Bit Oriented Protocols

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DataLink2-2Copyright December 21, 200 4 by Chaim Ziegler, Ph.D.

HDLC

!Type of Stations:1. Primary (issues commands)2. Secondary (issues responses)3. Combined

!Link Configurations:1. Unbalanced:

Consists of one primary and one or moresecondaries.

2. Balanced:Consists of two combined stations only.


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HDLC Data Transfer Modes:

!Normal Response Mode (NRM):This is an unbalanced configuration. The primaryuses polling and selection.

!

Asynchronous Response Mode (ARM):This is an unbalanced configuration. The secondarymay send response frames w ithout explicitpermission from the primary.

!Asynchronous Balanced Mode (ABM):This is a balanced configuration. Either combinedstation may initiate transmission.


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HDLC Frame Structure

!All transmissions use frames w ith following fields:- Flag: 8 bits (01111110) with bit stuffing- Address: One or more octets (for multipoint lines)- Control: 8 or 16 bits- Information: variable length- Frame Check Sequence (FCS): 16 or 32 bits

- Flag: 8 bits!Types of Frames:

- Information Frames (I Frames)- Supervisory Frames (S Frames)- Unnumbered Frames (U Frames) (IBM - NS)

!HDLC uses synchronous transmission


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HDLC Command/Response Repertoire

Format Commands Responses C-Field Format

8 7 6 5 4 3 2 1

InformationTransfer

I(Information)

I(Information)

N(R) PF

N(S) 0

Supervisory

RR(Receive Ready)

RR(Receive Ready)

N(R) PF

0 0 0 1

RNR(Receive NotReady)

RNR(Receive NotReady)

N(R) PF 0 1 0 1

REJ(Reject)

REJ(Reject)

N(R) PF

1 0 0 1

SREJ(Selective Reject)

SREJ(Selective Reject)

N(R) PF

1 1 0 1

Unnumbered

SNRM(Set Normal

Response Mode)

1 0 0 P 1 1 1 1

SARMSet Asynch.

Response Mode)

DM(Disconnect Mode)

0 0 0 PF

1 1 1 1

SABM(Set Asynch.

Balanced Mode)

0 0 1 P 1 1 1 1

SNRME(Set Normal

Response Mode)

1 1 0 P 1 1 1 1

SARMESet Asynch.

Response Mode)

0 1 0 P 1 1 1 1

SABME(Set Asynch.

Balanced Mode)

0 1 1 P 1 1 1 1

DISC(Disconnect)

RD(Request Disc.)

0 1 0 PF

0 0 1 1

UA(Unnumbered Ack)

0 1 1 F 0 0 1 1

SIM(Set

Initialization Mode)

RIM(Request

Initialization Mode)

0 0 0 PF

0 1 1 1

FRMR(Frame Reject)

1 0 0 F 0 1 1 1

RSET(Reset)

1 0 0 P 1 1 1 1

UP(Unnumbered Poll)

0 0 1 P 0 0 1 1

UI(Unnumbered Info)

UI(Unnumbered Info)

0 0 0 PF

0 0 1 1

XID(Exchange ID)

XID(Exchange ID)

1 0 1 PF

1 1 1 1

TEST(Test)

TEST(Test)

1 1 1 PF

0 0 1 1


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HDLC I Frame & S Frame Extended Numbering Formats

Format Commands Responses C-Field Format

8 7 6 5 4 3 2 1

InformationTransfer

I(Information)

I(Information)

N(S) 0

N(R) PF

Supervisory

RR(Receive Ready)

RR(Receive Ready)

0 0 0 0 0 0 0 1

N(R) PF

RNR(Receive Not

Ready)

RNR(Receive Not

Ready)

0 0 0 0 0 1 0 1

N(R) PF

REJ(Reject)

REJ(Reject)

0 0 0 0 1 0 0 1

N(R) PF

SREJ

(Selective Reject)

SREJ

(Selective Reject)

0 0 0 0 1 1 0 1

N(R) PF


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HDLC Operation

1. I-Frames:

!Contains user data

!Contains the sequence number of the transmittedframe

! Contains a piggybacked acknowledgement forreceived I-frames (N(r) = Next frame expected)

!Maximum window size is 7 or 127

!I-frame contains a poll/final (P/F) bit.

- In NRM the primary sets the P-bit to issue aPOLL. The secondary sets the F-bit in lastI-frame of a response.

- In ARM and ABM, the P/F bit is used to forcea response.


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2. S-Frames:

Used for both f low control and error control

a) Receive Ready (RR)

!

Used as posit ive acknow ledgement (thruN(r)-1) when no I-frame is available forpiggybacking.

!Primary can issue a POLL by setting P-bit.!Secondary response w ith F-bit set if it has no

data to send.

b) Receive Not Ready (RNR)

! Used as posit ive acknow ledgement (thruN(r)-1) and a request that no more I-framesbe sent until a subsequent RR is used.

!Primary or Combined station can set P-bit tosolicit the receive status of asecondary/combined station.

!Secondary/Combined station response to Pollwith F-bit set if the station is busy.

c) Reject (REJ)

!Go-Back-N technique (Retransmit from N(r))

d) Selective Reject (SREJ)

!Selective Repeat technique (Repeat N(r))


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3. U-Frames:

Categories:

!Mode Setting

SNRM, SNRME, SARM, SARME, SABM, SABMEUA, DMRIM, SIMRD, DISC

!Information Transfer

UPUI

!Recovery

FRMR, RSET

- Invalid Control Field- Data Field Too Long- Data Field Not Allow ed With Received

Frame Type- Invalid Receive Count

!Miscellaneous

XIDTEST


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HDLC Command/Response Repertoire Options

Basic Repertoire:Commands:

I, RR, RNR, (SNRM or SARM or SABM), DISCResponses:

I, RR, RNR, UA, DM, FRMRFunctional Extensions (Options):1. For Switched Circuits

Commands: ADD - XID; Responses: ADD - XID, RD2. For 2-way Simultaneous

Commands & Responses: ADD - REJ3. For Single Frame Retransmission

Commands & Responses: ADD - SREJ

4. For InformationCommands & Responses: ADD - UI

5. For InitializationCommands: ADD - SIM; Responses: ADD - RIM

6. For Group PollingCommands: ADD - UP

7. Extended Addressing8. Delete Response I Frames

9. Delete Command I Frames10. Extended Numbering11. For Mode Reset (ABM only)

Commands: ADD - RSET12. Data Link Test

Commands & Responses: ADD - TEST13. Request Disconnect

Responses: ADD - RD14. 32-bit FCS


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Link Establishment and Termination - NRM

Examples of HDLC Operation


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Data Transfer - NRM


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Link Establishment and Termination - ABM


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Data Transfer - Balanced Mode


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Busy Condition


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Reject Recovery


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Selective Reject Recovery


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Timeout Recovery


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LAPB State Diagram

!LAPB Addresses:


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LAPB Commands and Responses


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Various Data Link Control Frame Formats


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The Point-to-Point Protocol (PPP)

! Used on the Internet for router-to-router and homeuser-to-ISP data links.

!PPP provides three features:

- A framing method that delineates frames. the frameformat also handles error detection.

- A Link Control Protocol (LCP) for bringing lines up,testing them, negotiating options, and bringthem down gracefully. This protocol supportsboth synchronous and asynchronous circuits andboth byte-oriented and bit-oriented encodings.

- A w ay to negotiate netw ork layer options in a waythat is independent of the netw ork layer protocolused. A different Network Control Protocol

(NCP) is used for each netw ork layer supported.


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PPP State Diagram for Line Activation

!Dead (or IDLE):

No active carrier or connection exists.!Establish:LCP option negotiation begins.

!Authenticate (optional):The tw o end points authenticate each other.

!Network:The appropriate NCP protocol is invoked.

!Open:Data transport takes place.

!Terminate:The connection is closed.


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PPP Frame Format

!The PPP frame format is similar to HDLC.

!The address and control f ields are always constant.

!The Protocol f ield indicates what kind of packet is inthe Payload f ield (e.g., LCP, NCP, IP, IPX, AppleTalk,

etc.)

! The Payload field has a variable length and itsmaximum is negotiated. The default maximum is1500 bytes.


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The Link Control Protocol (LCP)

!LCP Packet Encapsulated in a PPP Frame:

- Code: Defines the type of LCP packet.- ID: Value used to match a request w ith a response.

16!LCP Packets (Protocol f ield = C021 ):

Options: packet size, authentication, compression. etc.


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The Password Authentication Protocol (PAP)!Authentication means validat ing a user who needs to

access a set of resources.! PPP has tw o protocols for authentication: Password

Authent ication Protocol (PAP) and ChallengeHandshake Authentication Protocol (CHAP).

!PAP:

16!PAP Packets (Protocol field = C023 ):


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Challenge Handshake Authentication Protocol (CHAP)!CHAP - 3-way handshake (password is never sent):

16!CHAP Packets (Protocol field = C223 ):


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The Network Control Protocol (NCP)

! NCP is a set of control protocols to allow theencapsulation of data from various network layerprotocols; such as, IP, IPX, AppleTalk, etc..


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The Internetwork Protocol Control Protocol (IPCP)

!IPCP is the protocol used to establish a netw ork layerconnection for carrying IP data.

!IPCP Packet Encapsulation in a PPP Frame:

16!IPCP Packets: (protocol field = 8021 ):

- Configure-request- Configure-ack- Configure-nak- Configure-reject- Terminate-request

- Terminate-ack- Code-reject

!Note:After conf iguration, the link is ready to carry IP datain the payload f ield of a PPP frame. The protocol f ield

16value is set to 0021 to indicate IP data is beingcarried.


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Example PPP Session

Documents

Bit Oriented Protocols