AppleTalk Network

Topology is bus. Cable type is STP. The connectors are specialized. The media access method is CSMA/CA . Maximum segment and network length is 300 meters. The maximum number of connected segments is 8. There are 32 maximum nodes per segment with 254 maximum number of nodes per network. Speed is 230.4Kbps. The cabling system used with AppleTalk is called LocalTalk.

Addressing

Addressing is dynamic with each computer, when powered on, choosing its last used address or a random address. The computer broadcasts that address to determine if the address is used. If it is used, it will broadcast another random address until it finds an unused address. EtherTalk and TokenTalk provide for use of AppleTalk network protocols on top of ethernet and token ring architectures respectively.

LocalTalk

LocalTalk uses STP cable and bus topology. Using CSMA/CA for media access, computers will first determine if any other computers are transmitting, before they transmit. A packet is transmitted prior to transmitting that alerts other computers that a transmission will be sent. Usually LocalTalk is only used in small environments.

ARCnet Network

ARCnet (Attached Resource Computer Network)(CR)

Topology is star and bus or a mixture. Cable type is RG-62 A/U coaxial (93 ohm), UTP or fiber-optic. A network can use any combination of this media. Connectors used include BNC, RJ-45, and others. It passes tokens passing for media access. Maximum segment length is 600 meters with RG-62 A/U, 121 meters with UTP, 3485 meters with fiber-optic, and 30 meters from a passive hub. The specification is ANSI 878.1. It can have up to 255 nodes per network. The speed is 2.5 Mbps. ARCnet Plus has operating speeds approaching 20Mbps.

Signals are broadcast across the entire network with computers processing only signals addressed to them. ARCnet tokens travel based on a station identifier (SID) which each computer has. Each network card has a DIP switch used to set the SID with an address between 1 and 255. Signals are generally sent from the lowest numbered station to the next until they wrap around back to SID of 1. To determine nonexistent stations, the station with the lowest ID indicates it has the token and begins querying IDs of higher value until it gets a response. Then the next computer does the same until the original station is queried. This procedure is done when a station is added or removed from the network or when the network is originally started. How does the network know when a station has been added or removed? How is the lowest numbered SID identified? Addresses assignment is based on proximity, which helps the network operate more efficiently.

The acronym SID is used for a station identifier with regard to ARCnet, but as used in the Windows NT and Windows 95 operating systems, it refers to the security identification number of a user or group.

Token Ring

Developed by IBM is standardized to IEEE 802.5. It uses a star topology, but it is wired so the signal will travel from hub to hub in a logical ring. These networks use a data token passed from computer to computer around the ring to allow each computer to have network access. The token comes from the nearest active upstream neighbor (NAUN). When a computer receives a token, if it has no attached data and the computer has data for transmission, it attaches its data to the token then sends it to its nearest active downstream neighbor (NADN). Each computer downstream will pass the data on since the token is being used until the data reaches its recipient.The recipient will set two bits to indicate it received the data and transmit the token and data. When the computer that sent the data receives the package, it can verify that the data was received correctly. It will remove the data from the token and pass the token to its NADN.

Characteristics

Maximum cable length is 45 meters when UTP cable is used and 101 meters when STP is used. Topology is starwired ring. It uses type 1 STP and type 3 UTP. Connectors are RJ-45 or IBM type A. Minimum length between nodes is 2.5 meters. Maximum number of hubs or segments is 33. Maximum nodes per network is 72 nodes with UTP and 260 nodes with STP. Speed is 4 or 16 Mps. Data frames may be 4,000 to 17,800 bytes long.

Hubs

A token ring network uses a multistation access unit (MAU) as a hub. It may also be known as a Smart
Multistation Access Unit (SMAU). A MAU normally has ten ports. Two ports are Ring In (RI) and Ring Out (RO) which allow multiple MAUs to be linked to each other. The other 8 ports are used to connect to computers.

Cables

UTP or STP cabling is used as a media for token ring networks. Token Ring uses an IBM cabling system based on American Wire Gauge (AWG) standards that specify wire diameters. The larger the AWG number, the small  diameter the cable has.

Token ring networks normally use type 1, type 3 or regular UTP like cable used on ethernet installations. If
electrical interference is a problem, the type 1 cable is a better choice. Cable types:

Type Description

1. Two 22 AWG solid core pair of STP cable with a braided shield. This cable is normally used between MAUs and computers.
2. Two 22 AWG solid core pair with four 26 AWG solid core of STP cable.
3. Four 22 or 24 AWG UTP cable. This is voice-grade cable and cannot transmit at a rate above 4Mbps.
4. Undefined.
5. Fiber-optic cable. Usually used to link MAUs.
6. Two 26 AWG stranded core pair of STP cable with a braided shield. The stranded-core allows more flexibility but limits the transmission distance to two-thirds that of type 1.
7. Undefined.
8. Type 6 cable with a flat casing to be used under carpets.
9. Type 6 cable with plenum-rating for safety.

Beaconing

The first computer turned on on a token ring will be the active monitor. Every seven seconds it sends a frame to its nearest active downstream neighbor. The data gives the address of the active monitor and advertised and sends it to its nearest active downstream neighbor. When the packet has traveled around the ring, all stations know the address of their upstream neighbor and the active monitor knows the state of the network. If a computer has not heard from its upstream neighbor after seven seconds, it will send a packet that announces its own address, and the NAUN that is not responding. This packet will cause all computers to check their configuration. The ring can thereby route around the problem area giving some fault tolerance to the network.

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.

Network WAN Connections

Three options for connecting over a telephone service:

• Dial-up connections.
• Integrated Services Digital Network(ISDN) - A method of sending voice and data information on a digital phone line.

1. Basic ISDN - Two 64Kbps B-channels with one 16Kbps D channel is provided. The Dchannel is used for call control and setup. Basic ISDN can provide 128Kbps speed capability.
2. Primary ISDN - 23 B-channels and one D channel is provided.

• Leased Lines - This involves the leasing of a permanent telephone line between two locations.

Remote Communication Protocols

• Serial Line Internet Protocol (SLIP) - Allows computers to connect to the internet with a modem. No error checking or data compression is supported. Only the TCP/IP protocols are supported.
• Point to Point Protocol (PPP) - Provides error checking and data compression. Also supports multiple network protocols such IPX/SPX and NetBEUI in addition to TCP/IP. Supports dynamic allocation of IP addresses.

Remote Access Service

Remote Access Service (RAS) with Windows NT allows users connecting to the network using a modem

to use network resources. RAS may be called dial up networking (DUN) depending on the version of

Windows you are using. The NT RAS server can handle 256 connections. Windows NT RAS servers

provide the following security features:

1. User account security

2. Encryption between the DUN (dial up networking) client and the server

3. Callback capability

The client software is called Dial up networking (DUN) in windows NT4 and Windows95. For NT 3.51

and Windows 3.1 it is called a RAS client. These clients may be used to connect to the internet through

an internet service provider (ISP).

Wireless Networking

This section may be skipped by all readers and used by those interested in wireless network technology.
Transmission of waves take place in the electromagnetic (EM) spectrum. The carrier frequency of the
data is expressed in cycles per second called hertz(Hz). Low frequency signals can travel for long
distances through many obstacles but can not carry a high bandwidth of data. High frequency signals can
travel for shorter distances through few obstacles and carry a narrow bandwidth. Also the effect of noise
on the signal is inversely proportional to the power of the radio transmitter, which is normal for all FM
transmissions. The three broad categories of wireless media are:

1. Radio - 10 Khz to 1 Ghz. It is broken into many bands including AM, FM, and VHF bands. The
Federal communications Commission (FCC) regulates the assignment of these frequencies.
Frequencies for unregulated use are:

• 902-928Mhz - Cordless phones, remote controls.
• 2.4 Ghz
• 5.72-5.85 Ghz

2. Microwave

• Terrestrial - Used to link networks over long distances but the two microwave towers must have a line of sight between them. The frequency is usually 4-6GHz or 21-23GHz. Speed is often 1-10Mbps. The signal is normally encrypted for privacy.
• Satellite - A satellite orbits at 22,300 miles above the earth which is an altitude that will cause it to stay in a fixed position relative to the rotation of the earth. This is called a geosynchronous orbit. A station on the ground will send and receive signals from the satellite. The signal can have propagation delays between 0.5 and 5 seconds due to the distances involved. The transmission frequency is normally 11-14GHz with a transmission speed in the range of 1-10Mbps.

3. Infared - Infared is just below the visible range of light between 100Ghz and 1000Thz. A light
emitting diode (LED) or laser is used to transmit the signal. The signal cannot travel through
objects. Light may interfere with the signal. The types of infared are

• Point to point - Transmission frequencies are 100GHz-1,000THz . Transmission is between two points and is limited to line of sight range. It is difficult to eavesdrop on the transmission.
• broadcast - The signal is dispersed so several units may receive the signal. The unit used to disperse the signal may be reflective material or a transmitter that amplifies and retransmits the signal. Normally the speed is limited to 1Mbps. The transmission frequency is normally 100GHz-1,000THz with transmission distance in 10's of meters. Installation is easy and cost is relatively inexpensive for wireless.

Terms:

1. AMPS - Advanced Mobile Phone Service is analog cellular phone service. TWireless Networking
2. CDMA - Code division multiple access allows transmission of voice and data over a shared part of radio frequencies. This is also called spread spectrum.
3. CDPD - Cellular Digital Packet Data will allow network connections for mobile users using satellites.
4. cellular - An 800 Mhz band for mobile phone service.
5. D-AMPS - Digital AMPS using TDMA to divide the channels into three channels.
6. FDMA - Frequency Division Multiple Access divides the cellular network into 30Khz channels.
7. GSM - Global System for Mobile Communications.
8. HDML - Handheld Device Markup Language is a version of HTML only allowing text to be displayed.
9. MDBS - Mobile Data Base Station reviews all cellular channels at cellular sites.
10. PCS - Personal communications Service is a 1.9 Ghz band.
11. TDMA - Time Division Multiple Access uses time division multiplexing to divide each cellular channel into three sub channels to service three users at a time.
12. wireless bridge - Microwave or infared is used between two line of site points where it is difficult to run wire.
13. WML - Wireless markup language is another name for HDML.

 Categories of LAN Radio Communications

• Low power, single frequency - Distance in 10s of meters. Speed in 1 10Mbps. Susceptible to interference and eavesdropping.
• High power, single frequency - Require FCC licensing and high power transmitter. Speed in 1-10Mbps. Susceptible to interference and eavesdropping.
• Spread spectrum - It uses several frequencies at the same time. The frequency is normally 902- 928MHz with some networks at 2.4GHz. The speed of 902MHz systems is between 2 and 6Mbps. If frequency-hopping is used, the speed is normally lower than 2Mbps. Two types are:

1. Direct sequence modulation - The data is broken into parts and transmitted simultaneously on multiple frequencies. Decoy data may be transmitted for better security. The speed is normally 2 to 6 Mbps.

2. Frequency hopping - The transmitter and receiver change predetermined frequencies at the same time (in a synchronized manner). The speed is normally 1Gbps.