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SDLC |
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SDLC ( synchronous data-link
control ) is IBM's version or rendition of HDLC.
SDLC uses the unbalanced normal response mode. In addition,
it uses several options of HDLC.
Its classification scheme would place it in UN - 1, 2,
4,5,6, and 12. Moreover, SDLC uses several
variations of the HDLC scheme, and one classification
within the HDLC schema would not fully describe the
capabilities and options, of SDLC products.
The term "superset" becomes blurred when discussing SDLC, because it uses several commands which are not found in the HDLC standard. These commands and responses provide the ability to establish a loop topology and perform loop or ring polling operation. Consequently, SDLC provides support for point - to - point, multipoint, or loop configurations. The latter topology uses the group-polling features.
Because of IBM's position in the industry, we will examine the similarities and differences of SDLC and HDLC. (Please note that IBM has several products that are specifically designed to operate with HDLC systems.) Also, an example of the HDLC function sued by IBM is provided to show a different mode of operation. The specific differences between SDLC and HDLC are as follows: |
- HDLC provides an option to extend the 8-bit address field by the use of extended bytes. The purpose is to address more terminals or more groups of terminals and devices in computer. SDLC implementations support only a single-byte address filed.
- HDLC permits the control field to be extended, as well. Under the extended format option, HDLC system may have the control field extended to 16 bits. This provides for extended sequence numbers for the N(R) and N(S) fields. IBM supports only the basic 8-bit format. This becomes an important consideration when using satellite links.
- SDLC implementations restrict the information field to an even integral number of bytes. HDLC does not have this restriction.
- As previously discussed, IBM's SDLC provides some additional commands and responses for loop operation.
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The below illustration shows as example of an IBM SDLC transmission process. Station A is the primary station in control of station B and C. notice the normal response mode under full duplex permits the primary station to transmit frames to one secondary station while receiving frames from another. Illustration assumes station C's mode has been set and is currently involved in data transmission. Under normal response mode, the address in the frame always contains that of the secondary station. |
Following are the times and events for the
process depicted in above said Illustration.
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n, n + 1, 2, 3, 4, 5 station A first polls B for status. B responds by requesting an initialization mode. Station A set B to an initialization mode and then normal response mode. B acknowledges both modes. |
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n + 6 Station A uses Receive Ready command to poll station C with poll bit set. |
n + 7, 8 Station A sends frames 0 and 1 to B, while station C responds to the previous poll and sends frames 0 an d1 to A on the other channel of the full-duplex circuit. |
n + 9 Station C send information frame 2 and sets final bit. |
n + 10 Station A polls B for a check point (confirmation.) |
n + 11 Station B responds by acknowledging A's 0 and 1 frames with a receive sequence of 2. Station B also sends information frame 0. |
n + 12 Station A acknowledges C's frames 0, 1, and 2 with a Receive Ready (RR) and a receive sequence of 3. Station B sends frame 1, and sets F to 1 in response to the P bit in n + 10. |
n + 13 Station A acknowledges B's frames 0 and 1 with a Receive Ready (RR) and a receive sequence of 2. Subsequent events would require stations A and B to respond with F bits set to 1. |
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Protocol Conversion. |
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