Telecommunications - SDLC Protocol

Abstract

1.0 - Introduction

SDLC is a protocol developed by IBM, unlike Bi-Sync it separates the link control functions from the character set and data content. HDLC was later defined as an international ISO standard and this was used as the basis of X.25 link layer.

SDLC ferforms the following functions:

The following types of physical link are supported by SDLC

X21 interface remote nodes can also be connected over a circuit switched network

1.1 - SNA

THE DATA-LINK LAYER

The data link layer provides functional and procedural means to establish, maintain and release data-link-connections among network-entities and to transfer data-link-service-data-units. A data-link-connection is built upon one or several physical-connections.

FUNCTIONS OF THE DATA LINK LAYER

The data-link converts the unreliable datastream into a process to make sure the bits that are sent are received correctly at the destination.

It beeaks up the stream of bits into blocks ("frames") of a reasonable size and adds some checking mechanism (usually a "cyclic redundancy check"). If the destination finds that the check is correct, it can assume that the data has been received without error; if the check is wrong, the destination assumes that there is an error in the data and requests the block to be retransmitted (either actively by sending an error indication or passively by not acknowledging the correct receipt of the block).

EARLY DATA-LINK PROTOCOLS

All the early protocols were " byte-orientated", that is, they saw the data as a sequence of 8-b bytes. They used special characters for control purposes (eg. to indicate the beginning and end of a block) and thus were committed to a specific character code.

One of the problems of any protocols is "data transparency". That is, the user wishes to send an arbitary data stream but the protocol itself needs to use control characters which could occur within that data stream.

There are a number of solutions to this problem:-

1) Do not use some characters for data, eg. ASCII, EBCDIC codes

2) Data Link Escape (DLE) (Character stuffing)

A special character DLE is used to indicate that th following character is to be interpreted as a control character. Thus if ETX ("end of text") is encountered in a block, not preceded by DLE, it is taken to be part of the data. If the sequence DLE EJX is found, then it is taken to indicate the end of text. If two DLEs are found, then it is assumed that only one DLE was in the data.

3) Count Field Mechanism

Used in DDCMP (Digital Data Communication Message Protocol) PROBLEM - If the count field is corrupted then this could lead to serious inefficiencies so, for this reason, protocols using a count field nearly always have a checksum appended to the block header in addition to the ordinary checksum at the end

of the block.

BIT-ORIENTED PROTOCOLS

The data-stream is considered to be a sequence of bits with no arbitrary division into bytes.

Bit-Stuffing - If the sender wishes to send a sequence of bits containing (at least) five consecutive "1" bits, it inserts a "0" bit after the five "1"s. Similarly, if the receiver detects five "1" bits followed by a "0", it removes the zero. Such an algorithm can very easily be implemented in hardware and is very efficient.

This transparency mechanism then allows sequences of more than five "1"s to be used for control purposes and bit-oriented protocols use the sequence "01111110" as a "flag" to delimit frames (and to synchronise sender and receiver).


2 - SDLC

A SDLC link can be configured as a point-to-point link or a multiple station loop configuration consisting of a primary station and a number of secondary stations.

All transmissions on a SDLC link are organized in the following form which is known as a frame.

Flag address control Information FCS Flag

2.1 - Address Field

This is a 1 byte field which identifies the secondary station(s), this can be:

The primary station does not need an address since SDLC links use Normal Response Mode (NRM). A frame from the primary to the secondary is known as a 'command' and contains the address of the secondary, a frame from the secondary is known as a response and also contains the address of the secondary station. Hence responses can only be sent as a result of a command.

2.2 - Control Field

This is an 8-bit field for Modulus 8 working or a 16-bit field for Modulus 128 working, this field identifies the frame type as follows:

Frame Types

Types of SDLC frame

I Information

I frames are numbered. The Ns count provides for numbering the frame being sent and the Nr provides acknowledgement for the I frames received. When duplex ifformation exchange is in continual process, each station reports its current Ns and/or Nr counts in each I or S frame exchanged.


Supervisory frames

RR - Receive Ready

Sent by either a primary or secondary station, RR confirms numbered frames through Nr-1 and indicates that the originating station is ready to receive.

RNR - Receive Not Ready

Sent by either a primary or secondary station, RNR indicates a temporarily busy condition due to buffering or other internal constraints.

REJ - Reject

This command/response may be transmitted to request transmission or retransmission of numbered information frames. REJ confirms frames through Nr-1 and requests the retransmission of numbered information frames starting ae Nr contained in the REJ frame.

Unnumbered Format

UI - unnumbered Information

As a command or a response, a UI frame is the vehicle for transmitting unnumbered information.

SNRM - set normal response mode

This command places the secondary station in normal response mode (NRM) for information transfer. UA is the expected response. The primary and secondary station Nr and Ns counts are reset to 0. No unsolicited transmissions are allowed from a secondary station that is in NRM. The secondary station remains in NRM until it receives a DISC or SIM command.


DISC - Disconnect

This command terminates other modes and places the receiving (secondary) staton in disconnected mode. The expected response is UA. A secondary station in disconnected mode cannot receive or transmit information or supervisory frames.


RD - Request Disconnect

This request is sent by a secondary station desiring to be disconnected (by the DISC command)

UA - Unnumbered Acknowledgment

This is the affirmative response to an SNRM, DISC or SIM command

RIM - Request Initialisation Mode

A RIM frame is transmitted by a secondary station to notify the primary station of the need for a SIM command

SIM - Set Initialisation Mode

This command initiates system-specified procedures for the purpose of initialising link-level functions. UA is the expected response. The primary and secondary Nr and Ns counts are reset to 0.

DM - Disconnected Mode

This response is transmitted by the secondary station to indicate that it is in the disconnected mode.

FRMR - Frame Reject

This response is transmitted by a secondary station in NRM only when it receives an invalid frame. A received frame may be invalid for several reasons:

TEST - test

As a command, a test frame may be sent to a secondary station in any mode to solicit a TEST response. If an information field is included with the command it is returned in the response.

XID - Exchange station Identification

As a command, XID solicits the identification of the receiving (secondary) station. An information field may be included in the frame to convey identification of the transmitting (primary) station. An XID response is required from the secondary station. An information field in the response may be used for identification of the responding secondary station.


coding of I field

Loop Commands and responses

UP - Unnumbered Poll

with P bit = 0:

An optional response poll is sent out by the primary station to poll one station, a group of stations, or all the stations on the loop. A response is not necessarily required.

with P-bit = 1:

A mandatory response poll is addressed to an individual station, group of stations, or all of the stations on a loop. It serves to perform an unnumbered poll of the addressed secondary stations. A polled station will respond either with frames it has waiting to transmit or retransmit, or, if no such frames exist, with another appropriate response (RR,RNR or DM).

CFGR - Configure

Command:

The configure command contains a function descriptor (a subcommand) in a single-byte information field.

Subcommands:

00000000 Clear
0000001x Beacon test
0000010x Monitor Mode
0000100x Wrap
0000101x Self-test
0000110x Modified Link test

RESPONSE:

The configure response is transmitted by secondary stations only in response to a configure command. The structure of the configure responses are identical to those of the configure commands. If the low order bit in the information field of the response is set to 1, the configure function in the information field has been set. If the low-order bit in the information field is set to 0, the configure function in the information field has been cleared.

BCN - Beacon - response

When the secondary station detects the loss of communication at its input, it begins to transmit a beacon response. This allows the primary station to locate the problem in the loop and to take appropriate action. In the beacon response, the F-bit can be either a 1 or a 0. As soon as the input resumes normal status the secodary stops transmitting the beacon response.

Link States

Bringing a link up

Primary __________________Secondary

------- SNRM P (command) ------>

------- SNRM P (command) ------>

------- SNRM P (command) ------>

<------ UA F (response) --------

Primary station polls with SNRM's with the P-BIT set to 1, when the secondary station is ready it responds with a UA with the P-BIT set to 1.

Information Transfer

primary to secondary -

------- I P (command) --------->

<------ RR F (response) -------

Primary can send an I frame at any time the link is up, the secondary can acknowledge this with either of RR,RNR,REJ or I frames.

secondary to primary -

------- RR P (command) --------> Poll
<------ RR F (response) -------- Nothing to send
------- RR P (command) --------> Poll
<------ I (response) -------- 1 to 7 I frames depending on
<------ I (response) -------- window size can be sent in response
<------ I F (response) -------- to poll final I frame has F bit to indicate turn round of line

Taking Line Down

Initiated by primary station -

Primary__________________ Secondary

------ DISC P (command) ------->

<----- UA F (response) --------

Initiated by secondary station -

------ RR P (command) --------->
<----- RD F (response) -------- | Request disconnect

------ DISC P (command) ------->

<----- UA F (response) --------

STANDARDS

ISO 3309 "HDLC Frame structure"

ISO 4335 "HDLC Elements of protocol"

ISO 6159 "HDLC - Elements of procedure - unbalanced classes"

ISO 6256 "HDLC - Elements of procedure - balanced classes"

CLITT X25 Level 2 (1984 revision) is known as LAP-B and this is

a balanced HDLC procedure with all the options determined.

1 - The link layer

* Link layer - could also be called, frame level

link level

frame layer

level 2

The link layer provides the following services:


.............................................

. .

. .

. ......... ......... . ------------

. .... PSE d ......... PSE f ..........-| |

----------- . . ......... ......... . | computer B |

| | ......... . . . |------------|

| computer A|--. PSE c . . . .

|-----------| ......... . . .

. . ......... .

. ............. PSE e . .

. ......... .

. .

.............................................

* At the link level each 'hop' through the network is independent.

that is -

If computer A has a packet to send to computer B

computer A 'wraps' packet into frame and sends it to PSE c

PSE c 'unwraps' the packet and decides where to send it - say PSE d

PSE c 'wraps' the packet in a completely new frame and sends it to PSE d

this continues until the packet reaches computer B

so the link header and trailer bytes will be different for each 'hop'

* Strictly standards such as X.25 only define the protocol between the network and customers computer, however in practice, the link level between two Packet Switching Exchanges will be similar or identical to X.25 level 2.

Slide LNK1 - Purpose of Link Layer

2 - Synchronisation

Bit Sychronisation

Bit synchronisation is derived by the modem from the incoming datastream.

There is a option in SDLC called Non Return to Zero Inverted (NRZI), this is designed to ensure that there are plenty of bit transitions in the data to help with bit synchronisation, this is not required by modern modems.

byte Synchronisation

Byte synchronisation is how the receiver knows where, in the serial datastream, one character stops and the next one starts.

In Asynchronous this is done by stop and start bits, this is simple but inefficient.

In Bisync it is done by the SYN character, this means that the SYN character cannot be used in normal data without using a special protocol.

In SDLC/HDLC/X.25 it is done by sending a flag:

Any sequence of six ones with a zero on either side is a flag -

eg flag = 01111110 = 'hex 7E'

* The flag has a dual purpose, it provides byte synchronisation and provides a marker of the beginning and end of a frame, so between frames there must be one or more flags.

* When there are no frames to send then flags are normally sent continuously the only exception is IBM SDLC which sometimes stops sending flags between frames.

* The sequence 01111110 could occur by chance in the data being sent which would cause a false flag to be detected. To stop this happening an extra zero is inserted after any sequence of five ones and this extra zero is removed at the distant end. This is known as 'bit stuffing'.

Slide LNK2 dividing the datastream into frames

Example 2 - flags and bit insert/delete

Analysing directly from the bits on the line is not something you will need to do in practice because all protocol testers do this automatically.

However try this example to get an insight into how the datastream is divided into frames.

Here is another example worked through

0100110111111011000000111110100110110100011011101111110110

----><------> ^ <------><---

end of this a zero flag start

last pattern following of next

frame must be five ones frame

a flag must have

wherever been inserted

it occurs for transparency

so remove it

this leaves the following,

1100000011111 1001101101000110111

divide up into bytes,

11000000 11111100 11011010 00110111

<---------------->

the sixteen bits

before the flag is

always the frame

check sequence

<------>

this is the

control byte

the LSB is

transmited

first so it

is 3F in hex

which identifies

this as a SABM

frame.

<------>

this is

03 in hex

which is

address A

Now try the example yourself - dont worry about what type of frame it is - just decode it to HEX

4 - Link States

Idle channel state - more than 15 ones

The action to be taken by a DCE upon detection of the idle channel state is not defined at this time.

active channel state - sending a frame or continuous flags

Disconnected phase - Set on DISC-UA exchange or expiry of timer T3. currently PSS polls with DISC frames in this state.

Information phase - Set by SARM-UA, SABM-UA, SABME-UA etc.

* when no data - time fill - continuous flags

- known as - active channel state

- useful test - flags shows RX line OK

Slide LNK14 - Link states

5 - Frame structure

All transmissions on a X.25/HDLC/SDLC link are organized in the following form which is known as a frame.

---------------------------------------------------

Flag | address | control | Information | FCS | Flag

|---------------------------------------------------|

8 bits 8 bits n bits 16 bits

see note 1 seeote 2

Note 1: Some recommendations define modulo 128 operation but PSS does not support these. When modulo 128 is used 16 bit control field is used for frame formats that contain sequence numbers.

Note 2: The information field is only used in some frame types. In the case of I-frames the length of the field is not restricted by the link layer, although restrictions may be imposed by higher layers in the protocol, eg maximum length or integral number of bytes.

Note 3: Addresses and control fields shall be transmitted with the low-order bit first (for example the first bit of the sequence number that is transmitted shall have the weight 2^0).

Slide LNK3 - Frame Structure

6 - Address Field

SDLC

Like other IBM protocols such as bi-sync, SDLC works in a polling environment this allows a point-to-point link or a multiple station loop configuration consisting of a primary station and a number of secondary stations.

The address field is a 1 byte field which identifies the secondary station(s), this can be:

The primary station does not need an address since SDLC links use Normal Response Mode (NRM). A frame from the primary to the secondary is known as a 'command' and contains the address of the secondary, a frame from the secondary is known as a response and also contains the address of the secondary station. Hence responses can only be sent as a result of a command.

8 - Information Field

In the case of I-frames the information field contains the packet level information, this is the point of the whole link layer - to get this information across the link safely, the contents of this field are covered in the packet layer.

In the case of FRMR frames the information field contains 3 byte s which contain information about the reason for the reject.

There is no theoretical maximum length of the I-field, the practical maximum length will depend on the error rate, if the error rate is high and the frames are long the retransmission overhead will become unacceptably high.

Parameter N1 defines the maximum length of the I-field, also the packet level allows a maximum length for packets to be negotiated. Some networks including PSS require that the I-field contain an integral number of bytes although there is nothing in the protocol that requires this.

slide LNK12 - error detection

slide LNK13 - error correction

FCS field - Error detection and recovery

During transmission bits can be corrupted due to causes such as Gaussian noise, thunderstorms at Reading exchange etc. These bit corruptions tend to happen in bursts, for example, a 10ms noise burst induced from electomechanical devices in a telephone exchange could zap 96 bits at 9600 baud.

The link layer needs to provide error free transmission in both directions, despite the possibility that any transmission could be hit. This also has to be done without slowing down the throughput.

The way this is done is to keep a copy of each frame at the sending end, while it is transmitted to the distant end with an error check, only when the receiving end sends back an acknowledgement that the frame has been received without error can the transmitting end delete its copy. This is much more efficient than duplicating or triplicating the data to correct errors.

Error detection

There are a number of possible methods of error detection:

* Parity bit on each character - This has the following disadvantages,

a) Uses 1 bit in every byte so only 7 bits are left for data, this means that it can only be used for ASCII characters without transparent 8-bit data.

b) An even number of errors will not be detected.

c) High overhead, ie 1 in 8 bits do not carry useful data.

d) There in no standard about whether even or odd parity is used.

* Longtitudinal parity byte - last byte in a block of data contains a parity byte, this still has the disadvantage that an even number of errors in any row would not be detected.

* Hamming codes - The principle of parity checks can be extended by adding more bits this can also allow errors to be corrected without being re-transmitted. This has the disadvantage of using more bits and therfore being even less efficient.

* Polynomial code - also known as a cyclic redundancy code (CRC) or frame check sequence (FCS). This uses a different form of arithmetic, the rules are the same as binary arithmetic except that there are no carries.

polynomial addition and subtraction thus become the same as 'exclusive or' on each corresponding pair of bits.

polynomial division is therefore done as follows:

1 1 0 0 0 0 1 0 1 0 <--- result

----------------------------

1 0 0 1 1 ) 1 1 0 1 0 1 1 0 1 1 0 0 0 0

1 0 0 1 1 | . . . . . . . .

--------- v

1 0 0 1 1 . . . . . . . .

1 0 0 1 1 | . . . . . . .

--------- v

0 0 0 0 1 . . . . . . .

0 0 0 0 0 | . . . . . .

--------- v

0 0 0 1 0 . . . . . .

0 0 0 0 0 | . . . . .

--------- v

0 0 1 0 1 . . . . .

0 0 0 0 0 | . . . .

--------- v

0 1 0 1 1 . . . .

0 0 0 0 0 | . . .

--------- v

1 0 1 1 0 . . .

1 0 0 1 1 | . .

--------- v

0 1 0 1 0 . .

0 0 0 0 0 | .

--------- v

1 0 1 0 0 .

1 0 0 1 1 |

--------- v

0 1 1 1 0

0 0 0 0 0

---------

0 1 1 1 0 <---- remainder

in practice this can be computed by using a shift register as follows:

msb lsb

bit-16 bit-15 bit-0

----- ------------------------------------

|-----| |------------check register----------| <-- data shifted in

| --^-- --------|

|-------------| & |--<-| + |

O

|-----| |--------

------------------------------------

|----------- crc constant -----------|

This is normally built into the SDLC/HDLC chip.

X25 link level Frame check sequence (FCS) field

The notation used to describe the FCS is based on the property of cyclic codes that a code vector such as 1000000100001 can be represented by a

12 5

polynomial P(x)=x + x + 1. The elements of an n-element code word are thus the coefficients of a polynomial of order n-1. In this application, these coefficients can have the value 0 or 1 and the polynomial operations are performed modulo 2. The polynomial representing the content of a frame is generated using the first bit received after the frame opening flag as the coefficient of the highest order term.

The FCS field shall be a 16-bit sequence. It shall be the ones complement of the sum (modulo 2) of:

k 15 14 13 12 11 10 9 8 7 6 5

1) the remainder of x (x + x + x + x + x + x + x + x + x + x + x

4 3 2

+ x + x + x + x + 1)

16 12 5

divided (modulo 2) by the generator polynomial x + x + x + 1, where k is the number of bits in the frame existing between, but not including, the final bit of the opening flag and the first bit of the FCS, excluding bits inserted for transparency, and

2) the remainder of the division (modulo 2) by the generator polynomial

16 12 5 16

x + x + x + 1 of the product of x by the content of the frame,

existing between but not including, the final bit of the opening flag and the first bit of the FCS, excluding bits inserted for transparency.

As a typical implementation, at the transmitter, the initial content of the register of the device computing the remainder of the division is preset to all 1s and is then modified by division by the generator polynomial (as described above) on the address, control and information fields; the ones compliment of the resulting remainder is transmitted as the 16-bit FCS.

At the receiver, the initial content of the register of the device computing the remainder is preset to all 1s. The final remainder, after

16

multiplication by x and then division (modulo 2) by the generator polynomial 16 12 5 of the serial incoming protected bits and the

x + x + x + 1 15 0

FCS, will be 0001110100001111 (x through x ,respectively) in the absence of transmission errors.

The 16-bit FCS is therefore generated using polynomial arithmetic as follows:

FFFF + (REM(FFFF/gen)+REM(gen/frame))

where gen = generator polynomial = 11005 hexadecimal

here is an exercise to do in the unlikely event of you having lots of time to spare: write a program to calculate the FCS.

REM integers must contain 32 bits

CRCCONST = %H1005

DEF PROCinit

DEF bitarray(128)

checkreg = %HFFFF

pointer=1

length = 16

ENDPROC

DEF PROCexor (checkreg,crcconst)

checkreg = (checkreg AND %HFFFF) XOR crcconst

ENDPROC

DEF PROCbit

checkreg=checkreg * 2 + bitarray (pointer)

IF checkreg > %HFFFF THEN PRCexor

ENDPROC

DEF PROCdivremain

FOR pointer=1 TO length

PROCbit

NEXT

PRINT "result = ",checkreg

ENDPROC

I Information frame (SDLC and HDLC)

Coding - bit: 8 7 6 5 4 3 2 1

value: N(R) P N(S) 0

modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

value: <-- N(S) ---> 0 <--- N(R) --> P

I frames are numbered. The Ns count provides for numbering the frame being sent and the Nr provides acknowledgement for the I frames received. When duplex information exchange is in continual process, each station reports its current Ns and/or Nr counts in each I or S frame exchanged.

Supervisory frames

RR - Receive Ready (SDLC and HDLC)

Coding - bit: 8 7 6 5 4 3 2 1

value: N(R) P 0 0 0 1

modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

value: 0 0 0 0 0 0 0 1 <--- N(R) --> P

Sent by either a primary or secondary station, RR confirms numbered

frames through Nr-1 and indicates that the originating station is

ready to receive.

RNR - Receive Not Ready (SDLC and HDLC)

Coding - bit: 8 7 6 5 4 3 2 1

value: N(R) P 0 1 0 1

modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

value: 0 0 0 0 0 1 0 1 <--- N(R) --> P

Sent by either a primary or secondary station, RNR indicates a

temporarily busy condition due to buffering or other internal

constraints.

REJ - Reject (SDLC and HDLC)

Coding - bit: 8 7 6 5 4 3 2 1

value: N(R) P 1 0 0 1

modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

value: 0 0 0 0 1 0 0 1 <--- N(R) --> P

This command/response may be transmitted to request transmission or

retransmission of numbered information frames. REJ confirms frames

through Nr-1 and requests the retransmission of numbered information

frames starting at the Nr contained in the REJ frame.

Unnumbered Format (SDLC only)

UI - unnumbered Information = 0 0 0 P 0 0 1 1 = X'03' or X'13'

As a command or a response, a UI frame is the vehicle for transmitting

unnumbered information.

SNRM - set normal response mode = 1 1 0 0 P 0 0 1 = X'83' or X'93'

This command places the secondary station in normal response mode

(NRM) for information transfer. UA is the expected response. The

primary and secondary station Nr and Ns counts are reset to 0. No

unsolicited transmissions are allowed from a secondary station that is

in NRM. The secondary station remains in NRM until it receives a DISC

or SIM command.

DISC - Disconnect = 0 1 0 P 0 0 1 1 = X'43' or X'53'

This command terminates other modes and places the receiving

(secondary) station in disconnected mode. The expected response is UA.

A secondary station in disconnected mode cannot receive or transmit information or supervisory frames.

RD - Request Disconnect = 0 1 0 F 0 0 1 1 = X'43' or X'53'

This request is sent by a secondary station desiring to be disconnected (by the DISC command)

UA - Unnumbered Acknowledgment = 0 1 1 F 0 0 1 1 = X'63' or X'73'

This is the affirmative response to an SNRM,DISC or SIM command

RIM - Request Initialisation Mode = 0 0 0 F 0 1 1 1 = X'07' or X'17'

A RIM frame is transmitted by a secondary station to notify the

primary station of the need for a SIM command

SIM - Set Initialisation Mode = 0 0 0 P 0 1 1 1 = X'07' or X'17'

This command initiates system-specified procedures for the purpose of

initialising link-level functions. UA is the expected response. The

primary and secondary Nr and Ns counts are reset to 0.

DM - Disconnected Mode = 0 0 0 F 1 1 1 1 = X'0F' or X'1F'

This response is transmitted by the secondary station to indicate that

it is in the disconnected mode.

FRMR - Frame Reject = 1 0 0 F 0 1 1 1 = X'87' or X'97'

This response is transmitted by a secondary station in NRM only when

it receives an invalid frame. A received frame may be invalid for

several reasons:

* its C field is not implemented at the receiving station. This

category includes unassigned commands.

* the information field is too long to fit the receiving station

buffers

* The C field in the received frame does not allow an I field to be

received with the frame.

* The Nr that was received from the primary station is invalid.

TEST - tst = 1 1 1 P 0 0 1 1 = X'E3' or X'F3'

As a command, a test frame may be sent to a secondary station in any mode to solicit a TEST response. If an information field is included with the command it is returned in the response.

XID - Exchange station Identification = 1 0 1 P 1 1 1 1 = X'AF' or X'BF'

As a command, XID solicits the identification of the receiving (secondary) station. An information field may be included in the frame to convey identification of the transmitting (primary) station. An XID response is required from the secondary station. An information field in the response may be used for identification of the responding secondary station.

Loop Commands and responses (SDLC only)

UP - Unnumbered Poll = 0 0 1 P 0 0 1 1 = X'23' or X'33'

with P bit = 0:

An optional response poll is sent out by the primary station to poll one station, a group of stations, or all the stations on the loop. A response is not necessarily required.

with P-bit = 1:

A mandatory response poll is addressed to an individual station, group of stations, or all of the stations ona loop. It serves to perform an unnumbered poll of the addressed secondary stations. A polled station will respond either with frames it has waiting to transmit or retransmit, or, if no such frames exist, with another appropriate response (RR,RNR or DM).

CFGR - Configure = 1 1 0 P 0 1 1 1 = X'C7' or X'D7'

Command:

The configure command contains a function descriptor (a subcommand) in a single- byte information field.

Subcommands:

00000000 Clear

0000001x Beacon test

0000010x Monitor Mode

0000100x Wrap

0000101x Self-test

0000110x Modified Link test

RESPONSE:

The configure response is transmitted by secondary stations only in response to a configure command. The structure of the configure responses are identical to those of the configure commands. If the low order bit in the information field of the response is set to 1, the configure function in the information field has been set. If the low-order bit in the information field is set to 0, the configure function in the information field has been cleared.

BCN - Beacon - response 1 1 1 F 1 1 1 1 = X'EF' or X'FF'

When the secondary station detects the loss of communication at its input, it begins to transmit a beacon response. This allows the primary station to locate the problem in the loop and to take appropriate action. In the beacon response, the F-bit can be either a 1 or a 0. As soon as the input resumes normal status the secondary stops transmitting the beacon response.

SDLC - procedures

Like other IBM protocols such as bi-sync, SDLC works in a polling environment this allows a point-to-point link or a multiple station loop configuration consisting of a primary station and a number of secondary stations.

Any commands sent by the host with the P bit set are polls and must be responded to by the terminal with the address specified. An I frame can be sent without the P bit, this indicates that there is more I-frame(s) to come and therefore the other end should not respond.

The P/F bit can therefore be thought of as indicating that the direction of transmission can be turned round. SDLC can therefore theorectically work

half-duplex.

SDLC - activating link

Bringing a link up (SDLC)

Primary Secondary

------- 01 SNRM P (command) ------>

primary ------- 02 SNRM P (command) ------> secondary

station stations

(host end) ------- 01 SNRM P (command) ------> (terminal

ends)

<------ 01 UA F (response) --------

------- 01 RR P (command) ------>

------- 02 SNRM P (command) ------>

This example shows the primary station polling devices 01 and 02 with

SNRM's with the P-BIT set to 1, secondary station then becomes ready it

responds with a UA with the P-BIT set to 1. station 01 will then be polled

with RRs and station 02 will continue to be polled with SNRMs.

Taking Line Down

Initiated by primary station -

Primary Secondary

------- ---------

------ DISC P (command) ------->

<----- UA F (response) --------

Initiated by secondary station -

------ RR P (command) --------->

<----- RD F (response) -------- | Request disconnect

------ DISC P (command) ------->

<----- UA F (response) --------

13 - SDLC Information Transfer

primary to secondary -

------- I P (command) --------->

<------ RR F (response) -------

Primary can send an I frame at any time the link is up, the secondary can

acknowledge this with either of RR,RNR,REJ or I frames.

secondary to primary -

------- RR P (command) --------> | Poll

|

<------ RR F (response) -------- | Nothing to

| send

------- RR P (command) --------> | Poll

|

<------ I (response) -------- | 1 to 7 I frames

| depending on

<------ I (response) -------- | window size can

| be seniin response

<------ I F (response) -------- | to poll

| final I frame has F

| bit to indicate

| turn round of line

Although X.25 frame types are similar to SDLC, commands and responses and the P-bit are used completely differently from SDLC.

The protocol is more either end can initiate transmission

\ 4 0 0 0 0 1 1 1 1

\ 3 0 0 1 1 0 0 1 1

\ 2 0 1 0 1 0 1 0 1

876 -------------------------------------------------------------

000 RR *UI RNR *RIM/SIM REJ *SRJ SARM

001 RR *UP RNR REJ *SRJ SABM

010 RR DISC RNR REJ *SRJ

011 RR UA RNR REJ *SRJ

100 RR *SNRM RNR FRMR REJ *SRJ *RSET

101 RR RNR REJ *SRJ

110 RR RNR *CGR REJ *SRJ

111 RR *TEST RNR REJ *SRJ *BCN

 

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