CIS 321 - Monday April 2, 2001

Data-Link Control Protocols

HDLC - High-Level Data Link Control

Three types of stations run the HDLC Protocol.

Stations running the HDLC can communicate in one of three modes.

HDLC is a bit-oriented protocol.

The format of a frame is similar to the format of the frames we have discussed.

The first 8 bits is a flag field. The next 8 or 16 bits is the address field. The next 8 or 16 is the control field. This is followed by data. The next 16 or 32 bits are for Frame Check Sequence. The last 8 bits are a flag field.

The flag fields are used to mark the beginning and end of the frame. In this field we have the pattern 01111110. When this pattern is seen, it is interpreted as the beginning or end of a frame.

If this sequence appears in our data, then before it is sent, whenever there are five consecutive 1s in the data, a 0 is added after them. This is called bit stuffing.

The control field is used to send status information or issue commands. Its content depends on the frame type.

There are three types of frames.

Information Frames
These are used primarily to transfer information.

Supervisory Frames
These are used by either station to indicate its status or to NAK frames received incorrectly.

Unnumbered Frames
These frames establish how the protocol will proceed. For example, will it use go-back-n or selective repeat? What size frames will it use? What mode will it use?

Other Bit-Oriented Protocols

Synchronous Data Link Control (SDLC)

HDLC was derived from SDLC, which was developed by IBM in the 1970s.

Advanced Data Communications Protocol (ADCCP)

IBM submitted SDLC to ANSI, and they modified it and renamed it ADCCP.

Link Access Protocols (LAP)

Link Access Protocols allow devices to be connected to packet-switched networks.

Logical Link Control (LLC)

LLC allows LANs to connect to other LANs or WANs.

Binary Synchronous Communications Protocol

This is a byte oriented protocol.

It uses synchronous, half-duplex communications.

In this protocol, the beginning of a frame is indicated by two special bytes called SYN characters. Two are used to reduce the probabilty of receiving a false frame.

After the SYN bytes are one or more control bytes.

One type of frame used with this protocol are control frames. In this type of frame most of the frame are the control bytes.

Another type of frame is a data frame. In a data frame the first control byte is SOH (start of header). It tells the receiving station that successive bytes in the arriving frame contain header information.

Header information varies but typically will contain information like source and destination addresses.

After the header information is an STX (start of text) byte. This indicates that next bytes received will be data. To indicate the end of the data a ETX (end of text) byte is used.

If we have to send more data than we can send in one frame, then we send multiple frames. However, at the end of the data in each frame except the last we use an ETB (end of transmission block) byte instead of an ETX byte.

This scheme works well if the data we are sending are simple text characters. What happens if we are sending binary data.

What happens if the ETB or ETX bytes are part of the binary data?

In order to solve this problem before we transmit any binary data we include a DLE (Data Link Escape) byte. When this is encountered, then no bytes are interpreted as control bytes until another DLE is encountered.

However, what happens if the DLE byte is part of the binary data. In order to solve this problem, we use byte stuffing. Before the frame is sent, the sending stations goes through the binary data, and if it finds a DLE byte it inserts another one. Then if a DLE byte is encountered, and the byte following it is another DLE byte, then the first is interpreted as data.

Data delimited this way is called transparent data.

Data delimited with ETB and ETX bytes is called non-transparent data.

Local Area Networking

In Chapter 6, Local Area Networking is discussed. Recall the various topologies we discussed for connecting local area networks.

In 1973, in his Ph. D. thesis, Robert Metcalfe described his research on LAN technology. After he graduated he went to work at the Xerox Corporation and worked with a group that eventually developed what became known as the Ethernet. It was named after ether which is an imaginary substance that many people believed occupied all space and was the medium through which light waves propogated. It was also a very real substance used in hospitals.

The concepts of the Ethernet were eventually written up as a proposal to the IEEE as a standard for LANs. It had the backing of Xerox, Intel, and DEC. General Motors and IBM also presented proposals to the IEEE. Instead of trying to decided between the proposals, the IEEE made all three of them standards. The proposal by Xerox, Intel, and DEC became known as IEEE 802.3

We will consider IEEE 802.3 which is discussed in section 6.2 and IEEE 802.5 which is discussed in section 6.3. Because of lack of time we will not consider IEEE 802.4 which is discussed in section 6.4.

We will also cover section 6.5 which deals with interconnection Local Area Networks.

Section 6.6 is a case of study of Novell Netware.

We do not have time to cover this topic in class so your assignment is to read section 6.6.

On the exam Friday, there will be several bonus questions totalling 15 points. These questions will cover section 6.6. You will be asked to write a system login script for a client of a Novell server and to discuss some of the following topics.