phones off (please)
Networks 4
Topologies and Protocols
Dr John Cowell
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Overview
 Network topologies
 ring, star, bus and mesh topologies
 combined topologies
 Communication protocols
 message switching
 LAN

Ethernet, Token Ring
 MAN
 FDDI, DQDB
 WAN
 ATM, ISDN
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Network Topologies
 Topology means physical layout of a network
 issues of:


hardware
 cabling, desking
software
 protocol, applications
 Each workstation attached to network is known as a node
 Types of Networks
 ring, star, bus, mesh/distributed
 combined
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Ring Networks
 Ring must be maintained
at all times
 network crashes if break in
cable ring
• Higher speeds available at cost
• Can be inflexible in wiring
• Addition/deletion cause problems
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Star Networks
• Large
amounts of
cable
 Server dependent - often a mainframe
 Good central control
 Robust - no problems if cable/node breaks
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(Common) Bus Networks





Also known as multidrop
Each node has a feed to the bus
Easy to add new nodes
Can be inflexible - limits to cable length
Coax connectors cause problems – only used with 10 MHz
Ethernet
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Mesh Networks
 Fully connected network
 Used in WANs, not LANs
 No competition for common lines
 fast connections
 Wasteful of connectivity potential
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Combined Topologies
Mainframe
Network Printer
Router
Bridge
Ethernet
File Server
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Combined Topologies 2
 Combinations of topologies are common
 ‘dynamically grown’ networks
 Common bus - backbone
 LAN’s cope with most users
 occasional access to mainframe is sufficient
 Bridges convert between protocols
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Protocols
 Set of rules governing communication issues
 a standard

IEEE define the 802 protocol standards
 Different protocols exist for different geographical area
topologies
 LANs
 Ethernet (802.3), Token Bus (802.4), Token Ring (802.5), Wireless - WiFi
(802.11[a|b|g|n])
 MAN
 FDDI (802.5), DQDB (802.6)
 WANs
 ATM, ISDN, Frame Relay
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Connection Strategies
B
D
A
F
C
 Circuit switched
E
 ‘permanent’ circuit established for call duration
 Message switched
 a message is sent from node to node
 message is stored temporarily at intermediate nodes
 problem if message is too long for buffer
 security issues
 Packet switched - 2 types
 datagram and virtual circuit
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Packet Switching
 Packet switched - 2 types
 datagram


each packet transmitted separately (e.g. TCP/IP)
connectionless service
 virtual circuit
 protocols establish a circuit, used for all packets (e.g. ATM)
 segments may be shared with other virtual circuits
 connection oriented
 There are possible routes between A and F
 each packet may have new route

packet 1: A  B  F
packet 2: A  C  E  F (could arrive last of three)
packet 3: A  C  F
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Ethernet and Token Ring
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Ethernet
 Node connects to bus cable via a transceiver
Sending Network interface card receives a frame from PC
 Transceiver says when cable clear
 frame is put onto cable by transceiver
 transceiver listens for collision (CSMA/CD)
 if collision occurs, waits random time, then tries again
Receiving Transceiver monitors all traffic




all incoming frames address field are read
if for node, bits are sent to NIC
NIC does error check
PC executes network software to move frame from NIC to
memory for further processing
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Ethernet Cable Specifications
 The 802.3 standard specifies limits
 maximum backbone length
 maximum number of nodes attached
 common data rates
Max distance
between nodes
Data Rate
Cable Type
Backbone
10 BASE 5
50 ohm coax,
10 mm diameter
500
10 Mbps
‘Thick’
10 BASE 2
50 ohm coax,
5 mm diameter
185
10 Mbps
‘Thin’
10 BASE-T
UTP
100
10 Mbps
star topology
100 BASE-T
UTP
100
100 Mbps
‘Fast’
1000 BASE-T
UTP
100
1000 Mbps
Gigabit Ethernet
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Distance limitations
 10 BASE 5 was limited to 500 metres
 signals degrade in copper cable
 Used repeaters (amplifiers) to join 500 meter sections together
 maximum of four repeaters (between two nodes)
 the limit comes from the collision detection protocol

if the transceiver hears a collision, it must still have part of frame to transmit, in
order to abandon it

minimum frame size of 64 bytes

maximum frame size to prevent network hogging
 Faster Ethernet reduces cable length
 e.g. 100 base T limited to 100 metre cables from central switch/hub
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Token Ring
 Used widely in industry
 originally developed by IBM
 time critical applications
 Data rates of 1 Mbps, 4 Mbps, 16
Mbps
 Nodes are connected in a ring
topology
 A token (special frame) is
constantly circling
 If network breaks, token
disappears
 need to restart (reboot) network
 avoid this with wire centre hub

‘star topology’, logical ring
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Sending Data on Token Ring
 Node wishing to send frame waits for token
 if token is ‘free’, it is replaced with data
frame
 Frame travels around ring
 each node reads destination address
 destination node
Workstation
Server



copies frame
resets status bits
puts frame back on ring
 transmitting node



Workstation
Token-ring
Network Printer
Workstation
removes frame from ring
creates a new token, and puts it on ring
waits a period before next grabbing token

prevents hogging
Workstation
Workstation
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Token Ring Features
 Unlike Ethernet, collisions cannot occur
 a node must wait until the token is free
 it is possible to calculate the maximum time that will pass before
any node will be able to transmit
 no wasted bandwidth from collisions
 Token ring uses a sophisticated priority scheme
 nodes can be assigned different priorities
 only nodes with high priority can seize the token
 If a sending device fails, its token may continue to circulate
forever and lock the network
 special monitor nodes can detect such a situation
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FDDI and DQDB
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Fibre Distributed Data Interface
 FDDI is essentially Token Ring over fibre
 covered by the same IEEE 802.5 standard



no electromagnetic interference
better security
faster data rates
 100 Mbps around 200 Km ring with 200 nodes
 Token is put back onto the ring immediately after a frame is
transmitted
 Capacity much greater than Token Ring
 Recently a CDDI (Copper Distributed Data Interface)
standard has also emerged
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Distributed Queue Dual Bus
Bus A
Head
Sink
Sink
Head
Bus B
 Attributed to the PhD thesis of Robert Newman (Uni. Western





Australia)
IEEE 802.6 is an example
High speed MAN (2 - 300 Mbps)
Range up to 34 miles.
Two parallel cables, if one breaks the signal can switch to the
other bus.
Highly resistance to cable damage
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ISDN, ATM & 10GbE
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Integrated Services Digital Network
 Now largely replaced by broadband technologies
 ISDN is digital network available over telephone
 integrates voice & non-voice transmission
 Basic installation uses three separate channels
 2 B channels = 64 Kbps - data channels
 D channel = 16 Kbps - control channel

can also used for telemetry
 remote (water/electricity) meter reading, alarm systems
 the three channels share time on a cable
 time-division multiplexing
 Industry installation uses thirty B channels + D
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Asynchronous Transfer Mode
 ATM is a very fast packet-switched protocol
 100 Mbps or greater, up to gigabits per second!
 Uses small fixed-size packets
 53 octets (5 header, 48 payload)

very similar to DQDB
 Connection oriented
 every packet with same destination travels same route
 virtual circuit number in header

requires dedicated routers
 CRC in header allows error checking at network nodes
 Protocol optimised for multimedia
 Will probably be replaced by Gigabit Ethernet technologies for
new implementations
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10 GbE
 10 Gigabit Ethernet
 10 times as fast as Gb Ethernet
 retains Ethernet frame format
 Over 1.2 million ports shipped in 2008
 Different standards available
 E.g. 10 GBASE-SR short range, 26m-85m. Low cost.
 E.g. 10 GBASE-LR range up to 10km, but 25lm often achievable.
 Fibre
 LAN PHY, etc
 Copper
 10GBaseT, etc
 Greatest distance between hosts currently 80 km (10GBASE-ZR), but this is a de-facto
standard only (not part of IEEE802)
 See
 http://en.wikipedia.org/wiki/10_gigabit_Ethernet
 http://www.ieee802.org/3/10GBT/public/nov03/10GBASE-T_tutorial.pdf
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Summary
 Network topologies
 ring, star, bus and mesh topologies
 combined topologies
 Communication protocols
 message switching
 LAN

Ethernet, Token Ring
 MAN
 FDDI, DQDB
 WAN
 ATM, ISDN
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