Computing – Communications and Networks - IP

IP


Modern networks have more than physical and data link. The network layer is
the set of mechanisms that can be used to send packets from one computer to
another in the world. There are two types of networks:
With Packet switching, data packets can be carried together on the same link.
They are differentiated by addressing information. Packet switching is the basis
for all data networks today, including the Internet, public data networks such as
Frame Relay, X.25, or ATM.
Circuit Switching is the way telephone networks operate. A circuit emulates
the physical signals of a direct end-to-end cable. When computers are
connected by a circuit switched network, they establish a direct data link over
the circuit. This is used today for modem access to a data network.
Modern circuit switches are based on byte multiplexing and are thus similar to
packet switches, with the main difference that they perform non-statistical
multiplexing (see later in this chapter).
A network has Intermediate systems (ISs): those are systems that send data to
next ISs or to the destination. Using interconnected ISs saves cable and
bandwidth. ISs are known under various terms depending on the context:
routers (TCP/IP, AppleTalk,…), switches (X.25, Frame Relay, ATM,
telephone), communication controllers (SNA), network nodes (APPN).
The Internet's network layer has two principle components. It has a protocol
that defines the fields in the IP datagram as well as how the end systems and
routers act on these fields. This protocol is the celebrated IP protocol. There is
only one IP protocol, and all Internet components that have a network layer
must run the IP protocol. The Internet's network layer also contains routing
protocols that determine the routes that datagrams take between sources and
destinations. The Internet has many routing protocols.


Physical Layer
Data Link Layer


 Physical transmission = Physical function
 bits <-> electrical / optical signals
 transmit individual bits over the cable: modulation, encoding
 Frame transmission = Data Link function
 bits <-> frames
 bit error detection
 packet boundaries
 in some cases: error correction by retransmission (802.11)
 Modems, xDSL, LANs



Physical Layer: The job of the physical layer is to move the individual bits within the frame
from one node to the next. The protocols in this layer are again link dependent, and further
depend on the actual transmission medium of the link (for example, twisted-pair copper wire,
single-mode fiber optics). For example, Ethernet has many physical layer protocols: one for
twisted-pair copper wire, another for coaxial cable, another for fiber, and so on. In each case, a
bit is moved across the link in a different way.
Link Layer: The services provided at the link layer depend on the specific link-layer protocol
that is employed over the link. For example, some protocols provide reliable delivery on a link
basis, that is, from transmitting node, over one link, to receiving node. The process is analogous
to the postal worker at a mailing center who puts a letter into a plane that will deliver the letter to
the next postal center along the route. Examples of link layers include Ethernet and PPP; in some
contexts, ATM and frame relay can be considered link layers. As datagrams typically need to
traverse several links to travel from source to destination, a datagram may be handled by
different link-layer protocols at different links along its route. For example, a datagram may be
handled by Ethernet on one link and then PPP on the next link. The network will receive a
different service from each of the different link-layer protocols.