CS422 Lecture Notes

Date: Monday 9/22/2003

Lecturer: Prof. Gustavo Rodriguez-Rivera

Taken By: Ammar T. Al-Sayegh

Ethernet Evolution

1. Thicknet

• Original Ethernet wiring.

• 10Base5.

• User transceiver devices at the point of attachment.

• Thick coaxial wiring.

2. Thinnet

• Thin coaxial cable.

• Computers are "daisy chained" using "T" Connectors.

• If you wanted to add a new computer, you needed to break the network.

• Terminator prevents the signal from bouncing back, causing noise in in later packets/transmissions.

Hubs

• A hub is a hardware device that behaves like a common Ethernet cable. All computers in the network will be attached to the hub.

• The cable used to connect the computers to the hub is a twisted pair cable (10BaseT).

• The twisted pair cable is done this way to cancel electromagnetic noise.

• This technology is:

• Physically is a star network.

• Logically is bus network.

• The total throughput is still divided among the computers connected. For example, if a sends to B at the same time that C sends to D and  if the total throughput is 10Mbps then this throughput will be divided between the two connections: (A->B = 5Mbps) + (C->D = 5Mbps) = 10Mbps.

Switches

• These devices allow simultaneous connections among not overlapping pairs of computers.

• There is a connection between each pair of ports.

• The throughput is not divided as long as the ports are different. For example, assume A transmits to B and at the same time C transmits to D. Then, (A->B = 10Mbps) + (C->D = 10Mbps) = 20Mbps (concurrent throughput).

• The throughput will still be divided if multiple computers talk to the same computer. For example, A transmits to D, B transmits to D, and C transmits to D. Then,  (A->D = 3.33Mbps) + (B->D = 3.33Mbps) + (C->D = 3.33Mbps) = 10Mbps. Still, switches are preferred to hubs because they allow simultaneous connections with the max throughput.

• Switches are more expensive than hubs. The cost of a switch is O(n^2) because the number of the internal connections is n(n-1)/2, where n = number of ports. The cost of a hub is O(n).

• Switches add an extra layer of security to the network since traffic between two machines is not sent across other ports.

• Another advantage of switches is that you can divide the switch into multiple independent networks.

Date: Wednesday 9/24/2003

Lecturer: Prof. Gustavo Rodriguez-Rivera

Taken By: Ammar T. Al-Sayegh

Types of Ethernet by Speed

• Standard Ethernet

• 10Mbps.

• Fast Ethernet

• 100Mbps.

• Ethernet cards can communicate in dual mode 10/100Mbps and they switch from one mode to the other depending on the hub they are connected to.

• Gigabit Ethernet

• 1000Mbps (1Bbps).

Ring Topology

• Second most popular LAN topology (after Bus topology).

• Bits flow in a single direction.

• A special short message called "token" is passed from computer to computer.

• Only the computer that has the token is allowed to transmit. This guarantees fair access.

Token Ring Mechanism

• A computer that is going to transmit waits for the token.

• The computer removes the token and sends the message.

• The computer passes the message until it reaches the destination.

• The receiver receives the message. Makes a copy and passes it to the OS and forwards the message to the next computer. The receiver sets a flag in the message.

• The message keeps circulating until it arrives to the sender. The sender verifies that the receiver set the flag and the message was received correctly.

• Advantage of Token Ring - Easy detection of:

• Broken ring.

• Hardware failures.

• Interference.

• Receiver not connected.

• Disadvantage of Token Ring:

• Broken ring disables entire ring.

• Difficult to add stations.

• Examples of Token Ring:

• Pronect-10 (10Mbps).

• IBM Token Ring (4Mbps/16Mbps).

• FDDI (Fiber Distributed Data Interconnect) 100Mbps.

• FDDI

• Uses optical fiber.

• High reliability.

• Immune to interference.

• CDDI

• FDDI Over Copper.

• Same frame format and data rate.

• Less noise immune.

• FDDI Hub

• Token circulates around the hub.

• The hub is called a "Star Shaped" ring. Physically the topology is a star, but logically the topology is a ring.

• FDDI Failure Recovery

• It uses two rings instead of one (called "dual attached").

• The token in the second ring rotates in the opposite directions (counter rotating).

• Outer ring is used to send data.

• Inner ring is used during failure.

• Neighbor stations detect when a station or link does not work and short-circuit the network (self-healing).

Star Topology

• Computers are attached to a single switch device.

• Example of star topology: ATM (Asynchronous Transfer Mode).

ATM

• Designed by telephone companies.

• Designed to accommodate:

• Voice.

• Video.

• Data.

• Building block of an ATM network is an ATM switch.

• Each station connects to the switch and switches are interconnected with optical fiber.

• For duplex connections, two fibers are required.

• Before sending data it's necessary to setup a connection across all switches involved and reserve bandwidth.

• The state of the connection is kept in each switch involved.

• Reserving bandwidth in a switch is good for real time data such as:

• Telephony.

• Video conferencing.

• High speed data rates

• e.g. 155Mbps.

• Fixed size packets called cells (53 long).

• 48 octets of data.

• 5 octets of header.

• smaller cells allow smaller delay.

Date: Friday 9/26/2003

Lecturer: Prof. Gustavo Rodriguez-Rivera

Taken By: Ammar T. Al-Sayegh

ATM (continued...)

• ATM was a response for telephone companies against the growth of the Internet.

• Telephone companies wanted to make the Internet run on top of a big ATM network that they could control. However, ATM was very expensive ($1000/computer interface + $1000/switch interface).

• Other alternative technologies that gave the same throughput at a fraction of the cost arrived (Fast/Gigabit Ethernet).

• ATM is no longer popular.

Extending Networks

• Motivation:

• Each LAN has a distance limitation.

• We need to connect a computer located across long distance.

• Fiber Modem.

• Repeater.

• Analog hardware device.

• Connects two line segments.

• Blindly copies the signal from one network segment to the other segment.

• Amplifies the signal before sending signal to the other side.

• Operates in both directions simultaneously.

• It also propagates noise and collisions.

• The repeater can be used to extend different Ethernet segments.

• Hub.

• Analog hardware device.

• It simulates an Ethernet cable using electronics.

• Propagates signal to all ports.

• The number of ports can typically range from 4 to 20.

• Inexpensive.

• It is possible to connect multiple hubs together to form a single network.

• Bridge.

• A digital hardware device.

• Alternative to a repeater.

• Connects different LAN Segments

• Smarter than repeater.

• Recognizes packets and will forward the packet and not noise or collisions.

• It's a computer with two Ethernet interfaces.

• Will listen to both segments in a promiscuous mode and will forward the packet only if the transmitter and the receiver are in different segments.

• Will keep a list of the computers that are attached to each segment. This list will be empty at boot time.

• Every time a packet is transmitted, the bridge checks if the transmitter is in the list. If it is not, the bridge adds the computers to the list.

• An entry in the list times out after some time (20 minutes, for example) if the computer does not transmit any packet.