Hello!
>From what I've read the ISO version of HDLC is bit synchronous. For the
purpuse of transmitting flag characters bit stuffing is performed. Is this
also true for Cisco HDLC? There are few resources on the web explaining
this. According to RFC1547:
4.3.1 Cisco Systems point-to-point protocols
The Cisco Systems gateway supports both asynchronous links using SLIP
and synchronous links using either simple HDLC framing, X.25 LAPB or
full X.25. The HDLC framing procedure includes a four byte header.
The first octet (address) is either 0x0F (unicast intent) or 0x8F
(multicast intent). The second octet (control byte) is left zero and
is not checked on reception. The third and fourth octets contain a
standard 16 bit Ethernet protocol type code.
>From this paragraph I can't conclude if we're doing bit or byte stuffing.
It also does not make sense to have a data link protocol without a FCS.
RFC1662:
3.1. Frame Format
A summary of the PPP HDLC-like frame structure is shown below. This
figure does not include bits inserted for synchronization (such as
start and stop bits for asynchronous links), nor any bits or octets
inserted for transparency. The fields are transmitted from left to
right.
+----------+----------+----------+
| Flag | Address | Control |
| 01111110 | 11111111 | 00000011 |
+----------+----------+----------+
+----------+-------------+---------+
| Protocol | Information | Padding |
| 8/16 bits| * | * |
+----------+-------------+---------+
+----------+----------+-----------------
| FCS | Flag | Inter-frame Fill
|16/32 bits| 01111110 | or next Address
+----------+----------+-----------------
Where character stuffing is performed in order to transmit flag
characters.
There are few resources about this on the web and elsewhere. A
clarification would be greatly appreciated. The main reason why I'm asking
is because I want to know how much possible stuffing would affect link
utilization.
Thanks!
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