Media, LAN Technologies, and Network Topologies Lecture 5

Introduction to Media Network traffic must flow through some form of media, whether it is a cable, or is wireless. The most common forms of network media are twisted-pair, coaxial, and fiber-optic cable.

Twisted-Pair Cable T-P cable is the most common of all of the media types in the average local area network (LAN) environment. Different categories of T-P cable exist. The different categories of cable specify the maximum data bandwidth that the cable can withstand. T-P comes in two forms, Unshielded (UTP) or Shielded (Plenum/STP).

Twisted-Pair Categories Category Maximum Data Rate Usual Application CAT-1< 1 MbpsPOTS & ISDN CAT-24 Mbps IBM Token Ring CAT-316 Mbps Voice/Data - 10baseT

Twisted-Pair Categories (cont.) CAT-420 Mbps 16Mbps Token Ring Networks CAT-5100 Mbps 100baseT, 155Mb ATM CAT-7 (in progress) 1000 Mbps 1000baseT, Gigabit Ethernet

Twisted-Pair Comparison Advantages Cheap Easy to implement Easy to manage LOTS of different applications Easy to terminate Disadvantages Susceptible to EMF,RF interference Limited distance – 100 meters

Twisted-Pair (cont.) Twisted-pair cable (CAT5 and up) consists of 4 separate pairs of wires, all wound separately. UTP is shown on the right.

Coaxial Cable Coaxial cable (coax) is almost the same thing that carries your cable TV signal. Data coax is just held to a higher quality. Historical Tidbit: Coax cable, although not commonly seen nowadays, was how Ethernet was developed!

Coax (cont.) The physical medium itself consists of an inner wire, surrounded by an insulator, which is also surrounded by a shield.

Coax Applications Local Area Networks (LANs) Thinnet (10base2) – 200 meters Thicknet (10base5) – 500 meters Baseband transmissions only Wide Area Networks (WANs) T3/DS3/E3 Broadband transmissions

Baseband v. Broadband Baseband is where the medium only carries one signal on the line. Broadband carries multiple signals on a single line.

Coax Comparison Advantages Highly shielded from EMF,RF interference Signals propagate much farther than TP cable. Conforms to standards. More channels than TP cable. Disadvantages One cable for all computers. To add additional computers, network must be taken down. MUST properly terminate. Expensive. Low channel count compared to fiber.

Fiber Optic Cable Fiber optic cable is where the future of LAN wiring exists. It is wicked fast. It is WICKED fast!

Fiber Optic Cable (cont.) Fiber comes in two different types: Multimode – a channelized fiber- optic circuit. Multiple carrier frequencies. Singlemode – a “clear channel” circuit. One carrier frequency.

Fiber Comparison Advantages Wicked fast! Handles lots of simultaneous B channels. Very reliable. Disadvantages Cost to implement.  Splicing kit.  Cable costs.  Redundancy (FDDI)? When disaster strikes, it’s a major ordeal. Point-to-point only

Fiber Applications High-bandwidth voice transmission. “Backbone” applications. Very fast data transfer between network devices.

Other Media Cellular Infrared Satellite Geosynchronous Low Earth Orbit Packet Radio Microwave

Applying It All To LANs Point to point (between all computers) is unmanageable. This only becomes apparent when looking at multiple machines.

If We Can’t Use Point-To- Point? That is where the concept of a shared medium was derived. If a shared medium was in use, then you would only have to connect all of the computers together, not each individual computer!

Locality of Reference A computer is more likely to communicate with computers that it is physically near than those that it is far away from. If a pair of computers communicates once, the pair is likely to communicate again in the near future, and then periodically.

LAN Topologies There are three main types of LAN systems: Star Topology Ring Topology Bus Topology Some topologies are actually hybrids of the above!

Star Topology All computers connect to a centralized point. The central point is called the hub.

Ring Topology One computer is connected to the two computers adjacent to it. In the traditional case, if a system is disconnected from the ring, the network fails.

Bus Topology A bus topology consists of a single cable to which each network device attaches. Bus topology has the same problems as a ring topology.

Ethernet This is the original Ethernet design by Bob Metcalfe in 1976.

Ethernet (cont.) The Ethernet specification details how devices are supposed to interact on the segment, distances between devices, and a whole other multitude of other things. Carrier Sense On Multi-Access Networks (CSMA) with Collision Detection (CD) was the most important part of the specification.

CSMA on Ethernet CSMA indicates that computers wait until the ether is free. In this case, no electrical signal denotes when the ether is “free.”

CSMA on Ethernet (cont.) The “signal” is the carrier of the ether. The act of waiting for the opportunity to send on the ether is carrier sense.

Collision Detection Since networks are not instantaneous, it is possible for two stations to check the ether, deem it time to send, and both send at the same time. This is called a collision. When a collision occurs, the sender immediately stops transmission, and waits some random length of time, and then begins transmission again.

Collision Detection (cont.) A busy segment will always have collisions. Collisions do not damage the equipment, but it forces the two machines that caused the collision to wait, and therefore it slows down the network. Fewer machines on a segment, smaller segments, can improve the collision count.

LocalTalk LocalTalk is another form of a bus network. LocalTalk differs from Ethernet in a few ways: When a machine senses the segment is free, it holds it until the frame transmission is over. All other machines know to wait for transmission to complete. Bandwidth is quite small at ~230Kbps. That’s 2.3% of the earliest Ethernet specification!

IBM Token Ring Access mechanism is called token passing. Once the sent information makes a complete turn around the ring, the sender passes the token to the next machine. This is a strict- alternation scheme where all machines have the same chance to transmit information.

Fiber Distributed Data Interconnect (FDDI)