– What is the problem to be addressed in this sublayer?
– Protocols that allow collision
• Pure ALOHA
• Slotted ALOHA
• CSMA
• CSMA/CD
– Collision free protocols:
• bitmap method, binary countdown and token

• Token pass.
– There is only one token in the network.
– The token is passed through every node in the network.
– Only the node that has the token can transfer data.

– collision based protocols (ALOHA,CSMA/CD) are good when the network load is low.
– collision free protocols (bit map, binary countdown) are good when load is high.
– How about combining their advantages -limited contention protocols.
• Behave like the ALOHA scheme under light load
• Behave like the bitmap scheme under heavy load.

– adaptive tree walk protocol
• trick: dynamic partition the stations into groups and limit the contention for each slot.
– under light load, every one tries for each slot like ALOHA
– under heavy load, only a small group can try for each slot
– how do we do it
» treat stations as the leaves of a binary tree.
» first slot, all stations (under the root node) can try to get the slot.
» if no conflict, repeat.
» if conflict, use depth first search to traverse the tree, only nodes of a sub-tree get to try for the next slot.

Example: 0
1
2
A
3
B C*
4
D
5
E* F* G
6
H*
Slot 0: C*, E*, F*, H* (all nodes under node 0 can try), conflict slot 1: C* (all nodes under node 1 can try), C sends slot 2: E*, F*, H*(all nodes under node 2 can try), conflict slot 3: E*, F* (all nodes under node 5 can try), conflict slot 4: E* (all nodes under E can try), E sends slot 5: F* (all nodes under F can try), F sends slot 6: H* (all nodes under node 6 can try), H sends.

– Invented at Xerox by Robert Metcalfe (founder of 3Com) and Dave Boggs
– background:
• ARPANet in late 60's, linking computers at different sites to central mainframe computers.
• By early 70's, the cost of computers went down, introduction of mini-computers PDP, which means each school can have more than one computer!
– Applications: share printers, share files, share cycles
• Factory automation: many computers on factory floor
• Need local area networks to link the computers

– Use shared medium instead of switched-based
– cost: one adaptor/machine + link
– performance: all hosts sharing one link.
– first Ethernet:
• 3 Mbps
– PDP-11 0.25 MIPS, 0.1 Mbps peak
– no all computers transmit at peak all the time meaning, easily support up to 100 computers at that time
– Now:
– 500MHz Pentium, around 200MIPS, 100Mbps
– 10Mbps cannot support as many machines.

– multiple stations may transmit on the medium at the same time, which may result in collisions
– Two solutions
• guarantee that only one station transfers at one time: (contention free protocol) FDDI, token ring, token bus use the first approach
• try the luck and re-transmit if there is a collision
(contention based protocol)
– need algorithm to reduce the probability of collision
• Ethernet uses CSMA/CD + binary exponential backoff to reduce the probability of collisions

– sense before send (CSMA)
– abort sending upon detecting collision (CD).
– adjust retransmission interval (binary exponential backoff)
• each time slot to be 51.2 us
• first collision, retransmission interval = random number between [0,1]
• second collision, interval = random number between
[0,1,2,3]
• kth collision, interval = random number between [0,
2^k-1]
• upper bound 1023 slots.

– Bandwidth: 10 Mbps
• Propagation Delay: limit the frame size.
– thin cable/thick cable/twisted pair/fiber
10Base5 500 meters thick (cable) Ethernet 100 nodes/seg
10Base2 200 meters thin (cable) Ethernet 30 nodes/seg
10BaseT 100 meters twist pair 1024 nodes/seg
10BaseF 2000 meters fiber optics 1024 nodes/seg
10Base5/10Base2, cable connected to each machine
10BaseT -- connecting to a hub
10BaseF -- between building Connecting

– Multiple segments can be connected through the repeaters (hubs).
– All segments connected by the repeaters are in the same collision domain .
• constraint: no two transceivers may be 2.5km apart and separated by 4 repeaters.
• frame format
| Preamble | Start| Dst Addr | Src Addr | length | Data |Pad |Checksum|
7 1 2/6 2/6 2 0-1500 0-46 4
– Header: 14 Bytes, CRC: 4 Bytes
– Minimum data (+ pad) length: 46 Bytes
– Maximum data length: 1500 Bytes

• Minimum frame size = ??
• Why? To run CSMA/CD, each frame must be large enough to detect collision.
– 2 * max propagation delay?
» standard: 2500m, 500m per segment, 4 repeaters.
» speed of light: 3*10^8m/s
» speed of signal propagation: 2*10^8m/s
» propagation delay: about 25us (on wire) +25 us in repeaters, total delay = 51.2us
» How many bytes do we need in each frame?
• Maximum frame size = ?
• Why?
– larger is better for bandwidth utilization

• How to find out your Ethernet address: "arp”
– /usr/sbin/arp xi --> xi (128.186.121.41) at 8:0:20:92:43:b1
• Ethernet Switch: Increase the bandwidth, segments connected by switch have different collision domain.
– Ethernet switch: data link layer device
– Ethernet hub (repeater): physical layer device
– Fast Ethernet
• Keep everything in Ethernet, make the clock faster 100Mbps.
• What are the problems?
– Cable
» 100Base-T4 100m category 3 UTP, 4 lines.
» 100Base-Tx 100m category 5 twisted pair
» 100Base-Fx 2000m Fiber optic

– Cable
– CSMA/CD?
• minimum frame size = 64byte = 512 bits,
• 5.12us using 100Mbps transmission rate.
• What can you do about this?
– Increase the minimum frame size.
– Reduce cable length
– Faster Ethernet:
» Reduce the cable length by a factor of 10, maximum length = 200 meters (100-Base-T, 100 meter cable).
• Full duplex mode: point to point connection, no contention. No CSMA/CD needed, can have longer cable.

– Gigabit Ethernet: make it even faster at 1Gbps.
• Cable: mainly fiber optics.
• CSMA/CD domain
– Shortening the cable? 20 meters
• Alternative: increase the minimum frame size to 512 bytes, CSMA/CD domain 200 meters (not much error margin)
– Experimental studies say that typical frame size are 200 -
300 bytes.
• backward compatibility:
– carrier extension -- short packet, stuff extra bits to make to
512 bytes
• improve performance: packet bursting -- transmit a burst of small frames, only the first one need carrier extension.