CS 422 Class Notes
September 22^nd – 26^th, 2003
By: Paolo Alandy

9/22/03

Types of Ethernet

1. Thicknet

• Original ethernet wiring
• 10 Base 5
• Used transceiver devices at the point of attachment
• Thick coaxial cable

2. Thinnet

• Thin coaxial cable
• 10 Base 2

• Computers are “daisy chained” using “T” connectors
• If you wanted to add a new computer, you needed to break the network
• The terminator prevents the signal from bouncing back, causing noise in later packets/transmissions

3. 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 throughput is still divided between computers in a hub

• The cable used to connect the computers to the hub is a twisted pair cable (10 Base T)
• Twisted cable is done this way to cancel electromagnetic noise
• This technology is:
• physically is a star network
• logically (the way it actually works) is a bus network
• The total throughput is still divided among the computers connected

• For example, if Comp 1 sends to Comp 2 at the same time that Comp 3 sends to Comp 4, then if the total throughput is 10 MBps then this throughput will be divided among the two connections.

Comp 1 à Comp 2                5 MBps

Comp 3 à Comp 4                5 MBps

10 MBps

4. Switches

• These devices allow simultaneous communication among not overlapping pairs of computers

• There is a connection between each pair of ports
• The throughput is not divided as long as the end points are different

• Assume Comp 1 transmits to Comp 2 and at the same time Comp 3 transmits to Comp 4.

Comp 1 à Comp 2                10 MBps

Comp 3 à Comp 4                10 MBps

20 MBps

• However, the throughput will still be divided if multiple computers talk to the same computer

• Example:

Comp 1 à Comp 4                10 MBps /3   = 3.33 MBps

Comp 2 à Comp 4                10 MBps /3   = 3.33 MBps

Comp 3 à Comp 4                10 MBps /3   = 3.33 MBps

                                                                  10 MBps

• Still the switch is preferred to the hub because it allows simultaneous connections with the max throughput
• Switches are more expensive than hubs. The cost of a switch is O(n²) (# of internal connectsioin is [n*(n-1)]/2, n = # of ports)
• Cost of a hub is O(n)
• Switches add more security to the network since traffic between the two machines is not sent across other ports
• Another advantage of switches is that you can divide the switch into multiple independent networks

9/24/03

Types of Ethernet by Speed

1. Standard Ethernet

• 10 Mbps

2. Fast Ethernet

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

3. Gigabit Ethernet

• 1000 Mbps (16 bps)

1. 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 message that has the token is allowed to transmit
• This guarantees fair access

2. Token Passing Mechanism

• A computer that is going to transmit waits for the token
• The computer removes the token and sends message
• The computers pass the message until it reaches the destination
• The receiver receives the message and makes a copy then passes it to the OS and forwards the message to the next computer
• 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. Token ring are slow and expensive when not in use

3. Advantages of Token Ring

• Easy detection of:
• Broken ring
• Hardware failures
• Interference
• Receiver is not connected
• Weaknesses
• Broken ring disables entire ring
• It is difficult to add stations
• Examples of token ring
• Pronet – 10 (10 Mbps)
• IBM token ring (4 Mbps / 16 Mbps)
• FDDI (Fiber Distributed Data Interconnect) 100 Mbps

4. FDDI

• Uses optical fiber
• High reliability
• Immune to interference

5. CDDI

• FDDI over copper
• Save frame format and data rate
• Less noise immune

6. FDDI Hub

• Stations attach to the hub
• Token circulates around the hub

• The hub is calles a “star shaped” ring star
• Physically the topology is a star
• Logically the topology is a ring

7. FDDI Failure Recovery

• It uses two rings instead of one (called “dual attached”)

• The token in the second ring rotates in the opposite direction
• Outer ring is used to send data
• Inner ring used during failure

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

8. Star Topology

• Computers are attached to a single switch device

• Example of star topology
• ATM – Asynchronous Transfer Mode
• Designed by telephone companies
• It is intended to accommodate
• Voice
• Video
• Data
• Building block for an ATM network is an ATM switch
• Each station connects to a switch
• Switches are interconnected

• Full duplex connections. Two fibers required
• Before sending data it was necessary to set up a connection across all switches involved and reserve bandwidth
• The state of the connection is kept in each switch involved
• Reserving bandwidth in the switches is really good for real time data
• Telephone
• Video conferencing
• High speed data rates
• E.g. 55 Mbps
• Fixed site packets called cells
• 53 bytes long
• 48 octets of data
• 5 octets of header
• Smaller cells allows smaller delays

9/26/03

1. ATM

• Asynchronous Transfer Mode
• Connection oriented technology
• Switches store the state of current connections
• It is necessary to establish a connection in all switches involved before sending data
• Bandwidth is guaranteed in the connection
• Cell size in 53 octets
• 48 bytes – data
• 5 bytes – header
• Small cells allow having small delays. This is important for real-time data
• Voice
• Teleconference
• ATM was a response of 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 / interface + 1000.00 interface switch
• Other alternative technologies that gave same throughput at a fraction of the cost arrived (Fast Ethernet / Gigabit Ethernet)
• ATM is not popular anymore

Extending Networks

1. Motivation

• Each LAN has a distance limitation
• We need to connect computer location across long distances

Options

1. Fiber Modem

2. Repeater

• Hardware Device (Analog)
• Connects two line segments

• Blindly copies the signal from one 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

3. Hub

• Analog device
• It simulates an Ethernet cable using electronics
• Propagates signal to all ports
• The number of ports can be 4 to 20 typically

4. Bridge

• Alternative to a repeater
• Connects different LAN segments

• The bridge is a digital device
• It is smarter than a repeater
• The bridge will forward the packet and not noise or collisions
• The bridge is a computer with two Ethernet interfaces
• The bridge will listen to both segments in promiscuous mode and it will forward the packets only if the transmitter and the receiver are in different segments
• The bridge will keep a list of the computers that are attached to each segment
• At boot time this list will be empty
• Every time a packet is transmitting the bridge checks if the transmitter is in the list. If it is not, the bridge adds the computer to the list
• An entry in the list times out after some time (Example 20 minutes) if the computer does not transmit any packets