A network with a large bandwidth-delay product is known as a long fat network (shortened to LFN and often pronounced "elephant"). As defined in RFC 1072 , a network is considered an LFN if its bandwidth-delay product is significantly larger than 105 bits (~12 kB ).
CSMA-CD (Ethernet) protocol: Normalized Delay-Bandwidth Product Propagation delay Time to transmit a frame
Typical Delay-Bandwidth Products The table below shows the number of bits in transit in one-way propagation delay assuming propagation speed of 3x10 8 m/s. (Max size Ethernet frame: 1500 bytes = 12000 bits) Distance 10 Mbps 100 Mbps 1 Gbps Network Type 1 m 3.33 x 10 -02 3.33 x 10 -01 3.33 x 10 0 Desk area network 100 m 3.33 x 10 01 3.33 x 10 02 3.33 x 10 03 Local area network 10 km 3.33 x 10 02 3.33 x 10 03 3.33 x 10 04 Metropolitan area network 1000 km 3.33 x 10 04 3.33 x 10 05 3.33 x 10 06 Wide area network 100000 km 3.33 x 10 06 3.33 x 10 07 3.33 x 10 08 Global area network
Normalized Delay versus Load E[T] = average frame transfer delay X = average frame transmission time
At low arrival rate, only frame transmission time
At high arrival rates, increasingly longer waits to access channel
Max efficiency typically less than 100%
Load Transfer delay E[T]/X max 1 1
Dependence on t prop R / L Transfer Delay Load E[T]/X max 1 1 max a a a > a
In heavily loaded networks collisions increase and every packet takes many attempts to get through and ultimately the network becomes collision dominated and throughput (S) goes down to zero. G is the total load.
For ALOHA peak throughput is 18.4% of channel capacity
Better channel usage if we detect the collision when it occurs rather than waiting until the end of the packet.
Carrier Sense Multiple Access with Collision Detection - CSMA-CD
Performed by the transmitting station listening to itself and if what it hears is different from what it sends then there is a collision.
If this occurs, transmitter sends a short jamming signal which notifies all stations there has been a collision – without this the receiver will not know there has been a collision and will continue to listen.
Then the transmission is aborted and a re-try scheduled.
At the time of writing the IEEE standards are available free on-line at http://standards.ieee.org/getieee802/portfolio.html
Wireless Computer Networks The task groups within 802.15 WPAN™ are: Task Group 1: (802.15.1) Bluetooth ; Task Group 2: Coexistence; Task Group 3: High data rate; Task Group 4: (802.15.4) Sensor networks .
For 10Base5 this is up to 1023x102.4 μs ~ 0.1 seconds
Then a further 6 retries at this limit.
After 16 retries it gives up and reports an error.
This is the standard… however, it is a fight for the network and both devices should choose a random number. Some vendors have been naughty in the past and have choosen lower numbers which makes them appear to be ‘faster’ network cards.
IEEE 802.3 Physical Layer IEEE 802.3 10 Mbps medium alternatives Thick Coax: Stiff, hard to work with T connectors transceivers 10base 5 10base 2 10base T 10base FX Medium Thick coax Thin coax T wisted pair Optical f iber Max. Segment Length 5 00 m 2 00 m 100 m 2 km Topology Bus Bus Star Point-to-point link
100baseT4 100baseT 100baseFX Medium Twisted pair category 3 UTP 4 pairs Twisted pair category 5 UTP two pairs Optical fiber multimode Two strands Max. Segment Length 100 m 100 m 2 km Topology Star Star Star
Frame bursting to allow stations to transmit burst of short frames
Frame structure preserved but CSMA-CD essentially abandoned
Extensive deployment in backbone of enterprise data networks and in server farms
1000baseSX 1000baseLX 1000baseCX 1000baseT Medium Optical fiber multimode Two strands Optical fiber single mode Two strands Shielded copper cable Twisted pair category 5 UTP Max. Segment Length 550 m 5 km 25 m 100 m Topology Star Star Star Star
WAN PHY for wide area interconnection using SONET OC-192c
Extensive deployment in metro networks anticipated
10GbaseSR 10GBaseLR 10GbaseEW 10GbaseLX4 Medium Two optical fibers Multimode at 850 nm 64B66B code Two optical fibers Single-mode at 1310 nm 64B66B Two optical fibers Single-mode at 1550 nm SONET compatibility Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band 8B10B code Max. Segment Length 300 m 10 km 40 km 300 m – 10 km
Typical Ethernet Deployment Server 100 Mbps links 10 Mbps links Server Server Server 100 Mbps links 10 Mbps links Server 100 Mbps links 10 Mbps links Server Gigabit Ethernet links Gigabit Ethernet links Server farm Department A Department B Department C Hub Hub Hub Ethernet switch Ethernet switch Ethernet switch Switch/router Switch/router
LAN Bridges and Ethernet Switches (Section 6.11 in the course text)
Thank You Recommended Private Study Read Chapter 6 of the course text. (Note: Content in 6.8 on Token Ring and 6.10 on Wireless LANs is not assessed. Source Routing Bridges and following sections are not assessed. )