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- 00:00In our OpenHPI course: A Half Century of Internet, we now consider the Ethernet
- 00:06the most essential LAN technology
- 00:10originally Ethernet was based on a bus topology
- 00:16it was developed in the early seventies last century, at Xerox
- 00:21PARC by a Metcalfe and Boggs.
- 00:24and these are the basic competence of an Ethernet
- 00:29on the one side we have sophistical media
- 00:32the carrier media, this is the Ethernet cable or only called Ether
- 00:41then a second basic component is a set of rules set of rules for
- 00:46how to deal with concurrent access to the transmission media
- 00:53and these are the CSMA /CD ruleset
- 00:59and the third basic component is a format of the data records
- 01:04to be sent to data packets. These are in that context called Ethernet frames.
- 01:11Have a closer look at each of these basic principles
- 01:16Data packets are sent via the Ethernet cable,
- 01:21so here we have the connecting cable, each of the systems is connected
- 01:28and when the sender here will send an information to a receiver then the package
- 01:36is broadcasted to all the systems that are connected to the Ethernet.
- 01:41This was the typical principle of a local area network,
- 01:46here in that case along with the Ethernet cable
- 01:54the data package is sent to all the connected computers.
- 01:59The receiver sees that the package was addressed to him, so receiver processes the package.
- 02:07All the other computers that received the network interface card detect
- 02:13that this package was not for the computer so it's discarded.
- 02:23A few rules about the CSMA/CD mechanism-
- 02:28The Ethernet multiple access algorithm:
- 02:32CSMA is Carrier Sense Multiple Access,
- 02:37CD is collision detection. This is the problem if there is
- 02:42broadcast on one cable, the data packages are broadcast along this one cable,
- 02:49the error can be collisions. So the question is how to direct them and
- 02:56it is not allowed for a participant of the network to start
- 03:01to send his package if there is a package sent to the computer.
- 03:06So when a computer sent a package, it's a broadcast mechanism.
- 03:11It is received by all the connected computers.
- 03:15so at any time, only one package can be transmitted over an Ethernet cable.
- 03:22If there are two package cables then the signals will interfere and destroy each other.
- 03:29So collision will occur and the electrical signals
- 03:36will be a super imposed or destroyed.
- 03:40With this CSMA/CD mechanism, this
- 03:46is successfully regulated.
- 03:49So concurrent send access of the computers and collision resolution are
- 03:56prevented or are dealt with this CSMA/CD ruleset.
- 04:04This ruleset consists
- 04:06as a basic rule the Carrier Sensing.
- 04:10It says a computer listens to the medium,
- 04:17to the Ethernet cable before it sends data.
- 04:21As long as there are other data transferred along this Ethernet cable,
- 04:25computer is not allowed to send a package.
- 04:30so if the media, if the Ethernet is occupied,
- 04:37computer has to wait for a random period of time. It is a
- 04:42mechanism with a bit off time to give him time till when
- 04:48he sees that there is some traffic on the cable, then he
- 04:53has to wait and then he has to soon perform
- 04:59carrier sensing and if it's free then he can send and if not
- 05:03then he has to wait till the next time.
- 05:06And the other is what happens if a very far distance computer
- 05:13listens to the cable and does not realize that there is any cable
- 05:18it looks like the cable is free and it simultaneously starts.
- 05:22So after a while say a collision is detected
- 05:26and the signals start to destroy each other, so computers have to
- 05:35listen for such collisions and if a collision has occurred
- 05:40the transmission is aborted and a so-called JAM signal is sent.
- 05:48More details you can find in our materials.
- 05:51So the Ethernet offers a connectionless service, that means
- 05:56two communication partners do not have their own connection.
- 06:01There are many computers connected and they all try the best way
- 06:08to care for communication. So the Ethernet provides a non-reliable service.
- 06:16It's not guaranteed that it works fine, it could come to a collision and then
- 06:22a retransmission is necessary.
- 06:25So the data packets include a checksum but after those
- 06:32at the receiver can see whether there are some of the signals
- 06:37was destroyed during the transport, but there is no acknowledgement mechanism
- 06:45so that the receiver sends back and acknowledges to the
- 06:48sender to say yes I got your packet in a right away.
- 06:52This is in this way a non-reliable service when we need it,
- 06:57and of course when send e-mail or when we want to see websites,
- 07:01we need such kind of service, then this acknowledgement mechanism
- 07:06has to be provided on the higher layer to take care of troubleshooting.
- 07:14The third basic of the internet was the format of such Ethernet data packets
- 07:20or an Ethernet frame.
- 07:23The data packet format is for the various different Ethernet technologies
- 07:30that occur over time. In principle, every time is the same, it starts with a preamble
- 07:37then an SFD area, the destination address, the source address
- 07:43the length of the package, and then the data which need to be transported, and then
- 07:49the checksum mechanism.
- 07:52So the Preamble is needed to tell the other computers connected
- 07:57to the cable that now a data transmission starts.
- 08:03So the Preamble is 7 or 8 bytes long.
- 08:07The 7 bytes start with such a sequence-101010
- 08:12to make clear for waking up a receiver and for synchronizing
- 08:20all the computers so that they interpret the sequence as a transmitted
- 08:25signal sequence in the same way.
- 08:28And the last byte of the Preamble ends with 10101011
- 08:33and this indicates that now the actual packet starts.
- 08:41This is the SFD- Start Frame Delimiter.
- 08:46then we have the destination and the source addresses
- 08:49as a package which makes clear who is the sender of the package
- 08:56and who is the receiver.
- 08:59We have a type field length here which in the device packet
- 09:04as a packet of the Ethernet protocol and then we have the data
- 09:09that we want to be transmitted with such a checksum.
- 09:14CRC number as checksum.
- 09:20What is a Collision Domain?
- 09:24The collision domain is a broadcast domain. A broadcast domain is when
- 09:28one sender sends the package and all the computer systems that are connected
- 09:34to this cable are the receivers of this broadcast package.
- 09:41So all hosts in the network that can receive the package by a broadcast,
- 09:49such broadcast domain, within such a broadcast domain
- 09:53there can be a collision.
- 09:55So a collision domain is when all hosts that could have a collision when sending simultaneously.
- 10:03So now we speak about an Ethernet segment and an Ethernet segment
- 10:09consists of a group of hosts connected via an Ethernet cable.
- 10:14Such an Ethernet segment is a collision domain, because here
- 10:19the broadcast message mechanism leads to the effect that when different systems start
- 10:26the transmission, then there will be such a collision occurring.
- 10:34What are the hardware components needed for such an Ethernet?
- 10:40we have the Ethernet segment as a first hardware component, it is a cable.
- 10:47The systems are connected to the cable.
- 10:53Then we have the Repeaters,
- 10:56which are connected in individual segments
- 11:00to build a larger network that operates as a singer collision domain.
- 11:05You remember the longer the cable the weaker is the signal,
- 11:11so a receiver is used to amplify and so we can prolong a system.
- 11:16Then we have Bridges to connect various physically separate collision domains
- 11:22in which parallel communication is possible.
- 11:26Then we have the Switches,
- 11:29to forward packets to the target segments of the LAN via the corresponding port.
- 11:35So switches can connect different Ethernet
- 11:40segments and can realize ok ok this package should be sent to a computer
- 11:46in the neighbouring collision domain in the neighbouring
- 11:52Ethernet segment and then the Switch are sending this out
- 11:57in this other transmission, in this other Ethernet segment.
- 12:02Then we have Router, which are necessary when there are crossings with different
- 12:09Ethernet LANs, to compute to which LAN of these
- 12:15interconnected LAN this is packet should be forwarded.
- 12:19A short look at the history of this Ethernet, because its
- 12:23long history you remember, it was it developed in the 70's last century,
- 12:29so fifty years ago and in that time of course computer technology, network technology
- 12:35has developed very fast. So the Ethernet types, different Ethernet types
- 12:41differ in terms of the medium used,
- 12:46there are different cablse: coaxial cables, twisted pair, fiber optic cable.
- 12:51There are limiting parameters which are changing, such as the number
- 12:56of connectable computers, the bandwidth, the topology.
- 13:00So a milestone in this development was 1980 wherein
- 13:0610 MBit per second Ethernet version was,
- 13:14was offered.
- 13:16Then in 1995, 15 years later, then already a 100 MBit
- 13:23per second fast Ethernet came to the market. The bandwidth,
- 13:30the number of bits that can be sent in a second of course it shows the
- 13:36power of this Ethernet. Then in 1999 the first 1 GBit
- 13:43per second Ethernet, the GigaBit Ethernet came to the market. Originally
- 13:48it was used as a backbone technology, today it's also used in Local Area Networks.
- 13:55Then the beginning of this century 2001, 10 GigaBits per second
- 14:01Ethernet came to the market, used in a Metropolitean
- 14:07Area Network and in WAN. You see in shorter and shorter time
- 14:12the amount of information that should be a transferred or such an
- 14:18network was exploding. 2014, then the first
- 14:242.5 GigaBits per second and 5 Gigabit per second Ethernet came,
- 14:30they came with cheaper cabling and hardware
- 14:34the Power Over Ethernet (PoE) was possible. So order through transmission.
- 14:39system was changed.
- 14:42So if we look to the most successful computer network
- 14:48technology world-wide, if we look to the Ethernet in a nutshell,
- 14:53Then Ethernet can connect several hundred computers within a
- 14:58radius of about 1 kilometer. There are relatively high data rates.
- 15:06A low delay due to the omission of any kind of memory and transport logic
- 15:13with this broadcast, thus is very fast
- 15:19This is a simple mechanism, simple algorithms for accessing
- 15:23transmission medias, as were the a rules we discussed. An efficient
- 15:29use and fair access to the network for all the
- 15:35computers connected to the network is possible.
- 15:39You see this in everyday life, for example here we in HPI of course
- 15:44we run such an Ethernet network that is high in reliability, no central control
- 15:51very simple very efficient, has a high stability even under heavy loads
- 15:59and this makes the Ethernet the most popular example
- 16:06of a Local Area Network.
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