Physical and Link Layers

Media

Twisted pair cable consists of 2 wires twisted together into a pair. Generally several pair are bundled together into a cable.
Currently two grades of twisted pair cable are in use: Catagory 3, which is usually limited to 10 Mb/s, and Category 5, which is usually limited to 100 Mb/s. The twisted pair cable for data in the new wing of Engineering Hall and in Materials Sciences is Category 5. Except for a few special category 5 runs, the rest of the data cable in the College of Engineering is category 3. The main difference between category 3 and category 5 wire is that the category 5 pairs are twisted together tighter.
Twisted pair cable can be either shielded where the pairs are enclosed in an electrical conductor or unshielded where the shield is omitted. In addition to the overall shield, each pair can be shielded separately. Most of the twisted pair cable from a closet into a room has 4 pairs and is unshielded. The data wiring in the new wing of Engineering Hall has an overall shield.
Coaxial cable consists of one wire with a second conductor around the wire. Coaxial cable is described by its characteristic impedance. Two common values of the characteristic impedance are 50-ohm, which is used for Ethernet, and 75-ohm, which is used for cable television. Each impedance comes in various diameter cables. The larger diameter cables can be used for longer cable runs.
Multi-mode fiber optic cable consists of a core of glass surrounded by another kind of glass with a different refractive index. Typical values for the core diameter are 50 µm, 62.5 µm, and 75 µm. The diameter of the outer layer is typically 125 µm. Multi-mode fiber is characterized by the bandwidth-distance product which tells how far a signal can be transmitted before it becomes too spread out. A typical value is 200 MHz-km.
The multi-mode fiber optic cable used on the campus is 62.5 µm core, 125 µm outer diameter fiber. The fiber inside buildings is 160 MHz-km and the fiber outside buildings is 200 MHz-km bandwidth-distance product. Multi-mode fiber can carry data up to about 100 Mb/s for about 2000 meters. The maximum length decreases for higher data rates. Multi-mode fiber can be driven by either a light emitting diode or a laser.
Single-mode fiber optic cable consists of a smaller glass fiber than multi-mode fiber. A typical single mode fiber is 8 µm. Single mode fiber can transmit data for longer distances at higher rates than multi-mode fiber. Single mode fiber is driven by a laser.

Arbitration schemes

Arbitration schemes are used to prevent more than one system from transmitting on the same transmission path at the same time or to recover from more than one system transmitting if it happens.

Full duplex can be used when there is a dedicated transmission path between two systems in each direction. Each system can transmit whenever it needs to without waiting for any other system.
Token systems pass a special message among themselves that gives the holder the right to transmit. A system not holding the token must not transmit.
Carrier sense/collision detect systems listen to the medium. When the system detects no other system transmitting on the medium, it goes ahead and transmits. Because of propagation delays, another system may start transmitting at the same time which results in a collision. Each system transmitting in the collision must detect the collision and retransmit the message later.
Ethernet requires that a collision be detected in the first 64 bytes of a packet. To guarantee collision detection, all packets must be at least 64 bytes long and the network propagation delay must be short enough so that all participants in a collision can detect the collision during the first 64 bytes of the packet.

Wiring topologies

Systems can be connected together in the following topologies:

Star Each system is connected to a central node.
Bus Each system is connected to a cable that forms a bus.
Ring Each system is connected by cable to one other system. The system is connected together into a closed loop.

Packet formats

A Localtalk packet includes an 8-bit source node address, an 8-bit destination node address, and a 16-bit checksum. It may be up to 605 bytes from the destination address through the checksum.
An Ethernet packet includes an 48-bit source address, a 48-bit destination address, and a 32-bit checksum. It may be up to 1512 bytes from the destination address through the checksum.
An FDDI packet includes a 48-bit source address, a 48-bit destination address, and a 32 bit checksum. It may be up to 4500 bytes long including control bytes (4495 bytes from the destination address through the checksum).
An ATM cell is 53 bytes long. It contains a 24 bit destination address and 48 bytes of data. Cells are combined into packets using the AAL5 (ATM Adaption Layer 5) protocol. An AAL5 packet includes no addresses and a 32 bit checksum. The AAL5 specification specifies no maximum packet length limit, but the computers using ATM to communicate must agree on a packet length limit. Typical length limits are about 1520 data bytes and about 9000 data bytes.

Local area network systems

Name	Data rate	Frame type	Medium	Arbitration	Topology	Segment length limit	Specification
Localtalk	230,400 b/s	Localtalk	Twisted pair (1 pair)	Similar to collision	Bus	300 m	Inside Appletalk
10Base5 Ethernet	10,000,000 b/s	Ethernet	Coaxial cable similar to RG-8	Collision	Bus	500 m	ISO 8803-3, section 8
10Base2 Ethernet	10,000,000 b/s	Ethernet	RG-58 Coaxial cable	Collision	Bus	185 m	ISO 8803-3, section 10
10BaseT Ethernet	10,000,000 b/s	Ethernet	Category 3 twisted pair cable (2 pair)	Collision or full duplex	Star	100 m	ISO 8803-3, section 14
10BaseFL Ethernet	10,000,000 b/s	Ethernet	Multi-mode fiber (2 fiber)	Collision or full duplex	Star	2000 m	ISO 8803-3, section 18
100BaseTX Ethernet	100,000,000 b/s	Ethernet	Category 5 twisted pair cable (2 pair)	Collision or full duplex	Star	100 m	IEEE 802.3u, section 25
100BaseFX Ethernet	100,000,000 b/s	Ethernet	Multi-mode fiber (2 fiber)	Collision or full duplex	Star	400 m (collision) or 2000 m (full duplex)	IEEE 802.3u, section 26
100BaseT4 Ethernet	100,000,000 b/s	Ethernet	Category 3 twisted pair cable (4 pair)	Collision	Star	100 m	IEEE 802.3u, section 23
100BaseT2 Ethernet	100,000,000 b/s	Ethernet	Category 3 twisted pair cable (2 pair)	Collision or full duplex	Star	100 m	IEEE 802.3y, section 32
1000BaseSX Ethernet	1,000,000,000 b/s	Ethernet	Multi-mode fiber (2 fiber)	Full duplex	Star	220 m (62 µm, 160 MHz-km fiber) or 275 m (62 µm, 200 MHz-km fiber)	Approved but not published
1000BaseLX Ethernet	1,000,000,000 b/s	Ethernet	Single-mode or Multi-mode fiber (2 fiber)	Full duplex	Star	550 m (62 µm multi-mode) or 5,000 m (single-mode)	Approved but not published
FDDI	100,000,000 b/s	FDDI	Single-mode or Multi-mode fiber (2 fiber)	Token	Double Ring	2000 m (multi-mode)	ANSI X3.166, X3.148, X3,139
CDDI	100,000,000 b/s	FDDI	Category 5 twisted pair (2 pair)	Token	Double Ring	100 m	ANSI X3.263, X3.148, X3,139
ATM, OC-3	155,520,000 b/s	ATM cell with AAL5	Single or multi mode fiber (2 fiber)	Full Duplex	Star	2000 m on multi-mode fiber	Many ITU and ATM Forum specs

Notes

FDDI uses a double ring topology. One ring is used for transferring data and the other ring is used as warm backup. The token rotates in opposite directions on the two rings. If there is a break in the active ring, FDDI automatically constructs a usable ring from parts of the primary and secondary ring. See the description of the concentrator in the equipment section.

ATM can be run at other speeds and on other media than the OC-3 specification given above.

In the College of Engineering, all network connections between rooms are 10BaseT, 100BaseTX, or 100BaseFX Ethernet, FDDI, or ATM-OC3. Within rooms, there is also some Localtalk and 10Base2 Ethernet. The rest of the network types are not used in the College of Engineering.