Direct Link Networks :

The first is encoding bits onto the transmission medium so that they can be understood by a receiving host.
Second is the matter of delineating the sequence of bits transmitted over the link into complete messages that can be delivered to the end node. This is called the framing problem, and the messages delivered to the end hosts are often called frames.
Third, because frames are sometimes corrupted during transmission, it is necessary to detect these errors and take the appropriate action; this is the error detection problem.
The fourth issue is making a link appear reliable in spite of the fact that it corrupts frames from time to time. Finally, in those cases where the link is shared by multiple hosts—as opposed to a simple point-to-point link—it is necessary to mediate access to this link. This is the media access control problem.

Although these five issues—encoding, framing, error detection, reliable delivery, and access mediation

They are very real problems that are addressed in different ways by different networking technologies.

This chapter considers these issues in the context of four specific network technologies:

point-to-point links,

carrier sense multiple access (CSMA) networks (of which Ethernet is the most famous example),

token rings (of which IEEE Standard 802.5 and FDDI are the most famous examples), and

wireless networks (for which 802.11 is the most widespread standard).

Networks in which there is no routing involved.We will look at the physical layer signal representation, error correction.We will look at the link layer Point to Point links Multiple-access Ethernet, Token Ring

Network Adaptor and Device Driver

Network adaptor sits on the Systems I/O and delivers data from the workstation’s memory to the network link.

Device driver is the software module that manages this adaptor.

Issues commands such as from what memory location should outgoing data be transmitted, where should the incoming data be stored etc.

Problem with NRZ

Receiver keeps an average of the signal received so far.

Compares incoming signal to this average -- if significantly higher -- high, if significantly lower, then low.

If too many zeroes or ones, difficult to track this average -- the average wanders -- called the baseline wander.

If there are clock drifts between the sender and receiver, this cannot be detected -- how many bits were transmitted ?

4B/5B encoding

Insert extra bits into bit stream to break long sequences of 0’s and 1’s.

Specifically every four bits of data encoded into a five bit code.

Codes such that no more than 2 trailing zeroes and no more than 1 leading zero. (When codes are transmitted back to back no more than 3 consecutive zeroes.

Resulting codes transmitted using NRZI.

Specific codes -- 11111 -- Line idle

00000 -- Line dead

00100 -- Halt