CSIS 625 1

CSIS 625 Week 6

Data Link Layer

Copyright 2001, 2002 - Dan Oelke

For use by students of CSIS 625 for purposes of this class only.

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Overview

• Data Link Layer– Line Discipline– Flow Control– Error Control

• Byte Oriented Link Control• Bit Oriented Link Control

– HDLC

• MAC Protocols– Wireless– Ethernet

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Line Discipline

• Line Discipline defines who can send and when they can send.

• Simplex system– Who is always known– When is usually any time

• In a duplex system, the sender should send only when it knows that the receiver is ready.– This is in theory, but in practice it only

sometimes works this way

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Line Discipline

• Three phases of communication– Establishment of communication

• Determines who is sending to whom

– Data transfer• Moving the data

– Termination• Tying up loose ends

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ENQ/ACK system

• Process• Sender sends a ENQ (Enquiry) to the intended

receiver.

• Receiver sends back an ACK (Acknowledgement) when it is ready to receive

• Sender sends data

• Sender sends EOT (End of Transmission) when done sending.

• Receiver may send a NAK (Negative Acknowledgement) if it isn’t ready.

• Used in point to point systems

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ENQ/ACK system

• Common to have one node a primary and another a secondary– Primary is responsible for initiating

communication

• Some systems have nodes as peers– Either can initiate a transfer

• In full-duplex system, data and control messages can be sent simultaneously

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Poll & Select system

• Always has a primary and secondary nodes

• Process - Poll– Primary node requests data from the secondary– If no data, secondary responds with a NAK– Otherwise secondary responds with the data

• Process - Select– Primary node selects a secondary node and

sends data to it– Secondary responds with ACK after successful

transfer of data

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Network Addresses

– In a multi-point system, addresses needed so that nodes know who is talking to who

– Simple in some systems where physical position or dip-switch sets it.

– Ethernet defines that all manufactures of devices include a unique 48-bit (6-byte) address in every device.

– Polling 2^48 devices isn’t practical• at 10Mbps, with a 60 byte poll it would take

– 2^24 / 10Mbps * 60 * 8 bytes = 1.35E+10 seconds

– or about 428 years

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Media Access Control

• MAC - Media access Control

• A multipoint network with no clear master and slave. – master == primary– slave == secondary

• In this system, there needs to be more complicated protocol than ENQ/ACK or select/poll to figure out who sends when

• See later on in this lecture….

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Flow Control

• Flow control - a protocol to ensure that the sender does not overwhelm a receiving station

• Incoming data must be checked and processed before it can be used– This is often slower than the transmission rate– Receivers need to buffer received data until it is

processed– Buffers are always limited

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Flow Control - Stop-n-Wait

• Stop and Wait flow control is simplest form of flow control

• Process– Sender sends one frame of data– Sender waits for ACK before sending more – Receiver can slow down process by waiting to

send ACK

• May be simple - but it is inefficient

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Stop-n-Wait efficiency

– Entire set of data is divided into n frames– tprop = time to go from sender to receiver

• depends on distance and medium

– tframe = time to send one data frame• depends on bit rate and frame size

– TD = n(2 tprop + tframe) = Time to send data– Actual transmission of data is n* tframe

– Efficiency is (n* tframe )/ n(2 tprop + tframe) – = 1 / (1 +2(tprop / tframe))– If tprop is small and tframefor frame is large, it

approaches 100%, but never reaches it.

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Sliding Window Protocol

• Sliding Window flow control allows for more than one frame to be sent before an acknowledgement is received

• frames are numbered from 0 to n-1• Sender sends frames up until it has sent n-1

frames• Receiver sends back an ACK with the number

of the next frame it expects• Sender can now send up to this new number • Maximum window size is n-1 frames

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Figure 10-14

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Sliding Window Example

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Sliding Window Efficiency

• If the sliding window is big enough, and the receiver is fast enough, then 100% efficiency can be achieved

• To get 100% - – 2 * tprop ((n - 1) * tframe )

– tprop = Time for a frame to propagate from one end to the other of the system

– tframe = Time to send a frame (based on frame size and bit rate)

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Error Control

• Error detection and retransmission

• Error detection is achieved using parity, checksums, CRC, etc– Extra data added to each frame

• ARQ - Automatic Repeat Request– This abbreviation isn’t really used much

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Stop-n-Wait Error Control

• Extension of Stop-n-Wait flow control– When a good frame is received, an ACK sent

back to originator.– When a damaged frame is received, a NAK is

sent back to originator– When a NAK is received, the same frame is

retransmitted

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Stop-n-Wait Error Control

• Any frame may be dropped - data or NAK or ACK.– If ACK isn’t received after some time, a NAK

is assumed– If a NAK is lost - timeout and retransmission – If a ACK is lost timeout and retransmission

• Frames must be numbered so receiver can throw away duplicates

– If data is lost - timeout and retransmission

• Has same efficiency issues

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Go-back-N Error Control

• Uses a Sliding window

• If a NAK is received or an ACK timeout occurs– all data since last ACK received is resent– This may be several frames

• ACKs contain next expected frame number

• NAKs contain errored frame number

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Selective-Reject Error Control

• Uses a Sliding window– If a NAK is received, only the damaged frame is

resent

• Selective Reject is more complicated to implement – Receiver must contain the sorting logic to enable it to reorder frames

– Sending device must contain searching mechanism to enable it to find and select only the requested frame for retransmission

– Buffer in the receiver must keep all previously received frames on hold until all retransmissions have been sorted, duplicates identified and discarded

– ACK numbers must refer to the frame received instead of next frame expected

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Selective-Reject Error Control

• More efficient as it resends less data – If errors don’t occur very often - there isn’t a

big difference in efficiency

• Selective Reject is rarely implemented over Go-back-N method

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Byte Oriented Link Control

• List here is all from Modem communications world– Normally people don’t address these as link layer

controls

• Not just data-link as most of these protocols addressed issues of file-transfer

• XMODEM– Stop-n-Wait error control

– Fixed data field of 128 bytes, CRC-8

• YMODEM– increased data field to 1024 bytes, CRC-16

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Byte Oriented Link Control

• ZMODEM– More features

– Sliding window

• Kermit– sliding window

– Also a terminal emulation package

• BSC - Binary synchronous communication– No-one cares about this one

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Bit Oriented Link Control

• Many bit Oriented protocols today are oriented around HDLC

• Ethernet, Token-Ring and other LAN technologies are also bit-oriented

• SONET, T1, and most telephony systems are bit-oriented.

• It is common to use HDLC over a SONET or T1 system

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HDLC node types

• Primary station– Has complete control over the link– Can be used in point-to-point or multi-link

• Secondary station– Receives commands from the primary and

responds accordingly

• Combined station– Can both command and respond to commands– Used in peer to peer (or balanced)

configurations

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HDLC Configurations

• Unbalanced – One primary node and one or more secondary

nodes– May be point-to-point or multi-point – Full or half-duplex

• Balanced– point-to-point topology only– Both nodes are combined stations

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HDLC Modes of Communication

• NRM - Normal Response mode– Standard Primary-secondary relationship– Secondary can transmit only when polled

• ARM - Asynchronous Response Mode– A secondary may initiate a transmission

without permission from the primary

• ABM - Asynchronous Balanced Mode– Combined stations - either can initiate – Point-to-point topology

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HDLC Frame structure

• Frame of data consists of:– Flag: 8 bits– Address: 1 or more bytes– Control: 8 or 16 bits– Information: many bits– Frame Check Sequence (FCS): 16 or 32 bits– Flag: 8 bits

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HDLC Flag byte

• The flag is a special bit pattern that signifies the start/end of a frame.

• A single flag byte may be used to end one frame and start another

• Flag bit pattern is 0111 1110

• Bit stuffing is used to make sure this pattern does not occur any where else.

• Bit stuffing occurs on all bytes (control or data) that are not flag bytes

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HDLC Address field

• First seven bits are used for address

• Last bit - if a 1 then there are no more address bytes

• Last bit - if a 0 then there is another address byte

• Last bit of last address byte is always a 1

• Number of address bits is a multiple of 7

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HDLC Control Field

• One or two byte segment for flow management

• Control fields change based on type of frame

• If first bit is 0 – frame is an I-Frame

• If first bit is 1 and second 0 – frame is an S-Frame

• If first bit is 1 and second 1 – frame is a U-Frame

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HDLC Control Field

• I-Frame - Information Frame– 3 bits for send sequence number

– 3 bits for receive sequence number (ACKs)

– 1-bit - Poll/Final bit• From secondary - set to 1 if last I-frame of response

• From primary - set to 1 if polling secondary for data

• S-Frame - Supervisory Frame– 3 bits for receive sequence number

– 2 bits for “supervisory” functions

– 1 bit - Poll/final bit

– S-Frames used for ACK when no data

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HDLC Control Field

• U-Frame - Unnumbered frame– 1 bit- Poll/Final bit– 5 bits - “Unnumbered function” bits

• S-frames can be used for:• reject

• selective rejects

• receiver ready

• receiver not ready

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HDLC Control Field

• U-frames can be used for• Mode setting (primary/secondary)

• unnumbered information transfer

• recovery - bad commands, etc

• initialization

• disconnect

• test command/response

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HDLC - Information field

• The information field is a series of bits until the next flag byte is found

• S-Frame has no information field

• The last 16 or 32 bits of the Information field is actually the FCS

• FCS - Frame check sequence– a 16 or 32-bit CRC on the data

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MAC - Media Access Control

• When there are multiple nodes that can all transmit on a given link, a means of controlling them is necessary

• The MAC layer is a sub-layer of data link layer in these situations.

• The Logical Link Control (LLC) Layer is the other sub-layer

• MAC is the lower sub-layer, LLC is the upper sub-layer

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MAC assumptions

1 - Station Model• There are N independent stations (aka nodes) that

generates frames of data for transmission

2 - Single Communications Channel3 - Collision detection

• All stations can detect collisions

4 - Time frame• Continuous time - any frames starts at any time• Slotted time - Time divides into discrete slots

5 - Carrier sense (or not)• The nodes can detect if the channel is busy

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ALOHA protocol

• Ground based radio broadcasting

• Developed at University of Hawaii

• Pure ALOHA– Any node starts transmitting at any time– Receiving node detects if a collision occurs

• Same if partially or completely clobbered

– If data not properly received, the transmitter waits a random amount of time and resends.

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ALOHA Protocol

• Slotted ALOHA– A special node emits a “pip” at the start of each

interval.– All frames start at the beginning of an interval

• Both Pure ALOHA and Slotted ALOHA have a point at which adding more traffic results in worse throughput– The collisions keep piling up and causing more

and more collisions– Slotted ALOHA will take about 2x as much

load as Pure ALOHA

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Carrier Sense Multiple Access (CSMA) Protocols

• Nodes listen to see if someone else is transmitting before they send their packet.– Better performance than ALOHA

• 1-Persistent CSMA– Transmitter waits until the the line is idle and

sends immediately

• Nonpersistent CSMA– Transmitter waits random time time if the line

isn’t idle– Leads to better utilization AND longer delays

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CSMA Protocols continued

• p-Persistent CSMA– Uses slotted channels - If slot is idle there is a

probability of p that transmitter will use the channel.

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CSMA/CD - Carrier Sense Multiple Access with Collision Detection

• Extend the idea so that if two nodes detect a collision they stop transmitting immediately

• Quickly terminating the transmission saves time and bandwidth

• To detect collision, need to transmit for 2x the maximum propagation time.– Time for signal to transmit from one end to the

other – PLUS Time for colliding signal to transmit

from far end back

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Wireless Protocols

• Typically nodes are all trying to talk with a central (wired) hub

• Sometimes (but not always) the nodes can “hear” one another

• Hidden station problem – When one node can’t hear another and tromps

on transmission to a third node

• Exposed station problem– When one node hears another node and so

won’t transmit to a third, even through it could.

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Wireless Protocols and CSMA

• CSMA tells the transmitter if there is activity near, but you want to know if there is activity by the receiver.

• MACA - Multiple Access with Collision Avoidance.

• MACAW - MACA with refinements• CSMA/CA - another name for MACA like

system– Carrier Sense Multiple Access with Collision

Avoidance

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Wireless Protocols continuted

• MACAW basics - A sending to B– A sends B an RTS (Request to Send) frame

with length– B sends A a CTS (Clear to Send) frame with

length– A sends B the data frame– Other nodes that hear CTS know to be quiet for

that time period (based on length)

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Further reference

• http://www.techfest.com/networking/index.htm