硬件实现SDLC控制器的相关背景知识
1. SDLC介绍
Although a subt of HDLC, SDLC was developed by IBM before HDLC, and was the first link layer protocol bad on synchronous, bit-oriented operation. IBM defined SDLC for managing synchronous rially transmitted bits over a data link, and the links can be full/half-duplex, switched or unswitched, point-to-point, point to multipoint or even looped. SDLC is designed for carrying SNA traffic.
In SDLC, a link station is a logical connection between adjacent nodes. Only one Primary Link Station is allowed on an SDLC line. A device can be t up as a Primary or a Secondary link station. A device configured as a Primary link station can communicate with both PU 2.0 nodes and PU 2.1 nodes (APPN) and controls the condary devices. If the device is t up as a condary link station then it acts as a PU 2.0 device and can communicate with Front End Processors (FEP), but only communicates with the primary device when the primary allows it, i.e. the primary ts up and tears down the connections
and controls the condaries.
铃声响起
In APPN configurations the device can support negotiable link stations where XID frames are exchanged to decide which station is to be condary and which is to be primary.
A primary station issues commands, controls the link and initiates error-recovery. A device t up as a condary station can communicate to a FEP, exist with other condary devices on an SDLC link and exist as a condary PU 2.0 device.
SDLC supports line speeds up to 64Kb/ V.24 (RS-232) at 19.2Kb/s, V.35 (up to 64Kb/s) and X.21.
征兵体重标准
The following diagram shows the frame format for SDLC, almost identical to HDLC.
∙ Flag - Begins and ends the error checking procedure with 0x7E which is 01111110 in binary.
∙ Address - This is only the condary address since all communication occurs via the single primary device. The address can be an individual, group or broadcast address.
∙ Control - this identifies the frame's function and can be one of the following:
o Information (I) - contains the Send Sequence Number which is the number of the next frame to be nt, and the Receive Sequence Number which is the number of the next frame expected to be received. The is also a Poll Final Bit (P/F) which performs error checking.
o Supervisory (S) - this can report on status, ask for and stop transmission and acknowledge I frames.
o Unnumbered (U) - this does not have quence numbers (hence 'unnumbered'), it can be ud to start up condaries and can sometimes have an Information field.
发热的症状
∙ Data - can contain Path Information Unit (PIU) or Exchange Identification (XID).
∙ Frame Check Sequence (FCS) - this check is carried out on the nding AND receiving of the frame.
In a poll, the address field identifies the station being polled, in a respon, the address field contains the station transmitting, so this field effectively is the condary station's address. The control field.
蒸馒头的做法和技巧
2. SDLC/HDLC投标书比特传送顺序
a) 从左到右顺序传送帧字节；
b) 每个字节先传送LSB；（MSB到LSB四叶草简笔画是从左到右）
c) FCS是两个字节， 也是先传送低8位，再传送高盐酸川芎嗪8位，先传LSB。（满江红的诗The FCS is transmitted least significant octet first, which contains the coefficient of the highest term.）
d) FCS的计算方法参考RFC1662中的例程。
