Ethernet

The IEEE 802.3 standard defines ethernet at the physical and data link layers of the OSI network model. Most
ethernet systems use the following:

• Carrier-sense multiple-access with collision detection (CSMA/CD) for controlling access to the network media.
• Use baseband broadcasts
• A method for packing data into data packets called frames
• Transmit at 10Mbps, 100Mbps, and 1Gbps.

Types of Ethernet

• 10Base5 - Uses Thicknet coaxial cable which requires a transceiver with a vampire tap to connect each computer. There is a drop cable from the transceiver to the Attachment Unit Interface (AIU). The AIU may be a DIX port on the network card. There is a transceiver for each network card on the network. This type of ethernet is subject to the 5-4-3 rule meaning there can be 5 network segments with 4 repeaters, and three of the segments can be connected to computers. It uses bus topology. Maximum segment length is 500 Meters with the maximum overall length at 2500 meters. Minimum length between nodes is 2.5 meters. Maximum nodes per segment is 100.
• 10Base2 - Uses Thinnet coaxial cable. Uses a BNC connector and bus topology requiring a terminator at each end of the cable. The cable used is RG-58A/U or RG-58C/U with an impedance of 50 ohms. RG-58U is not acceptable. Uses the 5-4-3 rule meaning there can be 5 network segments with 4 repeaters, and three of the segments can be connected to computers. The maximum length of one segment is 185 meters. Barrel connectors can be used to link smaller pieces of cable on each segment, but each barrel connector reduces signal quality. Minimum length between nodes is 0.5 meters.
• 10BaseT - Uses Unshielded twisted pair (UTP) cable. Uses star topology. Shielded twisted pair (STP) is not part of the 10BaseT specification. Not subject to the 5-4-3 rule. They can use category 3, 4, or 5 cable, but perform best with category 5 cable. Category 3 is the minimum. Require only 2 pairs of wire. Cables in ceilings and walls must be plenum rated. Maximum segment length is 100 meters. Minimum lengthbetween nodes is 2.5 meters. Maximum number of connected segments is 1024. Maximum number of  nodes per segment is 1 (star topology). Uses RJ-45 connectors.
• 10BaseF - Uses Fiber Optic cable. Can have up to 1024 network nodes. Maximum segment length is 2000 meters. Uses specialized connectors for fiber optic. Includes three categories:

1. 10BaseFL - Used to link computers in a LAN environment, which is not commonly done due to high cost.
2. 10BaseFP - Used to link computers with passive hubs to get cable distances up to 500 meters.
3. 10BaseFB - Used as a backbone between hubs.

• 100BaseT - Also known as fast ethernet. Uses RJ-45 connectors. Topology is star. Uses CSMA/CD media access. Minimum length between nodes is 2.5 meters. Maximum number of connected segments is 1024. Maximum number of nodes per segment is 1 (star topology). IEEE802.3 specification.

1. 100BaseTX - Requires category 5 two pair cable. Maximum distance is 100 meters.
2. 100BaseT4 - Requires category 3 cable with 4 pair. Maximum distance is 100 meters.
3. 100BaseFX - Can use fiber optic to transmit up to 2000 meters. Requires two strands of fiber optic cable.

• 100VG-AnyLAN - Requires category 3 cable with 4 pair. Maximum distance is 100 meters with cat 3 or 4 cable. Can reach 150 meters with cat 5 cable. Can use fiber optic to transmit up to 2000 meters. This ethernet type supports transmission of Token-Ring network packets in addition to ethernet packets. IEEE 802.12 specification. Uses demand-priority media access control. The topology is star. It uses a series of interlinked cascading hubs. Uses RJ-45 connectors.

The IEEE naming convention is as follows:

1. The transmission speed in Mbps

2. Baseband (base) or Broadband data transmission

3. The maximum distance a network segment could cover in hundreds of meters.

Comparisons of some ethernet types. distances are in meters.

Ethernet Type Cable                      Min length between nodes Max Segment length Max overall length

10Base2                   Thinnet 0.5             185            925

10Base5                   Thicknet 2.5           500            2500

10BaseF                   Fiber                      2000

10BaseT                   UTP 2.5                 100

Types of ethernet frames

• Ethernet 802.2 - These frames contain fields similar to the ethernet 802.3 frames with the addition of three Logical Link Control (LLC) fields. Novell NetWare 4.x networks use it.
• Ethernet 802.3 - It is mainly used in Novell NetWare 2.x and 3.x networks. The frame type was developed prior to completion of the IEEE 802.3 specification and may not work in all ethernet environments.
• Ethernet II - This frame type combines the 802.3 preamble and SFD fields and include a protocol type field where the 802.3 frame contained a length field. TCP/IP networks and networks that use multiple protocols normally use this type of frames.
• Ethernet SNAP - This frame type builds on the 802.2 frame type by adding a type field indicating what network protocol is being used to send data. This frame type is mainly used in AppleTalk networks.

The packet size of all the above frame types is between 64 and 1,518 bytes.

Ethernet Message Formats

The ethernet data format is defined by RFC 894 and 1042. The addresses specified in the ethernet protocol are 48 bit addresses.

The types of data passed in the type field are as follows:

1. 0800 IP Datagram

2. 0806 ARP request/reply

3. 8035 RARP request/reply

There is a maximum size of each data packet for the ethernet protocol. This size is called the maximum

transmission unit (MTU). What this means is that sometimes packets may be broken up as they are passed through networks with MTUs of various sizes. SLIP and PPP protocols will normally have a smaller MTU value than ethernet. This document does not describe serial line interface protocol (SLIP) or point to point protocol (PPP) encapsulation.

IEEE 802 Standard

The Data Link Layer and IEEE

When we talk about Local Area Network (LAN) technology the IEEE 802 standard may be heard. This

standard defines networking connections for the interface card and the physical connections, describing how they are done. The 802 standards were published by the Institute of Electrical and Electronics Engineers (IEEE). The 802.3 standard is called ethernet, but the IEEE standards do not define the exact original true ethernet standard that is common today. There is a great deal of confusion caused by this. There are several types of common ethernet frames. Many network cards support more than one type.

The ethernet standard data encapsulation method is defined by RFC 894. RFC 1042 defines the IP to link

layer data encapsulation for networks using the IEEE 802 standards. The 802 standards define the two

lowest levels of the seven layer network model and primarily deal with the control of access to the

network media. The network media is the physical means of carrying the data such as network cable. The

control of access to the media is called media access control (MAC). The 802 standards are listed below:

•  802.1 - Internetworking
•  802.2 - Logical Link Control *
•  802.3 - Ethernet or CSMA/CD, Carrier-Sense Multiple Access with Collision detection LAN *
•  802.4 - Token-Bus LAN *
•  802.5 - Token Ring LAN *
•  802.6 - Metropolitan Area Network (MAN)
•  802.7 - Broadband Technical Advisory Group
•  802.8 - Fiber-Optic Technical Advisory Group
•  802.9 - Integrated Voice/Data Networks
•  802.10 - Network Security
•  802.11 - Wireless Networks
•  802.12 - Demand Priority Access LAN, 100 Base VG-AnyLAN

*The Ones with stars should be remembered in order for network certification testing.

Network Access Methods

There are various methods of managing access to a network. If all network stations tried to talk at once, the messages would become unintelligible, and no communication could occur. Therefore a method of being sure that stations coordinate the sending of messages must be achieved. There are several methods listed below which have various advantages and disadvantages.

 Contention

1.  Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) - Used by Ethernet
2.  Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)

•  Token Passing - A token is passed from one computer to another, which provides transmission permission.
•  Demand Priority - Describes a method where intelligent hubs control data transmission. A computer will send a demand signal to the hub indicating that it wants to transmit. The hub sill respond with an acknowledgement that will allow the computer to transmit. The hub will allow computers to transmit in turn. An example of a demand priority network is 100VG-AnyLAN (IEEE 802.12). It uses a star-bus topology.
•  Polling - A central controller, also called the primary device will poll computers, called secondary devices, to find out if they have data to transmit. Of so the central controller will allow them to transmit for a limited time, then the next device is polled.

Token passing performs better when the network has a lot of traffic, while ethernet which uses CSMA/CD is generally faster but loses performance when the network has a lot of traffic. CSMA/CD is basically a method that allows network stations to transmit any time they want. They, however, sense the network line and detect if another station has transmitted at the same time they did. This is called a collision. If a collision happened, the stations involved will retransmit at a later, randomly set time in hopes of avoiding another collision.

IP to link layer encapsulation

The requirements for IP to link layer encapsulation for hosts on a Ethernet network

•  All hosts must be able to send and receive packets defined by RFC 894.
•  All hosts should be able to receive a mix of packets defined by RFC 894 and RFC 1042.
•  All hosts may be able to send RDC 1042 defined packets.

Hosts that support both must provide a means to configure the type of packet sent and the default must be

packets defined by RFC 894.

Ethernet and IEEE 802 Encapsulation formats

Ethernet (RFC 894) message format consists of:

1. 6 bytes of destination address.

2. 6 bytes of source address.

3. 2 bytes of message type which indicates the type of data being sent.

4. 46 to 1500 bytes of data.

5. 4 bytes of cyclic redundancy check (CRC) information.

IEEE 802 (RFC 1042) Message format consists of 3 sections plus data and CRC as follows:

•  802.3 Media Access Control section used to coordinate the sending of data between computers.

      1. 6 bytes of destination address.

      2. 6 bytes of source address.

      3. 2 bytes of length - The number of bytes that follow not including the CRC.

•  802.2 Logical Link control establishes service access points (SAPs) between computers.

      1. 1 byte destination service access point (DSAP).

      2. 1 byte source service access point (SSAP).

      3. 1 byte of control.

•  Sub Network Access Protocol (SNAP).

      1. 3 bytes of org code.

      2. 2 bytes of message type which indicates the type of data being sent.

•  38 to 1492 bytes of data.
•  4 bytes of cyclic redundancy check (CRC) information.

Some ethernet message types include:

•  0800 - IP datagram with length of 38 to 1492 bytes. 
•  0806 - ARP request or reply with 28 bytes and pad bytes that are used to make the frame long enough for the minimum length.
•  8035 - RARP request or reply of 28 bytes and pad bytes that are used to make the frame long enough for the minimum length.

These message types are the same for both formats above with the exception of the pad bytes. The pad bytes for the RFC 894 and RFC 1042 datagrams are of different lengths between the two message formats because the RFC 894 minimum message length is 46 bytes and the RFC 1042 minimum message length is 38 bytes. Also the two message formats above are distinguishable from each other. This is because the RFC 894 possible length values are exclusive of RFC 1042 possible type values.

Trailor Encapsulation

This is described in RFC 1122 and RFC 892, but this scheme is not used very often today. The trailer protocol [LINK:1] is a link-layer encapsulation method that rearranges the data contents of packets sent on the physical network. It may be used but only after it is verified that both the sending and receiving hosts support trailers. The verification is done for each host that is communicated with.

RFC 1122 states: "Only packets with specific size attributes are encapsulated using trailers, and typically only a small fraction of the packets being exchanged have these attributes. Thus, if a system using trailers exchanges packets with a system that does not, some packets disappear into a black hole while others are

delivered successfully."

Trailer negotiation is performed when ARP is used to discover the media access control (MAC) address

of the destination host. RFC 1122 states: "a host that wants to speak trailers will send an additional "trailer ARP reply" packet, i.e., an ARP reply that specifies the trailer encapsulation protocol type but otherwise has the format of a normal ARP reply. If a host configured to use trailers receives a trailer ARP reply message from a remote machine, it can add that machine to the list of machines that understand trailers, e.g., by marking the corresponding entry in the ARP cache."