(GRAC)Networking - Chapter 7

HDLC frame - S-frame

Supervisory frame Provide the ARQ mechanism when piggybacking is not used

Piggybacking

Support for the requirement that, if two stations exchange data, each needs to maintain two windows, one for transmit and one for receive, and each side needs to send the data and acknowledgements to the other Sending data and acknowlegment in one

Sliding window - 3-bit sequence number field. What does it mean when B sends RR 3?

"I have received frames up through frame number 2 and am ready to receive frame number 3; in fact I am prepared to receive seven frames, beginning with frame number 3"

What does RNR 5 mean?

"I have received frames up through number 4 but am unable to accept any more at this time."

What are the 3 data transfer modes of HDLC?

*Normal response mode (NRM)* - used with unbalanced configuration, 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 balanced configuration - either combined station may initiate transmission without receiving permission from the other combined station (most widely used) *Asynchronous response mode (ARM)* - used with unbalanced configuration, the secondary may initiate transmission without explicit permission from the primary

Flag field

...

Frame

...

Sliding window - Range of sequence numbers

0 through 2^k - 1

Sliding window - Max window size

2^k - 1, for k bit field

Automatic repeat request (ARQ)

A layer of control in each communicating device that provides functions: Error detection Positive acknowledgement Retransmission after timeout Negative acknowledgement and retransmission Effect of ARQ is to turn a potentially unreliable data link into a reliable one. Three versions of ARQ have been standardized: Stop-and-wait ARQ Go-back-N ARQ Selective-reject ARQ

Bit length of a link

B = R x (d / V) B = number of bits present on the link at an instance in time when a stream fully occupies the link R = data rate of the link, in bps d = length, or distance, of the link in meters V = velocity of propagation, in m/s When bit length of a link is greater than the frame length, serious inefficiencies result

RNR

Acknowledges former frames but forbids transfer of future frames Most data link control protocols allow a station to cut off the flow of frames from the other side by sending a RNR message Later, the station must send a normal acknowledgement to reopen the window.

Sliding-window flow control

Addresses inefficiencies of stop-and-wait when a > 1 by allowing multiple frames to be in transit at the same time Procedure: Stations A and B, via full-duplex link. Buffer space for W frames - B can receive W frames and A can send W frames without waiting Frames are numbered B acknowledges frame by sending sequence number of next frame, which means B is ready for W frames, and lets B acknowledge multiple frames A has list of sequence numbers its allowed to send B has list of sequence numbers it's prepared to receive Each of these lists is a window of frames

HDLC frame - control field

All control field formats contain the poll/final (P/F) bit In command frames - its referred to as the P bit and is set to 1 to solicit (poll) a response frame from the peer HDLC entity In response frames - it is referred to as the F bit and is set to 1 to indicate the response frame transmitted as a result of a soliciting command Basic control field for S- and I-frames use 3-bit sequence numbers but can employ 7-bit sequence numbers U-frames always contains an 8-bit control field

Go-back-N-ARQ

Based on sliding window flow control A station may send a series of frames sequentially numbered modulo some maximum value. The number of unacknowledged frames is determined by window size, using the sliding-window-form technique

Stop-and-wait ARQ

Based on the stop-and-wait flow control technique outlined previously -> source station transmits a single frame and must await an acknowledgement - no other data frames can be sent until the destination station's reply arrives at the sourc estation Advantage is simplicity, disadvantage is inefficiency

HDLC operation

Consists of 3 phases: Initialization Exchange crap Termination

HDLC - Disconnect

Either HDLC module can initiate a disconnect, either on its own or at the request of its higher-layer user Issues a disconnect by sending a disconnect (DISC) frame

Flow control

Enables a receiver to regulate the flow of data from a sender so that the receiver's buffers do not overflow Sending station must not send frames at a rate faster than the receiving station can receive them Technique for assuring that a transmitting entity does not overwhelm a receiving entity with data In absence of flow control, the receivers' buffer may fill up and overflow while it is processing old data

Structure of HDLC frame

Flag (8 bits), Address (8 bits, extendable), Control (8 or 16 bites), Information (variable number of bits), FCS (16 or 32 bits), Flag (8 bits) (page 224)

HDLC frame - flag fields

Flag fields delimit the frame at both ends with the unique pattern 01111110 - a single flag may be used for both closing and opening a frame Receivers are continuously hunting for the flag sequence to synchronize on the start of a frame. While receiving a frame, a station continues to hunt for that sequence to determine the end of a frame But no assurance that the pattern 01111110 will not appear inside of the frame, destroying synchronization. Solution - bit stuffing

HDLC frame - FCS

Frame check sequence field Error-detecting code calculating from the remaining bits of a frame, exclusive of flags

Trailer

In HDLC frame - Frame check sequence and flag fields following the data field

Header

In HDLC frame - the flag, address, and control fields that precede the information field

Error control

In a data link control protocol, error control is achieved by retransmission of damaged frames that have not been acknowledged or for which the other side requests a transmission Refers to mechanisms to detect and correct errors that occur in the transmission of frames Two types of errors: Lost frame and damaged frame

Data frame

Includes a field that holds the sequence number of that frame plus a field that holds the sequence number used for acknowledgement

HDLC frame - I-frame

Information frames Carry the data to be transmitted for the user (the logic above HDLC that is using HDLC) Flow and error control data, using ARQ mechanism, are piggybacked on an information frame 2 fields that we care about: Sequence number and Acknowledgement

High-data-level-control (HDLC)

Is a widely used standardized data link control protocol, and contains virtually all of the features found in other data link control protocols Serves as a baseline from which virtually all other important data link control protocols are derived Uses synchronous transmission - all transmission are in the form of frames

Data link control protocol

Layer of logic added above the physical layer

HDLC - Busy condition

May arise because HDLC entity is not able to process I-frames as fast as they are arriving or they can't accept data that fast So entity's receive buffer fills up and must halt the incoming flow of I-Frames, using RNR command. So the other station will periodically poll the station with a P and requires the other side to respond with RR or RNR

Frame synchronization

One of the requirements and objectives for effective data communication between two directly connected transmitting-receiving stations Data are sent in blocks called frames - the beginning and end of each frame must be recognizable

Problem with stop-and-wait

Only one frame at a time can be in transit

HDLC frame - Information field

Present only in I-frames and some U-frames Can contain any sequence of bits but must consist of an integral number of octets (what does that mean?) Length is variable

What are the 3 station types of HDLC?

Primary station - responsible for controlling the operation of the link (frames issued here are called commands) Secondary Station - (frames issued here are called responses) Combined Station - combines features of primary and secondary, and may issue both commands and responses Primary maintains a separate logical link with each secondary station on the line

Bit stuffing

Procedure that solves the 01111110 flag field problem in HDLC frame For all bits between the starting and ending flags, the transmitter inserts an extra 0 bit after each occurrence of five 1s in the frame Receiver detects a starting flag - monitors the bit stream When 5 1's appear - 6th bit is examined. If it's 0, it's deleted. If it's a 1 and 7th bit is 0, the combination is accepted as a flag. If sixth and seventh bits are both 1, the sender is indicating an abort condition And now arbitrary bit patterns can be inserted into the data field of a frame (page 225)

Acknowledgment frame

RR or RNR What a station sends if it has an acknowledgement but no data (must repeat the last acknowledgement sequence number that it sent)

Stop-and-wait flow control

Simplest form of flow control Source entity transmits a frame After destination entity receives a frame, it indicates its willingness to accept another frame by sending back an acknowledgement to the frame just received Source must wait until it receives the acknowledgement before sending the next frame Destination can stop flow of data by withholding acknowledgement Problem: only one frame at a time can be in transit

Selective-reject ARQ

The only frames retransmitted are those that receive a negative acknowledgement, called SREJ, or those that timed out Useful choice for satellite link because of the long propagation delay involved

REJ x

The sender has to resend all frames starting with x

Propagation time

Time it takes for a bit to traverse the link between source and destination Equal to distance of the link divided by the velocity of propagation

Transmission time

Time it takes for a station to emit all of the bits of a frame onto the medium Equal to length of frame in bits divided by the data rate

What are the 2 link configurations of HDLC?

Unbalanced configuration - consists of one primary and one or more secondary stations and supports full-duplex and half-duplex transmission Balanced configuration - consists of two combined stations and supports both full-duplex and half-duplex transmission

HDLC frame - U-frame

Unnumbered frame Provide supplemental link control functions First one or two bits of the control field serves to identify the frame type

Data link

When a data link control protocol is used, the transmission medium between systems is referred to as a data link

Data transparency

When arbitrary bit patterns can be inserted into the data field of a frame (using bit stuffing)

What does it mean if propagation delay is less than 1?

a < 1 The propagation time is less than the transmission time and the frame is sufficiently long that the first bits of the frame have arrived at the destination before the source has completed its transmission of the frame Line is inefficiently underutilized

Propagation delay

a = B / L where B = length of the link in bits, and L = number of bits in the frame

What does it mean if propagation delay is greater than 1?

a > 1: The propagation time is greater than the transmission time The sender completes transmission of the entire frame before the leading bits of that frame arrive at the receiver. Here, serious inefficiencies result Line is always underutilized