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.

Address Resolution Protocol

ARP and RARP Address Translation

Address Resolution Protocol (ARP) provides a completely different function to the network than Reverse Address Resolution Protocol (RARP). ARP is used to resolve the ethernet address of a NIC from an IP address in order to construct an ethernet packet around an IP data packet. This must happen in order to send any data across the network. Reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network.

Address Resolution Protocol (ARP)

In an earlier section, there was an example where a chat program was written to communicate between

two servers. To send data, the user (Tom) would type text into a dialog box, hit send and the following

happened:

1. The program passed Tom's typed text in a buffer, to the socket.

2. The data was put inside a TCP data packet with a TCP header added to the data. This header

    contained a source and destination port number along with some other information and a

    checksum.

3. The TCP packet was be placed inside an IP data packet with a source and destination IP address

    along with some other data for network management.

4. The IP data packet was placed inside an ethernet data packet. This data packet includes the

    destination and source address of the network interface cards (NIC) on the two computers. The

    address here is the hardware address of the respective cards and is called the MAC address.

5. The ethernet packet was transmitted over the network line.

6. With a direct connection between the two computers, the network interface card on the intended

     machine, recognized its address and grabbed the data.

7. The IP data packet was extracted from the ethernet data packet.

8. The TCP data packet was extracted from the IP data packet.

9. The data was extracted from the TCP packet and the program displayed the retrieved data (text) in

     the text display window for the intended recipient to read.

In step 4 above, the IP data was going to be placed inside an ethernet data packet, but the computer

constructing the packet does not have the ethernet address of the recipient's computer. The computer that is sending the data, in order to create the ethernet part of the packet, must get the ethernet hardware (MAC) address of the computer with the intended IP address. This must be accomplished before the ethernet packet can be constructed. The ethernet device driver software on the receiving computer is not programmed to look at IP addresses encased in the ethernet packet. If it did, the protocols could not be independent and changes to one would affect the other. This is where address resolution protocol (ARP) is used. Tom's computer sends a network broadcast asking the computer that has the recipient's IP address to send it's ethernet address. This is done by broadcasting. The ethernet destination is set with all bits on so all ethernet cards on the network will receive the data packet. The ARP message consists of an ethernet header and ARP packet. The ethernet header contains:

1. A 6 byte ethernet destination address.

2. A 6 byte ethernet source address.

3. A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 for RARP

The encapsulated ARP data packet contains the following:

1. Type of hardware address (2 bytes). 1=ethernet.

2. Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IP address.

3. Byte size of the hardware address (1 byte). 6

4. Byte size of the protocol address (1 byte). 4

5. Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARP reply.

6. The sender's ethernet address (6 bytes)

7. The sender's IP address (4 bytes)

8. The recipient's ethernet address (6 bytes)

9. The recipient's IP address (4 bytes)

When the ARP reply is sent, the recipient's ethernet address is left blank.

In order to increase the efficiency of the network and not tie up bandwidth doing ARP broadcasting, each computer keeps a table of IP addresses and matching ethernet addresses in memory. This is called ARP cache. Before sending a broadcast, the sending computer will check to see if the information is in it's ARP cache. If it is it will complete the ethernet data packet without an ARP broadcast. Each entry normally lasts 20 minutes after it is created. RFC 1122 specifies that it should be possible to configure the ARP cache timeout value on the host. To examine the cache on a Windows, UNIX, or Linux computer type "arp -a".

If the receiving host is on another network, the sending computer will go through its route table and determine the correct router (A router should be between two or more networks) to send to, and it will substitute the ethernet address of the router in the ethernet message. The encased IP address will still have the intended IP address. When the router gets the message, it looks at the IP data to tell where to send the data next. If the recipient is on a network the router is connected to, it will do the ARP resolution either using it's ARP buffer cache or broadcasting.

Reverse Address Resolution Protocol (RARP)

As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.

Network Topology

A network consists of multiple computers connected using some type of interface, each having one or more
interface devices such as a Network Interface Card (NIC) and/or a serial device for PPP networking. Each
computer is supported by network software that provides the server or client functionality. The hardware used to transmit data across the network is called the media. It may include copper cable, fiber optic, or wireless transmission. The standard cabling used for the purposes of this document is 10Base-T category 5 ethernet cable. This is twisted copper cabling which appears at the surface to look similar to TV coaxial cable. It is terminated on each end by a connector that looks much like a phone connector. Its maximum segment length is 100 meters.

Network Categories

There are two main types of network categories which are:
1. Server based
2. Peer-to-peer

In a server based network, there are computers set up to be primary providers of services such as file service or mail service. The computers providing the service are are called servers and the computers that request and use the service are called client computers. In a peer-to-peer network, various computers on the network can act both as clients and servers. For instance, many Microsoft Windows based computers will allow file and print sharing. These computers can act both as a client and a server and are also referred to as peers. Many networks are combination peer-to-peer and server based networks. The network operating system uses a network data protocol to communicate on the network to other computers. The network operating system supports the applications on that computer. A Network Operating System (NOS) includes Windows NT, Novell Netware, Linux, Unix and others.

Three Network Topologies The network topology describes the method used to do the physical wiring of the network. The main ones are bus, star, and ring.

1. Bus - Both ends of the network must be terminated with a terminator. A barrel connector can be used to       extend it.

2. Star - All devices revolve around a central hub, which is what controls the network communications, and

    can communicate with other hubs. Range limits are about 100 meters from the hub.

3. Ring - Devices are connected from one to another, as in a ring. A data token is used to grant permission       for each computer to communicate.

There are also hybrid networks including a star-bus hybrid, star-ring network, and mesh networks with

connections between various computers on the network. Mesh networks ideally allow each computer to have a direct connection to each of the other computers. The topology this documentation deals with most is star topology since that is what ethernet networks use.