9 Replies Latest reply: Jun 14, 2009 8:14 AM by Chris RSS

    Serial Interfaces




      If I'm correct, ethernet is a type of serial communication (e.g. one wire within the UTP cable to send data and one wire to receive data). Therefore, an ethernet interface is a serial interfae. So why only WAN interfaces are called "serial" interfaces ?





        • 1. Re: Serial Interfaces

          Hi Harris,


          Technically you're correct.  It's serial transmission due to the fact that each bit is put onto the wire one bit at a time and in sequence vs. parallel transmission which is what the PCs system bus is or printer cables.  The distinction is there simply to have an differention when discussing LANs vs. WANs.  Networking topics/definitions when residing at the same OSI layer must have a distinct connotation; otherwise we wouldn't be able to differentiate one technology from another, or make the proper distinctions when learning/discussing their differences.



          • 2. Re: Serial Interfaces
            Chris Buscemi

            I agree.


            Even though they both communicate in the same Serial manner, it is just easier to refer to the style of ports with different nomenclature, otherwise you would always have to specify which type of serial communication you are using.




            • 3. Re: Serial Interfaces

              Hi Harris


              You are correct it is a strange term.

              Image a serial port would be connected to another serial port by a serial cable so it is 1 to 1.

              Ethernet originally was a long piece of co-axial cable which daisy chained computers together but it was 1 to n because the PC tapped into the cable so the signal was presented to everybody. There has been much water under the bridge since those days and even the term LAN and WAN may need to be reviewed.


              Regards Conwyn

              • 4. Re: Serial Interfaces
                Scott Morris - CCDE/4xCCIE/2xJNCIE

                heheheh...   i'd agree this ranks in  a strange question.


                With switches, while your connection may APPEAR serial in nature (point to point to point to point) the medium itself is not designed in that fashion and allows for other connectivity.


                Following the generic definition of serial communications though, I suppose it does follow the logic.  But I've never considered it that way. 


                Shake the foundations...  It's always fun!



                • 5. Re: Serial Interfaces
                  Priscilla Oppenheimer

                  WANs and LANs come from two different worlds. WANs came first and back then there was a bigger focus on the physical layer, including whether to use multiple wires (parallel) or a single wire (serial). When LANs came out most of the discussion focused on how to make them sharable by many stations.


                  Yes, Ethernet uses serial transmission. But nobody made a big deal out of this. I checked my ancient copy of IEEE 802.3 from 1985 and it does mention that Ethernet uses serial communications, but it doesn't discuss it in any detail. There's much more focus on CSMA/CD and the frame format. Token Ring used serial communications too, but there's little mention of it in the standard which instead focuses on the protocol for token passing.


                  In the WAN world and the SNA world people made a bigger deal out of serial versus parallel communications. If you look at old telecommunications and data communications books, somewhere in the first couple chapters there will be a discussion about these characteristics of links:


                  parallel versus serial

                  analog versus digital

                  asynch versus synch versus isochronous

                  simplex, half duplex, full duplex

                  line coding (NRZ, B8ZS, HDB3, etc.)


                  If you look at old LAN books, you don't see any of that.


                  I think it's still a good discussion to have. Is parallel communications used these days just for ancient printers and for buses on the mother board of a single computer? Couldn't we consider Wave-Division Multiplexing (WDM) parallel communications? It sends multiple signals on a single fiber-optic cable using different wavelengths. In wireless networks, is Multiple Input/ Multiple Output (MIMO) parallel communications?


                  Anyway, to make a long story short: it's a tad silly that Cisco calls their WAN interfaces serial interfaces. Talk to a non-Cisco person and they'll think you're babbling about Cheerios or Wheeties (cereal) until you sheepishly admit that Cisco calls their WAN interfaces "serial interfaces" and their LAN interfaces e, fa, and gi (or to :-)



                  • 6. Re: Serial Interfaces

                    Hi Priscilla,


                    Great post!!!


                    Is WDM using multiple light waves to transfer the same frame/conversation(akin to a stream of parallel transmission), or is it transmitting the frames of multiple distinct transmissions?


                    This thread is getting very interesting!



                    • 7. Re: Serial Interfaces

                      Hi Vlan


                      Simply WDM sends a wave of data stream one and then a second wave of data stream two .....     and then a final wave by which time it is time for the second occurence of the first wave. Imagine a three lane motorway but only one custom post at each end. The cars arrive at the speed the customs official can process them but once on the motor way they travel in their own lane.  So rather than 155Mb you can get 256 * 155Mb = OC768-STM256=38,486Mb/s


                      Regards Conwyn

                      • 8. Re: Serial Interfaces

                        Hi Conwyn,


                        Great explanation!


                        However, am I riding in one car, in one lane, i.e. same frame/one wave akin to serial, or am I riding in more than one car at the same time on multiple lanes, i.e. same singular data frame/stream traversing multiple waves/lanes as in parallel?



                        • 9. Re: Serial Interfaces
                          Priscilla Oppenheimer

                          Oh, good point. :-) Since WDM is multiple streams of traffic, perhaps it's not really similar to parallel communications which is one stream spread across multiple channels. OK, for some other modern examples of parallel communications, check out this quote from Wikepedia:


                          There has been a resurgence of parallel data links in RF communication. Rather than transmitting one bit at a time (as inMorse code and BPSK), well-known techniques such as PSM, PAM, and Multiple-input multiple-output communication send a few bits in parallel. (Each such group of bits is called a "symbol"). Such techniques can be extended to send an entire byte at once (256-QAM). More recently techniques such as OFDM have been used in Asymmetric Digital Subscriber Line to transmit over 224 bits in parallel, and in DVB-T to transmit over 6048 bits in parallel.


                          OK, off to do more typical Sunday things. :-)