by Gerald Boerner
[Continued… ] Ethernet has become the standard for Local Area Networking. Please Note that this part covers some of the more detailed (and tedious) aspects of Ethernet. Yes, it is necessary to configure a network correctly, but it is more for technicians than for the home user!
But is was not always so; in the early days of networking, mainframe computers would use this technology, but it was expensive, slow and not supported by a major vendor. IBM was pushing its Token Ring technology while office networks often used ARCNet because it was less expensive. But 3Com developed this technology into a viable player, who has become the dominant technology in this area.
The old saying when dealing with networks is this: “In a network, you need to consider speed, cost, and reliability. But you can only have two!” Ethernet provides an inexpensive network that transfers data reliably. In recent specifications, it has also achieved speeds into the Gbit/second range.
Be aware that this technology was the enabling technology for Local Area Networks which today pervades our offices and homes. The topic is so large that this is the third installment of four parts. GLB
“That was actually on the drawing board for MCI last year. The current Ethernet capabilities that MCI has do not allow for dynamic bandwidth.”
— Lisa Pierce
“We are looking to leverage technology, specifically Time Warner Telecom’s metro Ethernet services, to deliver exceptional service to our patients. Both from a quality and administrative standpoint, I believe these services will help us achieve that goal.”
— Rod Roeser
“We are looking to leverage technology, specifically Time Warner Telecom’s metro Ethernet services, to deliver exceptional service to our patients. Both from a quality and administrative standpoint, I believe these services will help us achieve that goal.”
— Rod Roeser
“Since our official launch earlier this month, numerous companies have decided to join the Ethernet Alliance. We are pleased to see the value proposition of the alliance being so well recognized by so many companies, and look forward to welcoming many more new members in 2006 and beyond.”
— Blaine Kohl
“Cisco has laid out a vision and architecture for Carrier Ethernet that provides service providers a broad selection of flexible options across all network domains. Cisco announced products today that deliver a number of important network services at cost-effective price points, adding to Cisco’s already large portfolio.”
— Michael Howard
“As organizations continued to build out and upgrade their aging infrastructure, they spent more than $19 billion on enterprise routers and switches last year. Overall, the market for enterprise networking gear remains healthy, with Ethernet switches doing particularly well.”
— Matthias Machowinski
“Based on our recent service provider studies, we see an intertwining of MPLS and Ethernet — most providers of metro services are using or planning MPLS as a standard approach for augmenting and scaling their Layer 2 services. A cost-effective solution that has a mix of Layer 2, routing, and MPLS capabilities will help give these service providers the flexibility to expand their customer reach and increase revenue.”
— Michael Howard
“Andy is one of the most experienced individuals in telecommunications sales. He has a proven track record and has consistently demonstrated the highest degree of integrity and professionalism while generating great successes for the companies he has worked for, many of which have become major players in our space. We are confident that Andy’s leadership and expertise will help drive our global sales force to further permeate the market with World Wide Packet’s cutting edge Carrier Ethernet products.”
— Dave Curry
Wizards of the Internet: Ethernet, Part 3
Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model, through means of network access at the Media Access Control protocol (a sub-layer of Data Link Layer), and a common addressing format.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980 to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.
Physical layer
The first Ethernet networks, 10BASE5, used thick yellow cable with vampire taps as a shared medium (using CSMA/CD). Later, 10BASE2 Ethernet used thinner coaxial cable (with BNC connectors) as the shared CSMA/CD medium. The later StarLAN 1BASE5 and 10BASE-T used twisted pair connected to Ethernet hubs with 8P8C (RJ45) modular connectors.
Currently Ethernet has many varieties that vary both in speed and physical medium used. The most common forms used are 10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize Category 5 cables and 8P8C modular connectors. They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively.
Fiber optic variants of Ethernet are commonly used in structured cabling applications. These variants have also seen substantial penetration in enterprise datacenter applications, but are rarely seen connected to end user systems for cost/convenience reasons. Their advantages lie in performance, electrical isolation and distance (up to tens of kilometers with some versions). 10 gigabit Ethernet is becoming more popular in both enterprise and carrier networks, with development starting on 40 Gbit/s and 100 Gbit/s Ethernet. Metcalfe now believes commercial applications using terabit Ethernet may occur by 2015 though he says existing Ethernet standards may have to be overthrown to reach terabit Ethernet.
A data packet on the wire is called a frame. A frame viewed on the actual physical wire would show Preamble and Start Frame Delimiter, in addition to the other data. These are required by all physical hardware. They are not displayed by packet sniffing software because these bits are removed by the Ethernet adapter before being passed on to the host.
Ethernet Frame Types and the EtherType Field
There are several types of Ethernet frames:
In addition, all four Ethernet frames types may optionally contain a IEEE 802.1Q tag to identify what VLAN it belongs to and its IEEE 802.1p priority (quality of service). This encapsulation is defined in the IEEE 802.3ac specification and increases the maximum frame by 4 bytes to 1522 bytes.
The different frame types have different formats and MTU values, but can coexist on the same physical medium.
The most common Ethernet Frame format, type II
Versions 1.0 and 2.0 of the Digital/Intel/Xerox (DIX) Ethernet specification have a 16-bit sub-protocol label field called the EtherType. The new IEEE 802.3 Ethernet specification replaced that with a 16-bit length field, with the MAC header followed by an IEEE 802.2 logical link control (LLC) header. The maximum length of a frame was 1518 bytes for untagged (1522 for 802.1p or 802.1q tagged) classical Ethernet v2 and IEEE802.3 frames. The two formats were eventually unified by the convention that values of that field between 64 and 1522 indicated the use of the new 802.3 Ethernet format with a length field, while values of 1536 decimal (0600 hexadecimal) and greater indicated the use of the original DIX or Ethernet II frame format with an EtherType sub-protocol identifier. This convention allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.3 frame, allowing the coexistence of both standards on the same physical medium. See also Jumbo Frames.
By examining the 802.2 LLC header, it is possible to determine whether it is followed by a SNAP (subnetwork access protocol) header. Some protocols, particularly those designed for the OSI networking stack, operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented network services. The LLC header includes two additional eight-bit address fields, called service access points or SAPs in OSI terminology; when both source and destination SAP are set to the value 0xAA, the SNAP service is requested. The SNAP header allows EtherType values to be used with all IEEE 802 protocols, as well as supporting private protocol ID spaces. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.
Frame Types that are outside of the mainstream…
Novell’s "raw" 802.3 frame format was based on early IEEE 802.3 work. Novell used this as a starting point to create the first implementation of its own IPX Network Protocol over Ethernet. They did not use any LLC header but started the IPX packet directly after the length field. This does not conform to the IEEE 802.3 standard, but since IPX has always FF at the first two bytes (while in IEEE 802.2 LLC that pattern is theoretically possible but extremely unlikely), in practice this mostly coexists on the wire with other Ethernet implementations, with the notable exception of some early forms of DECnet which got confused by this.
Novell NetWare used this frame type by default until the mid nineties, and since Netware was very widespread back then, while IP was not, at some point in time most of the world’s Ethernet traffic ran over "raw" 802.3 carrying IPX. Since Netware 4.10, Netware now defaults to IEEE 802.2 with LLC (Netware Frame Type Ethernet_802.2) when using IPX. (See "Ethernet Framing" in References for details.)
Mac OS uses 802.2/SNAP framing for the AppleTalk V2 protocol suite on Ethernet ("EtherTalk") and Ethernet II framing for TCP/IP.
The 802.2 variants of Ethernet are not in widespread use on common networks currently, with the exception of large corporate Netware installations that have not yet migrated to Netware over IP. In the past, many corporate networks supported 802.2 Ethernet to support transparent translating bridges between Ethernet and IEEE 802.5 Token Ring or FDDI networks. The most common framing type used today is Ethernet Version 2, as it is used by most Internet Protocol-based networks, with its EtherType set to 0×0800 for IPv4 and 0x86DD for IPv6.
There exists an Internet standard for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers. It is almost never implemented on Ethernet (although it is used on FDDI and on token ring, IEEE 802.11, and other IEEE 802 networks). IP traffic cannot be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is an LLC protocol type for IP, there is no LLC protocol type for ARP. IP Version 6 can also be transmitted over Ethernet using IEEE 802.2 with LLC/SNAP, but, again, that’s almost never used (although LLC/SNAP encapsulation of IPv6 is used on IEEE 802 networks).
The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two bytes of the tag are the Tag Protocol Identifier (TPID) value of 0×8100. This is located in the same place as the EtherType/Length field in untagged frames, so an EtherType value of 0×8100 means the frame is tagged, and the true EtherType/Length is located after the Q-tag. The TPID is followed by two bytes containing the Tag Control Information (TCI) (the IEEE 802.1p priority (quality of service) and VLAN id). The Q-tag is followed by the rest of the frame, using one of the types described above.
Runt Frames
A runt frame is an Ethernet frame that is less than the IEEE 802.3 minimum length of 64 bytes. Possible causes are collision, underruns, bad network card or software.
References:
Katie Hafner & Matthew Lyon. (1998) Where Wizards Stay Up Late: The Origins of the Internet. Simon & Schuster
Background and biographical information is from Wikipedia articles on:
Wikipedia: ARPANet…
http://en.wikipedia.org/wiki/ARPAnet
Wikipedia: The Internet…
http://en.wikipedia.org/wiki/The_Internet
Wikipedia: Ethernet…
http://en.wikipedia.org/wiki/Ethernet
Wikipedia: OSI Model…
http://en.wikipedia.org/wiki/OSI_Model
Web Sites and Blogs:
ThinkExist.com: Ethernet Quotes…
http://thinkexist.com/search/searchQuotation.asp?search=Ethernet
How Stuff Works: How OSI Works/The Layers…
http://computer.howstuffworks.com/osi1.htm
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