent)—the sender can send a frame whenever the link is idle. The more interesting case is when multiple nodes share a single broadcast link—the so-called multiple access problem. Here, the MAC protocol serves to coordinate the frame transmissions of the many nodes.

ARP

Address Resolution Protocol. Resolves IP addresses to MAC addresses.

Framing

Almost all link-layer protocols encapsulate each network-layer datagram within a link-layer frame before transmission over the link. A frame consists of a data field, in which the network-layer datagram is inserted, and a number of header fields. The structure of the frame is specified by the link-layer protocol. We'll see several different frame formats when we examine specific link-layer protocols in the second half of this chapter.

Possible services that can be offered by a link-layer protocol?

Framing Link access Reliable delivery Error detection and correction

Nodes

Hosts and routers

The communication channels that connect adjacent nodes along the communication path?

Links

CSMA/CD

Short for carrier sense multiple access/collision detection, CSMA/CD is a MAC (media access control) protocol. It defines how network devices respond when two devices attempt to use a data channel simultaneously and encounter a data collision.

Error detection and correction

The link-layer hardware in a receiving node can incorrectly decide that a bit in a frame is zero when it was transmitted as a one, and vice versa. Such bit errors are introduced by signal attenuation and electromagnetic noise. Because there is no need to forward a datagram that has an error, many link-layer protocols provide a mechanism to detect such bit errors. This is done by having the transmitting node include error-detection bits in the frame, and having the receiving node perform an error check. Recall from Chapters 3 and 4 that the Internet's transport layer and network layer also provide a limited form of error detection—the Internet checksum. Error detection in the link layer is usually more sophisticated and is implemented in hardware. Error correction is similar to error detection, except that a receiver not only detects when bit errors have occurred in the frame but also determines exactly where in the frame the errors have occurred (and then corrects these errors).

Reliable delivery

When a link-layer protocol provides reliable delivery service, it guarantees to move each network-layer datagram across the link without error. Recall that certain transport-layer protocols (such as TCP) also provide a reliable delivery service. Similar to a transport-layer reliable delivery service, a link-layer reliable delivery service can be achieved with acknowledgments and retransmissions (see Section 3.4). A link-layer reliable delivery service is often used for links that are prone to high error rates, such as a wireless link, with the goal of correcting an error locally—on the link where the error occurs—rather than forcing an end-toend retransmission of the data by a transport- or application-layer protocol. However, link-layer reliable delivery can be considered an unnecessary overhead for low bit-error links, including fiber, coax, and many twisted-pair copper links. For this reason, many wired link-layer protocols do not provide a reliable delivery service.

Detecting and correcting the corruption of bits in a link-layer frame sent from one node to another physically connected neighboring node.

bit-level error detection and correction

Links

communication channels that connect adjacent nodes along communication path - wired links - wireless links - LANs

data-link layer

has responsibility of transferring datagram from one node to physically adjacent node over a link

An MPLS-capable router is often referred to as a __________, since it forwards an MPLS frame by looking up the MPLS label in its forwarding table and then immediately passing the datagram to the appropriate output interface.

label-switched router

frame

layer-2 packet: encapsulates datagram

Over a given link, a transmitting node encapsulates the datagram in a __________ and transmits it into the link.

link-layer frame

For the most part, the link layer is implemented in a __________.

network adapter

A network adapter is also sometimes known as a __________.

network interface card (NIC)

flooding is?

the forwarding by a router of a packet from any node to every other node attached to the router except the node from which the packet arrived.

Even with the use of error-detection bits there still may be __________; that is, the receiver may be unaware that the received information contains bit errors.

undetected bit errors