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DataLink2-1Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Bit Oriented Data Link Protocols
Examples:
! High-level Data Link Control (HDLC) - ISO
! Advanced Data Communications Control Procedures(ADCCP) - ANSI
! Synchronous Data Link Control (SDLC) - IBM
! Link Access Procedure, Balanced (LAPB) - ITU-T (forX.25)
! Link Access Procedure, D-Channel (LAPD) - ITU-T (forISDN)
! Link Access Procedure for Frame-Mode Bearer Services(LAPF)
! Logical Link Control (LLC) - IEEE 802.2
DataLink2-2Copyright © December 21, 2004 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 tw o combined stations only.
DataLink2-3Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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 init iate transmission.
DataLink2-4Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
HDLC Frame Structure
! All transmissions use frames w ith follow ing f ields:- Flag: 8 bits (01111110) w ith bit stuff ing- Address: One or more octets (for mult ipoint 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
DataLink2-5Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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 Not
Ready)
RNR(Receive Not
Ready)
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
DataLink2-6Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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
DataLink2-7Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
HDLC Operation
1. I-Frames:
! Contains user data
! Contains the sequence number of the transmittedframe
! Contains a piggybacked acknow ledgement forreceived I-frames (N(r) = Next frame expected)
! Maximum w indow size is 7 or 127
! I-frame contains a poll/f inal (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.
DataLink2-8Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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) w hen no I-frame is available forpiggybacking.
! Primary can issue a POLL by sett ing 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 t he receive st at us of asecondary/combined station.
! Secondary/Combined station response to Pollw ith F-bit set if the stat ion is busy.
c) Reject (REJ)
! Go-Back-N technique (Retransmit from N(r))
d) Selective Reject (SREJ)
! Selective Repeat technique (Repeat N(r))
DataLink2-9Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
3. U-Frames:
Categories:
! Mode Sett ing
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
DataLink2-10Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
HDLC Command/Response Repertoire Options
Basic Repertoire:Commands:
I, RR, RNR, (SNRM or SARM or SABM), DISCResponses:
I, RR, RNR, UA, DM, FRMR
Functional Extensions (Options):1. For Sw itched Circuits
Commands: ADD - XID; Responses: ADD - XID, RD2. For 2-w ay Simultaneous
Commands & Responses: ADD - REJ3. For Single Frame Retransmission
Commands & Responses: ADD - SREJ4. For Information
Commands & Responses: ADD - UI5. For Init ialization
Commands: ADD - SIM; Responses: ADD - RIM6. For Group Polling
Commands: ADD - UP7. Extended Addressing8. Delete Response I Frames9. 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
DataLink2-11Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Link Establishment and Termination - NRM
Examples of HDLC Operation
DataLink2-13Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Link Establishment and Termination - ABM
DataLink2-19Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
LAPB State Diagram
! LAPB Addresses:
DataLink2-21Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Various Data Link Control Frame Formats
DataLink2-22Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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, negotiat ing options, and bringthem dow n 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 w aythat is independent of the netw ork layer protocolused. A dif ferent Netw ork Control Protocol(NCP) is used for each netw ork layer supported.
DataLink2-23Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
PPP State Diagram for Line Activation
! Dead (or IDLE):No active carrier or connection exists.
! Establish:LCP option negotiat ion begins.
! Authenticate (optional):The tw o end points authenticate each other.
! Netw ork:The appropriate NCP protocol is invoked.
! Open:Data transport takes place.
! Terminate:The connection is closed.
DataLink2-24Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
PPP Frame Format
! The PPP frame format is similar to HDLC.
! The address and control f ields are alw ays constant.
! The Protocol f ield indicates w hat kind of packet is inthe Payload f ield (e.g., LCP, NCP, IP, IPX, AppleTalk,etc.)
! The Payload f ield has a variable length and itsmaximum is negotiated. The default maximum is1500 bytes.
DataLink2-25Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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.
DataLink2-26Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
The Password Authentication Protocol (PAP)! Authentication means validating a user w ho needs to
access a set of resources.! PPP has tw o protocols for authentication: Passw ord
Authentication Protocol (PAP) and ChallengeHandshake Authentication Protocol (CHAP).
! PAP:
16! PAP Packets (Protocol f ield = C023 ):
DataLink2-27Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Challenge Handshake Authentication Protocol (CHAP)! CHAP - 3-w ay handshake (passw ord is never sent):
16! CHAP Packets (Protocol f ield = C223 ):
DataLink2-28Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
The Network Control Protocol (NCP)
! NCP is a set of control protocols to allow theencapsulation of data from various netw ork layerprotocols; such as, IP, IPX, AppleTalk, etc..
DataLink2-29Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
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 f ield = 8021 ):
- Configure-request- Configure-ack- Configure-nak- Configure-reject- Terminate-request- Terminate-ack- Code-reject
! Note:After configuration, 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.