1. MODULE I
NETWORKING CONCEPTS

2. HDLC The most important data link control protocol is HDLC
The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors.
Basic charscteristics
HDLC defines
3 types of stations
2 link configurations
3 data transfer modes of operation

3. Station Types
Primary station:
Has the responsibility of controlling the operation of data flow the link.
Handles error recovery
Frames issued by the primary station are called commands

4. HDLC Secondary station:
Operates under the control of the primary station.
Frames issued by a secondary are called responses.
The primary maintains a separate logical link with each secondary station on the line.
Combined station:
Combines the features of primary and secondary.
A combined station may issue both commands and responses.

5. HDLC HDLC Link Configurations Unbalanced configuration:
Consists of one primary and one or more secondary stations
supports both full-duplex and half-duplex transmission.
Balanced configuration:
Consists of two combined stations

6. HDLC HDLC Data Transfer Modes Normal response mode (NRM):
Used with an unbalanced configuration.
The primary may initiate data transfer to a secondary, but a secondary may only transmit data in response to a command from the primary.
Asynchronous balanced mode (ABM):
Used with a balanced configuration.
Either combined station may initiate transmission without receiving permission from the other combined station.

7. HDLC HDLC Data Transfer Modes Asynchronous response mode (ARM):
Used with an unbalanced configuration.
The secondary may initiate transmission without explicit permission of the primary.
The primary still retains responsibility for the line, including initialization, error recovery, and disconnection.

10. HDLC HDLC Frame Structure All transmissions are in the form of frames
HDLC defines three types of frames: information frames
(I-frames), supervisory frames (S-frames), and unnumbered frames (U-frames).
Each type of frame serves as an envelope for the transmission of a different type of message.

11. I-frames are used to transport user data and control information relating to user data (piggybacking).
Additionally, flow and error control data, using the ARQ mechanism, are piggybacked on an information frame
S-frames are used only to transport control information.
U-frames are reserved for system management.
Information carried by U-frames is intended for managing the link itself.
The first one or two bits of the control field serves to identify the frame type.
The remaining bit positions are organized into subfields

13. HDLC HDLC Frame Structure Diagram
The flag, address, and control fields that precede the information field are known as a header.
The FCS and flag fields following the data field are referred to as a trailer.

14. HDLC
Flag Fields
Flag fields delimit the frame at both ends with the unique pattern
Bit stuffing used to avoid confusion with data containing

15. HDLC Flag Fields Bit Stuffing
0 inserted after every sequence of five 1s
If receiver detects five 1s it checks next bit
If 0, it is deleted
If 1 and seventh bit is 0, accept as flag

16. HDLC
With the use of bit stuffing, arbitrary bit patterns can be inserted into the data field of the frame.
This property is known as data transparency.
Address Field
The second field of an HDLC frame contains the address of the secondary station.
If a primary station created the frame, it contains a to address.
If a secondary creates the frame, it contains a from address.
An address field can be 1 byte or several bytes long, depending on the needs of the network

17. HDLC
Control Field
HDLC defines three types of frames, each with a different control field format.

18. HDLC

19. Control Field for I-Frames
The subfields in the control field are used to define these functions.
The first bit defines the type.
If the first bit of the control field is 0, this means the frame is an I-frame.
The next 3 bits, called N(S), define the sequence number of the frame.

20. The last 3 bits, called N(R), correspond to the acknowledgment number when piggybacking is used.
The single bit between N(S) and N(R) is called the P/F bit.
The P/F field is a single bit with a dual purpose.
It has meaning only when it is set (bit = 1) and can mean poll or final.
It means poll when the frame is sent by a primary station to a secondary (when the address field contains the address
of the receiver).
It means final when the frame is sent by a secondary to a primary (when the address field contains the address of the sender).

21. Control Fieldfor S-Frame
Supervisory frames are used for flow and error control whenever piggybacking is impossible
S-frames do not have information fields.
If the first 2 bits of the control field is 10, this means the frame is an S-frame.
The last 3 bits, called N(R), corresponds to the acknowledgment number (ACK) or negative acknowledgment number (NAK) depending on the type of S-frame.
The 2 bits called code is used to define the type of S-frame itself
With 2 bits, we can have four types of S-frames

22. With 2 bits, we can have four types of S-frames
Receive ready (RR). If the value of the code subfield is 00, it is an RR S-frame
Receive not ready (RNR). If the value of the code subfield is 10, it is an RNR S-frame
Reject (REJ). If the value of the code subfield is 01, it is a REJ S-frame.
Selective reject (SREJ). If the value of the code subfield is 11, it is an SREJ S-frame

23. Control Field for U-Frames
Unnumbered frames are used to exchange session management and control infonnation between connected devices.
Unlike S-frames, U-frames contain an information field, but one used for system management information, not user data.
As with S-frames, however, much of the information carried by U-frames is contained in codes included in the control field.

24. HDLC

25. Control Fieldfor S-Frames
Supervisory frames are used for flow and error control whenever piggybacking is impossible.
S-frames do not have information fields.
If the first 2 bits of the control field is 10, this means the frame is an S-frame.
The last 3 bits, called N(R), corresponds to the acknowledgment number (ACK) or negative acknowledgment number (NAK) depending on the type of S-frame.
The 2 bits called code is used to define the type of S-frame itself

26. HDLC Information Field
The information field is present only in I-frames and some U-frames.
The field can contain any sequence of bits
The length of the information field is variable

27. HDLC Frame Check Sequence Field
The frame check sequence (FCS) is an error detecting code calculated from the remaining bits of the frame, exclusive of flags.
The normal code is the 16-bit
An optional 32-bit FCS

28. HDLC
Operation

29. HDLC
Operation
HDLC operation consists of the exchange of I-frames, S-frames, and U-frames between two stations
Involves 3 phases
First, one side or another initializes the data link so that frames may be exchanged
After initialization, the two sides exchange user data and the control information to exercise flow and error control.
Finally, one of the two sides signals the termination of the operation.

30. HDLC
Initialization
initialization request by issuing one of the six set mode commands.
This command serves three purposes:
1. It signals the other side that initialization is requested.
2. It specifies which of the three modes (NRM,ABM,ARM) is requested.
3. It specifies whether 3- or 7-bit sequence numbers are to be used.
If the other side accepts this request, then the HDLC module on that end transmits an unnumbered acknowledged (UA) frame back to the initiating side.

31. HDLC
If the request is rejected, then a disconnected mode (DM) frame is sent.
Data Transfer
When the initialization has been requested and accepted, then a logical connection is established
Both sides may begin to send user data in Iframes, starting with sequence number 0.
The N(S) and N(R) fields of the I-frame are sequence numbers that support flow control and error control
An HDLC module sending a sequence of I-frames will number them sequentially and place the sequence number in N(S)

32. HDLC
N(R) is the acknowledgment for I-frames received; it enables the HDLC module to indicate which number I-frame it expects to receive next.
The receive ready (RR) frame acknowledges the last I-frame received by indicating the next I-frame expected.
Receive not ready (RNR) acknowledges an I-frame, as with RR, but also asks the peer entity to suspend transmission of I-frames

33. HDLC When the entity that issued RNR is again ready, it sends an RR.
REJ indicates that the last I-frame received has been rejected and that retransmission of all I-frames beginning with number N(R) is required.
Selective reject (SREJ) is used to request retransmission of just a single frame.
Disconnect
HDLC issues a disconnect by sending a disconnect (DISC) frame.
The remote entity must accept the disconnect by replying with a UA

35. LINK ACCESS PROTOCOL-BALANCED (LAPB)
X.25 is a network interface defined for accessing packet-switched public networks.
It is commonly used for interconnecting LANs.
The X.25 defines the lowest three layers of the OSI Reference Model Physical layer, Data link Layer and Network Layer.
LAPB is a bit-oriented synchronous protocol that provides complete data transparency in a full-duplex point-to-point operation.
It is a data link layer protocol used to manage communication between data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) devices.

36. Data terminal equipment devices are end systems that communicate across the X.25 network.
They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.
DCE devices are communications devices, such as modems and packet switches

38. LAPB has been derived from HDLC and shares the same frame format, frame types, and field functions as HDLC.
It differs from HDLC in the representation of address field.
The address field can contain only one of two fixed (DTE or DCE) addresses.
LAPB is restricted to the ABM transfer mode and is appropriate only for combined stations

39. LINKED ACCESS PROTOCOL - D CHANNEL (LAPD)
ISDN emerged as an alternative to traditional dialup networking .
It is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone networks.
It provides a single, common interface with which to access digital communications services that are required by varying devices, while remaining transparent to the user.
ISDN standards are constructed using the Open System Interconnection seven-layer reference model.

40. Linked Access Protocol (D Channel) is a Layer 2 (data link) protocol.
D channel is the data or signalling channel which is used for communications (or "signalling") between switching equipment in the ISDN network and the ISDN equipment at your site.
The LAPD handle the handshaking (commands and responses), signalling, and control for all of the voice and data calls that are setup through the ISDN D channel.

41. LAPD works in the Asynchronous Balanced Mode (ABM)
LAPD works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced (i.e., no master/slave relationship).
Each station may initialize, supervise, recover from errors, and send frames at any time.
The objective of LAPD is to provide a secure, error-free connection between two end-points so as to reliably transport Layer 3 messages.
The control field of LAPD frame is identical to HDLC, but the address field differs.

43. The first Address-field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3.
The C/R bit indicates whether the frame contains a command or a response.
The Terminal Endpoint Identifier (TEI) field identifies either a single terminal or multiple terminals compatible with the ISDN network.
Example: Telephones, personal computers.