HP 180 Degree Turn Product Review Download Page 24

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Hints

Q1: On fiber its difficult to deal with

asynchronous transmission, photons

cannot be buffered easily, store and

forward problems
Q2: No, autonegotiation on fiber

does not care for data rates
Q3: "normal" parallel links would be

disabled by STP, Etherchannel

supports up to 8 links

Summary of Contents for 180 Degree Turn

Page 1: ...1 2005 03 11 C Herbert Haas The Ethernet Evolution The 180 Degree Turn ...

Page 2: ...void wrapping coax around sources of strong electric or magnetic fields Do not wrap the cable around flourescent light ballasts or cyclotrons for example Ethernet Headstart Product Information and Installation Guide Bell Technologies pg 11 ...

Page 3: ...like topology Coax cables were used in baseband mode thus allowing only unicast transmissions Therefore CSMA CD was used to let Ethernet operate under the events of frequent collisions Another important point No intermediate network devices should be used in order to keep latency as small as possible Soon repeaters were invented to be the only exception for a while An Ethernet segment is a coax ca...

Page 4: ...e grade twisted pair cables already installed in buildings 10BaseT had been specified to support Cat3 cables voice grade or better for example Cat4 and today Cat5 Cat6 and Cat7 Hub devices were necessary to interconnect several stations These hub devices were basically multi port repeaters simulating the half duplex coax cable which is known as CSMA CD in a box Logically nothing has changed we hav...

Page 5: ...age per station bandwidth by 1 n On the other hand the benefit of sharing a medium for communication should be still maintained which was expressed by Metcalfe s law Bridges are store and forwarding devices introducing significant delay that can filter traffic based on the destination MAC addresses to avoid unnecessary flooding of frames to certain segments Thus bridges segment the LAN into severa...

Page 6: ...le full duplex communication on twisted pair cables different frame rates on different ports special forwarding techniques e g cut through or fragment free Content Addressable Memory CAM tables and much more Of course marketing rules demand for another designation for this machine the switch was born Suddenly a collision free plug and play Ethernet was available Simply use twisted pair cabling onl...

Page 7: ...ecent experiments demonstrated the interconnection of two Ethernet Switches over a span of more than 100 km Thus Ethernet became a WAN technology Today many carriers use Ethernet instead of ATM SONET SDH or other rather expensive technologies GE and 10GE is relatively cheap and much simpler to deploy Furthermore it easily integrates into existing low rate Ethernet environments allowing a homogeneo...

Page 8: ...d be about 25 meters which is rather impracticable for professional usage Although some countermeasures had been specified in the standard such as frame bursting and carrier extension no vendor developed an GE hub as for today Thus Forget GE Hubs The 10 GE specification does neither consider copper connections nor hubs 10 GE can only run over fiber At this point please remember the initial idea in...

Page 9: ... links by enforcing collisions to occur and hereby triggering the truncated exponential backoff algorithm Just let a collision occur and the aggressive sender will be silent for a while A much finer method is to send some dummy frames just before the backoff timer allows sending This way the other station never comes to send again Both methods are considered as ugly and only work on half duplex li...

Page 10: ...re means disconnected Several Ethernet operating modes had been defined which are incompatible to each other including different data rates 10 100 1000 Mbit s half or full duplex operation MAC control frames capabilities etc Original Ethernet utilized so called Normal Link Pulses NLPs to verify layer 2 connectivity NLPs are single pulses which must be received periodically between regular frames I...

Page 11: ...ch representing a 16 bit word GE sends several pages A series of FLPs constitute an autonegotiation frame The whole frame consists of 33 timeslots where each odd numbered timeslot consists of a real NLP and each even timeslot is either a NLP or empty representing 1 or 0 Thus each FLP sequence consists of a 16 bit word Note that GE Ethernet sends several such pages ...

Page 12: ...5B block coding over only two pairs of regular Cat 5 twisted pair cables or two strand 50 125 or 62 5 125 Fm multimode fiber optic cables 100Base4T signaling has been specified to support 100 Mbit s over Cat3 cables This mode allows half duplex operation only and uses a 8B6T code over 4 pairs of wires one pair for collision detection three pairs for data transmission One unidirectional pair is use...

Page 13: ...f the 4B5B block coding principle which is used by 802 3u and also by FDDI The basic idea is to transform any arbitrary 4 bit word into a relatively balanced 5 bit word This is done by a fast table lookup Balancing the code has many advantages better bandwidth utilization better laser efficiency constant temperature better bit synchronization PLL etc Note that the signaling overhead is 5 4 12 5 ...

Page 14: ...in March 1996 by the working group IEEE 802 3z The GMII represents a abstract interface between the common Ethernet layer 2 and different signaling layers below Two important signaling techniques had been defines The standard 802 3z defines 1000Base X signaling which uses 8B10B block coding and the 802 3ab standard uses 1000Base T signaling The latter is only used over twisted pair cables UTP Cat ...

Page 15: ...ex MAC PMD IEEE 802 3 Ethernet ANSI X3T11 Fibre Channel IEEE 802 3z Gigabit Ethernet Reconciliation Sublayer PHY Gigabit Ethernet layers have been defined by adaptation of the LLC and MAC layers of classical Ethernet and the physical layers of the ANSI Fiber Channel technology A so called reconciliation layer is used in between for seamless interoperation The physical layer of the Fiber Channel te...

Page 16: ...d 10 bit codewords Actually there are not enough balanced 10 bit codewords available Note that there are 256 8 bit codewords which need to be mapped on 1024 10 bit codewords But instead of using a fully balanced 10 bit codeword for each 8 bit codeword some 8 bit codewords are represented by two 10 bit codewords which are sent in an alternating manner That is both associated 10 bit words are bit co...

Page 17: ...nsmitted over various types of fiber Currently at least two types are specified short and long wave transmissions using 850 nm and 1300 nm respectively The long wave can be used with both single mode SMF and multimode fibers MMF Only SMF can be used for WAN transmissions because of the much lower dispersion effects Note that there are several other implementations offered by different vendors such...

Page 18: ...ansmission techniques ensure that this high data rate can be transmitted over a UTP Cat5 cable For example all 4 pairs are used together for both directions Echo cancellation ensures that the sending signal does not confuse the received signal 5 level PAM is used for encoding instead of 8B10B because of its much lower symbol rate Now we have only 125 Mbaud x 4 instead of 1250 Mbaud The interface d...

Page 19: ...PLS Physical Layer Signaling MDI Medium Dependent Interface PCS Physical Coding Sublayer MII Media Independent Interface GMII Gigabit Media Independent Interface PMA Physical Medium Attachment MAU Medium Attachment Unit PMD Physical Medium Dependent The diagram above shows various physical media designs supported by the official GE standard Each modern GE card could theoretically support the old 1...

Page 20: ...layer anymore because optical switching at that bit rate only allows synchronous transmissions The GMII has been replaced or enhanced by the so called XAUI known as Zowie Note At the time of writing this module the 10 GE standard was not fully finished Though some vendors already offer 10 GE interface cards for their switches These interfaces are very expensive but the investment ensures backward ...

Page 21: ...use GE over copper wires anymore although 802 3ab would specify it This is because the whole electrical hardware cables and connectors are re used from older Ethernet technologies and have not been designed to support such high frequencies For example the RJ45 connector is not HF proof Furthermore shielded twisted pair cables require a very good grounding seldom found in reality The Bit Error Rate...

Page 22: ... 11 Summary Ethernet evolved in the opposite direction Collision free WAN qualified Switched Several coding styles Complex PHY architecture Plug play through autonegotiation Much simpler than ATM but no BISDN solution might change ...

Page 23: ...as 2005 03 11 Quizz Why tends high speed Ethernet to synchronous PHY Can I attach a 100 Mbit s port to a 1000 Mbit s port via fiber What is the idea of Etherchannels Maximum bit rate difference to multiple parallel links ...

Page 24: ...difficult to deal with asynchronous transmission photons cannot be buffered easily store and forward problems Q2 No autonegotiation on fiber does not care for data rates Q3 normal parallel links would be disabled by STP Etherchannel supports up to 8 links ...

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