LAN Topology, Ethernet

• LAN Topology:
  • Aloha
  • Ethernet
  • Wireless LANs
  • ATM
• Ethernet
  • Ethernet Wiring
  • Ethernet Encoding
  • Ethernet Frames
  • Hardware Addressing
  • Ethernet Interface Hardware
  • Frame Types

Local Area Networks

• point-to-point links to completely connect n computers need n-1 interfaces on each computer, for a total cost of O(n²)
• instead, we can connect each computer to 1 shared network for a total cost of O(n)
• such a network is especially useful when the cost of the cabling is small compared to the cost of the interfaces: in Local Area Networks (LANs)
• these networks use TDM over a shared medium

LAN Topologies

• Star Topology: each computer attaches to a central point, which redistributes the data (e.g. Ethernet with hubs/switches)
• Ring Topology: each computer attaches to the next, and the last one back to the first. Each interface must forward and/or keep messages (e.g. FDDI)
• Bus Topology: each computer is directly connected to a physically shared medium (e.g. older Ethernet, Aloha)
• in practice, most installations tend to be star topology, leading back to a wiring closet
• star topology tends to be more robust, leading to easier fault isolation

Aloha

• wireless star topology
• frequency f₁ carries data from end systems to hub, f₂ carries data from hub back to end systems
• original Aloha (1960s) used military radios, all traffic received by hub was rebroadcast to all the end systems
• the only problem is if two end systems transmit at the same time -- a collision which keeps the hub from hearing either packet
• a 1-bit overlap is sufficient to corrupt an entire packet

Aloha Collisions

• if the hub rebroadcasts each packet, each sender can listen for its transmission and retransmit if the packet is missing or wrong
• the maximum theoretical traffic that can be sent is about 18% of the channel capacity (assuming equal-sized packets): sending more means more collisions and less useful traffic
• slotted Aloha constrains sender to start sending at the beginning of a slot, reducing overlap and allowing up to about 30% of the bandwidth to be used
• Aloha is used for communication from earth stations to satellites: the satellite can be the hub, or the satellite can relay traffic back to an earth station which is the hub

Ethernet Shared Medium

• Ethernet is similar to Aloha, except it uses a wire as its shared medium
• on a short enough wire, senders can sense others' transmission and wait: Carrier Sense Multiple Access, CSMA
• if two senders are waiting for a frame to complete, the might start sending almost simultaneously, so we can still have collisions
• on ethernet, if we have a collision we wait a random time then try again
• every successive time we collide, we double the range of our random waiting time: exponential backoff
• after 16 collisions, we give up

Ethernet Collision Detection

• on a radio, we cannot listen and transmit at the same time
• on a wire, we can!
• if the data I am sending is being received incorrectly by my own receiver, there must be a collision
• if so, I jam the wire (put an illegal electrical pattern on it) to make sure everyone detects the collision -- this is CSMA/CD
• CD requires a small enough "diameter" that everyone sees the same collisions
• Ethernet will start rejecting bursty packets at around 50% average load

Modern Ethernet

• point-to-point links from end systems to hubs (or switches)
• a hub is the only place where collisions and medium sharing take place
• hubs can be connected to other hubs, up to about 2 (diameter) -- more than 2, and collision detection becomes unreliable
• switches work like hubs, but optimize by only sending on the wire connecting to the destination
• speeds of 10Mb/s, 100Mb/s, 1Gb/s, and soon 10Gb/s
• different media for the point-to-point links: copper (CAT-5), fiber

Wireless LANs

• station A and station C might both be in range of station B, but not in range of each other
• A sends a short Request To Send packet (RTS), including the length S of the packet it really wants to send
• B sends a short Clear To Send (CTS) packet, also including the same length S
• if A gets the CTS back, then anyone within range of either A or B knows to keep quiet for S bit times
• this is Carrier Sense Multiple Access with Collision Avoidance, CSMA/CA

ATM Topology

• Asynchronous Transfer Mode, based on ideas from telephony: connections, point-to-point links
• central ATM switch accepts data from each end system, forwards it to the correct end-system (non-broadcast)
• one fiber for each connection in each direction
• star topology for end systems
• ATM switches can be interconnected arbitrarily, will forward data from one to another to the final destination

Ethernet Wire Types

• Thickwire (10Base5) -- coaxial cable is the shared medium, connected to transceivers (using vampire taps), connected to Network Interface Cards
• Thinwire (10Base2) -- coaxial cable is the shared medium, plugged into BNC connectors connected to Network Interface Cards
• CAT-5 (10BaseT) -- twisted pair connects Network Interface Card to hubs, which are the actual shared medium.

Ethernet Encoding

• 10Mb/s Ethernet is encoded using Manchester Encoding
• "1" bit is encoded by a rising edge:
  • if the voltage was low, in the middle of the bit period, raise the voltage
  • if the voltage was high, lower the voltage at the beginning of the bit period, then raise it in the middle of the bit period.
  "0" bit is encoded by a falling edge
• there is at least one transition per bit, allowing the receiver to keep its clock synchronized
• 1010101010... (64 bits) bit pattern is sent before every frame: preamble, has a transition in the middle of each bit slot

Ethernet Frames

• preamble allows receiver clocks to synchronize
• destination comes first: very little buffering/intelligence needed to discard a frame that is not for me
• source address is generally ignored
• ethertype identifies next higher protocol
• minimum data length is required to insure that everyone sees the same collisions

Ethernet Addresses

• each ethernet address is 6 bytes long
• each ethernet NIC (Network Interface Card) carries a unique ethernet address in hardware
• blocks of 2²⁴ ethernet addresses are sold to companies for 4-digit (dollar) figures.
• it is up to the company to assign the low-order 24 bits as they please
• because each address is globally unique, computers plugged into the same network are guaranteed to have distinct addresses

Broadcast and Multicast

• each NIC generally looks at the first 6 bytes of the header to determine whether to read or ignore the rest of the packet
• NICs are also configured to accept the address ff:ff:ff:ff:ff:ff -- the broadcast address
• many NICs can also be configured to recognize other (arbitrary) addresses -- this allows for multicasting, sending data to a restricted set of hosts without bothering the others
• many NICs can also be configured into promiscuous mode, where they forward all packets to the host

Network Analyzers, tcpdump

• analyzers can make visible (blinking lights :-) or report any number of events or statistics:
  • collisions
  • frames per second
  • average frame size
  • network utilization
  • broadcast frames
• analyzers can also be full computers running software to classify each packet
• tcpdump on Unix will print all the headers of all received packets
• most LANs are not very secure!

Network Interface Cards

• a Network Interface Card (NIC) connects the computer (CPU and memory) to the network
• a NIC has an analog side which connects to the LAN and a digital side connected to the computer
• a NIC will:
  • recognize frames
  • check the destination address
  • if the frame is for us, store it in memory (DMA) and interrupt the computer
  • also check the CRC
  • DMA outgoing frames, insert CRC after sending

Ethernet Type, LLC/SNAP

• ethertype is 0x0800 for IP packets, 0x0806 for ARP packets
• if ethertype is 1500 or less, it encodes the frame length, and other protocols (LLC/Logical Link Control) must be used to determine next higher layer
• LLC starts with bytes AA, AA, 03
• followed by 3 bytes of organizational unique ID identifying the issuer of the type field -- all zeros for Ethernet types
• followed by the type field itself