Lecture 3B

Home - Network - Lec1 and 2 - Lec3 - Lec4 - Lec5 and 6 - Lec7 - Lec8 - Lec9 - Lec10 - Lec11 - Lec12 1. Bus:

• A single transmission medium, no switches, no repeaters.
• Broadcasting style
• Signal transmits in both direction, terminators are needed at both end.
• Problem: How to control the access & How the recipients know the message is for them
• Solution: Frame header.
• Adv: Very flexible, easy to layout, no single-point failure
• Disadv: Not suitable for high speed.

2.Tree:

• A group of buses joins at the head end
• Just like the bus
• Adv: Very flexible, easy to layout, no single-point failure
• Disadv: Not suitable for high speed, short distance

3.Ring

• A set of repeaters joined by point-to-point link in a closed loop.
• The link are unidirectional, data circulate around the ring and remove by the sender ( using the frame header ).
• The repeater receives and transmits bit by bit, no buffering.
• Adv: Very high speed, high throughput over a larger distance
• Disadv: Single-point failure

4.Star

• Each station connects to a common central node (a.k.a star coupler)
• Point-to-point link to the central node
• If the central node broadcast a frame from one station to the rest, then the network physically is a start but logically is a bus.
• If the central node buffers and switches the frame from station to another then the network is a real star configuration.
• Adv: Easiest to layout in a building, best for short distance and high data rate.
• Disadv: Short distance, small number of nodes

Relationship b/w medium and topology

• Bus: All media are suitable.
• Ring: All except broadband coaxial cable. The ring topology requires point-to-point link on multiple channels for broadband signal which is difficult and expensive to implement.
• Tree: Only broadband coax and twisted-pair are suitable .
• Star: All media except coaxial ( baseband & broadband ). The star requires point-to-point link on multiple channels for broadband signal which is difficult and expensive to implement. For baseband ?????

Bus/Tree for coax

• Multiple connection ==> interference if 2 stations transmit at the same time.
• Required a medium access control (MAC) protocol
• Signal balancing is difficult for multiple connection, the number of constrained signal is n x (n-1) where n = number of station on the same subnet. For RF signal, it’s even more difficult to balance the signal.
• Solution: Segment the networks, use amplifier or repeaters between segments.

A. Baseband coaxial cable

• Bus is the most popular for baseband, bidirectional signal.
• Digital signaling ==> no frequency multiplexing.
• Most baseband use 50 ohm CATV ==> suffer less reflection from the tap and better EMI at low frequency.
• 10BASE5 : 10 Mbps, baseband , 500 meters max segment length, max 100 taps with spacing 2.5m, network span 2500 m
• 10BASE2 : 10 Mbps, baseband , 185 meters max segment length, max 30 taps with spacing 0.5m, network span 1000m

B.Broadband coaxial cable

• Dual configuration : inbound and outbound are separate cable join by a passive headend.
  Single frequency band is used ==> High bandwidth than slit configuration, suitable for small system.
• Slit configuration : inbound and outbound are on different bands and share the same cable(FDM). The headend is active bidirectional frequency translator.

Broadband coax use 75 ohm CATV, 10 Km radius, 1000 devices. Main components are:

• Cable: trunk cable, distributed cable or feeder cable, drop cable.
• Amplifier: Use on trunk and distributed cable
• Terminator: Use to absorb the signal at the end ==> No reflection.
• Modem: Convert digital data from the station to analog signal to transmit on the medium.
• Directional coupler: Use to divide the outputs or join the inputs together. Splitter: Use to divide the input into 2 equal outputs.

C.Carrier band

• Single channel broadband ( no FDM )
• Bidirectional transmission ( bus topology )
• No amplifier, no headend
• The signal energy concentrate on low frequency ==> less attenuation at low freq.
• Wide output spectrum ==> cheap modem and electronic part. Carrier band provide the same performance as baseband at comparable price.

Optical fiber bus

Optical fiber tap
Can be either passive or active. For active tape these steps occur:

1. Optical energy enter the tap
2. Clock is recovered, the optical signal is converted to electrical signal.
3. This converted signal enter the node ( may be modified by the node later )
4. The node send out an electrical signal which modulate the light beam and the optical signal is launched back to the bus. ( see fig. 3.8 )

So the bus topology of optical fiber consists of a chain of point-to-point link. Active tap is expensive and delay is accumulated for each tap ==> clock jittering in ring topology.

For passive tap, the tap extract a portion of the optical energy from the bus for the node reception. The lossy nature of pure optical tap limits the number of devices and the max length of the fiber.

For optical bus, see fig 3.9 Star topology

• Multilevel configuration use: IHUB, HHUB
• Popular at 10 Mbps, for 100 Mbps encounters these problem
  • Existing telephone is inadequate for 100Mbps
  • Twisted-pair cable are tightly pack together in the conduits ==> crosstalk is significant at 100 Mbps.
• These problems can be overcome by use of signal processing technique and better transceiver.

Optical fiber star
One of the first commercial hub for optical fiber is passive-star coupler which employed 2 techniques: biconic fused coupler and mixng rod coupler. See fig 3.19 The optical start coupler can support only a few tens of stations and the distance up to 1Km.

Ring topology

Ring consists of a number of repeaters connect by unidirectional link to form a closed path. See fig 3.10.
Repeater regenerates and retransmits each bit sequentially around the ring. Three basic functions performed by the repeater for the ring to operate as a network:

• Data insertion
• Data reception
• Data removal

MAC protocol is need for the data insertion and data reception.
Data removed by the addressed repeater or by the transmitting repeater ( multicast )
On the physical layer, the repeater has 3 states:

• Listen state:
  • Scan the passing bit stream
  • Copy each incoming bit and send it to the attached station while continuing to retransmit each bit ( hit condition )
  • Modify a bit as it pass by ( acknowledge )
• Transmit state:
  • If the bit is from the same packet the repeater is still sending, the repeater pass the bits back to the station for some form of checking.
  • If bit is from another packet, the repeater while transmitting, buffers them to be transmit later. ( More than one packet at the same time on the ring, in some control states )
• Bypass state: Inactive station just bypass the packet.

Ring benefit:

• Point-to-point link : Cover greater distance ( due to the repeater at each node ), easy maintenance
• Fault isolation and recovery is easy (discuss later)
• Efficient sharing the medium ==> the best throughput among other topologies.

Ring problem:

• Single point failure on the link or repeater.
• Perambulation: Locating the failed repeater or link required a full search on the network.
• Installation: Need to shut down 2 adjacent repeaters to insert a new node.
• Size limitation : limited number of repeaters on the ring before timing problem occurs.
• Coordination problem: No node is assigned as the controller ==> need a very strict cooperation and initialization.
• Timing problem: Clock jittering