2 Hui Zhang
Aloha Network
3 Hui Zhang
Original Ethernet
4 Hui Zhang
High Level View
Goal: share a communication medium among multiple hosts connected to it
Problem: arbitrate between connected hosts Solution goals:
High resource utilization
Avoid starvation
Simplicity (non-decentralized algorithms)
5 Hui Zhang
Medium Access Protocols
Channel partitioning Divide channel into smaller “pieces” (e.g., time slots,
frequency) Allocate a piece to node for exclusive use
Random access Allow collisions “recover” from collisions
Taking-turns Tightly coordinate shared access to avoid collisions
6 Hui Zhang
Random Access Protocols
When node has packet to send Transmit at full channel data rate R. No a priori coordination among nodes
Two or more transmitting nodes -> “collision”, Random access MAC protocol specifies:
How to detect collisions How to recover from collisions
Examples of random access MAC protocols: Aloha Slotted ALOHA CSMA and CSMA/CD
8 Hui Zhang
Aloha
Nodes sends the message when it has data to send.
If it receives an ack, it considers the transmission completed, otherwise it retransmits after a random delay.
Simple, distributed protocol, but not very efficient 18% maximum utilization
Slotted Aloha: more efficient. Reduces chances of collision
37% maximum utilization
CentralComputer
9 Hui Zhang
Slotted Aloha
Time is divided into equal size slots (= packet transmission time) Node with new arriving pkt: transmit at beginning of next slot If collision: retransmit pkt in future slots with probability p, until
successful.
Success (S), Collision (C), Empty (E) slots
10 Hui Zhang
CSMA/CD
Broadcast technology
Carrier-sense multiple access with collision detection (CSMA/CD). MA = multiple access
CS = carrier sense
CD = collision detection
host host host host
host host host host
hub
11 Hui Zhang
CSMA/CD Algorithm
Sense for carrier. If carrier present, wait until carrier ends.
Sending would force a collision and waste time
Send packet and sense for collision. If no collision detected, consider packet delivered. Otherwise, abort immediately, perform “exponential back
off” and send packet again. Start to send at a random time picked from an interval
Length of the interval increases with every retransmission
12 Hui Zhang
Collision DetectionTime
A B C
13 Hui Zhang
Collision Detection: Implications
All nodes must be able to detect the collision.
Any node can be sender
The implication is that either we must have a short wires, or long packets.
Or a combination of both
Can calculate length/distance based on transmission rate and propagation speed.
Messy: propagation speed is media-dependent, low-level protocol details, ..
Minimum packet size is 64 bytes
– Cable length ~256 bit times Example: maximum coax cable length is
2.5 km
A B C
14 Hui Zhang
CSMA/CD: Some Details
When a sender detects a collision, it sends a “jam signal”. Make sure that all nodes are aware of the collision
Length of the jam signal is 32 bit times
Exponential backoff operates in multiples of 512 bit times. Longer than a roundtrip time
Guarantees that nodes that back off longer will notice the earlier retransmission before starting to send
15 Hui Zhang
Ethernet Frame Format
Preamble marks the beginning of the frame. Also provides clock synchronization
Source and destination are 48 bit IEEE MAC addresses. Flat address space Hardwired into the network interface
Type field is a demultiplexing field. What network layer (layer 3) should receive this packet? Is actually a length field in the 802.3 standard
CRC for error checking.
Preamble Type PadDest Source Data CRC
8 6 6 2 4
16 Hui Zhang
Minimum Packet Size
Why put a minimum packet size? Give a host enough time to detect collisions In Ethernet, minimum packet size = 64 bytes (two
6-byte addresses, 2-byte type, 4-byte CRC, and 46 bytes of data)
If host has less than 46 bytes to send, the adaptor pads (adds) bytes to make it 46 bytes
What is the relationship between minimum packet size and the length of the LAN?
17 Hui Zhang
Minimum Packet Size (more)
propagation delay (d)a) Time = t; Host 1 starts to send frame
Host 1 Host 2
propagation delay (d)
Host 1 Host 2b) Time = t + d; Host 2 starts to send a frame just before it hears fromhost 1’s frame
propagation delay (d)
Host 1 Host 2c) Time = t + 2*d; Host 1 hears Host 2’s frame detects collision
LAN length = (min_frame_size)*(light_speed)/(2*bandwidth) = = (8*64b)*(2*108mps)/(2*107 bps) = 5.12 km
18 Hui Zhang
Ethernet Physical Layer
10Base2 standard based on thin coax.
Thick coax no longer used Nodes are connected using
thin coax cables and “T” connectors in a bus topology
10-BaseT uses twisted pair and hubs.
Hub acts as a concentrator
The two designs have the same protocol properties.
Key: electrical connectivity between all nodes
Deployment is different
host host host host
host host host host
Hub
Host
19 Hui Zhang
Ethernet Technologies: 10Base2
10: 10Mbps; 2: under 200 meters max cable length Thin coaxial cable in a bus topology
Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!
20 Hui Zhang
10BaseT and 100BaseT
10/100 Mbps rate; later called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus
“star topology”
21 Hui Zhang
802.3u Fast Ethernet Apply original CSMA/CD medium access protocol at
100Mbps Must change either minimum frame or maximum diameter:
change diameter Requires
2 UTP5 pairs (4B5B) or 4 UTP3 pairs (8B6T) or 1 fiber pair
No more “shared wire” connectivity. Hubs and switches only
4B/5B encoding
22 Hui Zhang
Gbit Ethernet
Use standard Ethernet frame format Allows for point-to-point links and shared broadcast
channels In shared mode, CSMA/CD is used; short distances
between nodes to be efficient Uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links
23 Hui Zhang
Traditional IEEE 802 Networks:MAC in the LAN and MAN
Ethernet defined as IEEE 802.3. Not quite identical
The IEEE 802.* set of standards defines a common framing and addressing format for LAN protocols.
Simplifies interoperability Addresses are 48 bit strings, with no structure
802.3 (Ethernet) 802.5 (Token ring) 802.X (Token bus) 802.6 (Distributed queue dual bus) 802.11 (Wireless)
24 Hui Zhang
LAN Properties
Exploit physical proximity. Typically there is a limitation on the physical distance between the
nodes, for example,
– to collect collisions in a contention based network
– to limit the overhead introduced by token passing or slot reservations
Relies on single administrative control and some level of trust. Broadcasting packets to everybody and hoping everybody (other
than the receiver) will ignore the packet
Token passing protocols assume everybody plays by the rules
25 Hui Zhang
Why Ethernet?
Easy to manage. You plug in the host and it basically works No configuration at the datalink layer
Broadcast-based. In part explains the easy management Some of the LAN protocols (e.g. ARP) rely on broadcast
– Networking would be harder without ARP Not having natural broadcast capabilities adds complexity to a LAN
– Example: ATM
Drawbacks. Broadcast-based: limits bandwidth since each packets consumes the
bandwidth of the entire network Distance
26 Hui Zhang
802.3z Gigabit Ethernet
Same frame format and size as Ethernet. This is what makes it Ethernet
Full duplex point-to-point links in the backbone are likely the most common use.
Added flow control to deal with congestion
Choice of a range of fiber and copper transmission media.
Defining “jumbo frames” for higher efficiency.
Top Related