Just so I make sure I'm not lost, a bit-sync. protocol is one that has predefined fields that signify SOF/delimiters/protocol type (like Ethernet) and a bi-sync. protocol does not? It just sends characters, and after a predetermined number of bytes have been sent receives an ack of some sort (L2). Do bi-synch protocols have fields?
If a L2 protocols sends acks, does that make it a bi-synch protocol? So, wouldn't PPP still be a bit-sync. protocol because of the fields it does have fields (address, control, etc,.)? Or am I confused. ""Priscilla Oppenheimer"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > At 06:53 PM 2/6/02, s vermill wrote: > > >I wonder if you, and others, would comment on ppp as a character-oriented > >protocol. I did a search on the internet and found some university teaching > >papers that characterize synchronous ppp as character-oriented while at the > >same time acknowleging the fact that it is based on the bit-synchronous HDLC > >frame format. I guess the LCP is the culprit? > > PPP has many components, including framing (encapsulation), LCP, and NCP. > It's best to consider the components separately. The short answer to your > question, though, is that all the components are character-oriented > (byte-oriented). In other words, they identify their operations with entire > bytes, rather than using bits within bytes. And here's the long answer: > > PPP encapsulation is based on HDLC framing, except that PPP adds two bytes > for a Protocol field. The Protocol field is not a bit-oriented field. The > two bytes are taken together to mean IP, AppleTalk, DECnet, etc. > > Also, the other fields in the PPP header, even though based on HDLC, are > also not really bit-oriented because PPP doesn't do much compared to other > HDLC derivatives. (sort of like Cisco's HDLC which also doesn't do much) > > Take the Control field, for example. In PPP, it's always a single byte that > contains the binary sequence 00000011, which calls for transmission of user > data in an unsequenced frame. > > Other HDLC-based protocols allow for a few different binary values for this > field and, in fact, the field can be two bytes. These protocols can send > Supervisory, Information (with sequence numbers), and Unnumbered Frames. > They offer reliability and are bit-oriented. LLC2 is an example. Other > examples are LAPB and LAPD. > > PPP acts like LLC1 and doesn't really do much and doesn't really need to be > bit-oriented, (with the exception that PPP devices have to know the one > valid binary value of the Control field.) > > Now, as far as LCP is concerned.... It's in the control plane. It's pretty > complex, but you could learn more about it by turning on debug ppp or > reading RFC 1548. > > Per RFC 1548, "The LCP is used to automatically agree upon the > encapsulation format options, handle varying limits on sizes of packets, > authenticate the identity of its peer on the link, determine when a link is > functioning properly and when it is defunct, detect a looped-back link and > other common misconfiguration errors, and terminate the link." > > Despite the complexity, the frame format for LCP appears to be simple and > character-oriented. Here's probably way more than you ever wanted to know: > > The Code field is one octet and identifies the kind of LCP packet. This > specification concerns the following values: > > 1 Configure-Request > 2 Configure-Ack > 3 Configure-Nak > 4 Configure-Reject > 5 Terminate-Request > 6 Terminate-Ack > 7 Code-Reject > 8 Protocol-Reject > 9 Echo-Request > 10 Echo-Reply > 11 Discard-Request > > Identifier > > The Identifier field is one octet and aids in matching requests and > replies. When a packet is received with an invalid Identifier field, the > packet is silently discarded. > > Length > > The Length field is two octets and indicates the length of the LCP packet > including the Code, Identifier, Length and Data fields. Octets outside the > range of the Length field are treated as padding and are ignored on > reception. When a packet is received with an invalid Length field, the > packet is silently discarded. > > Data > > The Data field is zero or more octets as indicated by the Length field. The > format of the Data field is determined by the Code field. > > > > And we haven't even gotten to the other major component of PPP, the NCP > part. ;-) > > Priscilla > > > > > > ________________________ > > Priscilla Oppenheimer > http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=34762&t=34629 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
