C SC 481.20 Lecture 14: Ethernet
major resource: Computer Networking (4th Edition), Kurose and Ross, Addison Wesley, 2008

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Prequel: IEEE 802 project

• Collection of LAN standards
• Major sections are:
  • 802.1 : internetworking standards (network layer)
  • 802.2 : LLC specifications
  • 802.3 : Ethernet specifications
  • 802.4 : Token Bus specifications
  • 802.5 : Token Ring specifications
  • 802.11 : wireless LANs
• We will focus on IEEE 802.3, Ethernet

Ethernet Overview

• Originally developed at Xerox by Bob Metcalfe.
• Named after the "ether", a term describing the medium through which scientists once thought electromagnetic radiation flowed.
• Ether refers to the cable. Originally coax, now typically twisted pair or fiber optic
• Uses digital signal with Manchester encoding: each bit period has a signal transition in middle (low-to-high for 0, high-to-low for 1), to facilitate synchronization.
• Transmission rates have increased by orders of magnitude: originally 1Mbps (bits per second), then 10Mbps; Fast Ethernet is 100Mbps, Gigabit Ethernet is 1Gbps, latest (2007) is 10Gbps.
• Note that M and G here are literally 1 million and 1 billion, not 2²⁰ and 2³⁰ as they are for storage capacity
• A number of standards exist
  • 10Base-T : 10Mbps, baseband, Twisted pair cable
  • 100Base-T : 100Mbps, baseband, Twisted pair cable
  • 100Base-FX : 100Mbps, baseband, Fiber optic cable
  • 1000Base-T : figure it out -- this is achieved using 4 twisted pairs
  • 1000Base-LX : uses fiber, L is for Long haul
  • 10GBase-T : 10Gbps, baseband, Twisted pair cable (2007)
  • Note: "baseband" means the signal uses entire bandwidth
• Characterized by bus topology and CSMA/CD for coordinating transmission.
• Connectionless! Unreliable! No ACKs!
• Reliability has to be implemented in higher layers (e.g. TCP) if desired

CSMA/CD : Frames Collide on the Common bus

• Consider how collisions can occur, since a station will not begin transmitting until it senses an idle carrier.
• If collision occurs, how to avoid repeating it, since both stations will try again? Solution is to have each station wait a different random amount of time before retrying.
• Specific Ethernet solution:
  • Think of "bit time" to be a small time slot - the time to transmit one bit
  • After i'th collision involving a frame, station waits random period based on i.
  • Range of random period is from 0 to 2ⁱ-1 bit times
  • This is called binary exponential backoff. Can you see why?
  • After first collision, they wait 0 or 1. With 2 stations, 50% probability of second collision.
  • After second collision, they wait 0 to 3. With 2 stations, 25% probability of third collision.
  • Etc. The maximum i is 10, the range 0 to 1023.
  • This is compromise, if period too short collisions reoccur; if too long bandwidth is wasted

Ethernet Addresses

• Ethernet uses 48-bit (6 byte) addresses.
• Recall that frames are broadcast.
  • NIC reads destination address as frame comes by.
  • If it matches the NIC i.d. number, the frame is saved and its payload passed on to the network layer of its station.
  • Otherwise, remainder of frame is ignored. Destination address is at the beginning of the frame.
• Some frames are truly broadcast, e.g. intended for all stations. A special destination address, all 1’s, is reserved for this purpose, and all NIC cards will match it.

Ethernet Frame

• Each frame contains both source and destination MAC address.
• Generic: consists of frame header part, with addressing and type information, and frame data part with the bits/bytes being transmitted (payload).

• field	bytes	description
  preamble	8	alternating 1 and 0 for synch (Manchester sine wave), last byte ends in "11"
  destination	6	all 1’s means broadcast, first bit 1 = group address, else individual
  source	6	physical address of source station
  type/length	2	#data bytes, or predefined frame type (mutually exclusive ranges)
  data	46-1500	If less than 46 bytes of data, is padded out to 46 bytes with zeroes. Necessary for minimum 512 bit frame length.
  CRC	4	Cyclic Redundancy Code

LAN extending device: Hub

• Ethernet defines maximum distance between stations
• Hubs are one means to extend
• Is a repeater: regenerates and retransmits input signals
• Physical layer device -- doesn't interpret bits it is carrying
• Multiple Ethernet stations plug into one hub to form star.
• Unless buffers provided, all stations must use same transmission protocols (e.g. 10Mbps)
• All stations connected through hub(s) comprise a collision domain, a.k.a. segment

LAN extending device: Ethernet switch

• Conceptually like a router but has differences
  • Performs store-and-forward at Link layer (router does this at Network layer)
  • Operates at only Physical and Link Layer (router also has Network layer)
  • Maintains forwarding table
  • Forwards based on MAC address, not IP address
  • Has input interfaces and buffers, switching fabric, output interfaces and buffers
  • Scope is limited to LAN
• Ethernet stations are not aware of switch
• Due to store-and-forward nature, no collisions occur across a switch
• Possibly only one station attached to each interface
  • In this situation, each station forms its own segment (i.e. collision domain)
  • Since NIC can only process one frame at a time, collision is not possible!
  • This renders the CSMA/CD protocols unnecessary
• Ideal for connecting heterogeneous segments (e.g. 10Mbps, 100Mbps, 1000Mbps)
  • Different interfaces (ports) can have different operating speeds or cabling
  • Internal memory buffers data for slower interfaces
• Can switch multiple transmissions at once, if each destination is different segment.
• Switch autonomously builds and maintains switch table, its forwarding table
  • This is called self learning
  • Each table entry has: MAC address, interface leading to it, and TTL
  • When frame arrives on interface X, it inspects frame's source address SA and adds, or updates the TTL for, the table entry for (SA,X).
  • This is called backwards learning.
  • Analogy: If I'm at a fork on an unmarked hiking trail and want to go to Sunbury, I wait for someone to come toward me then I ask them where they came from. If they came from Sunbury, I know to take the fork they came from.
  • Beware of cycles!
    • It is possible, particularly with hubs, that a segment will be connected to more than 1 switch
    • In this case, a loop of alternating switches and segments may be possible
    • Ethernet frame, unlike IP datagram, does not have TTL field to halt a cycle
    • This an infinite loop is possible!
    • Protocol is needed to construct spanning tree over the LAN and route only on it.
      • To construct spanning tree, consider segment as node, switch interface as edge
      • Prune edges such that all nodes are "covered" with no cycles (tree = no cycles)
      • Do not route across a pruned edge -- this basically shuts down the corresponding switch interface
• What the switch does when a frame from SA addressed to destination DA arrives on interface X:
  • Adds or updates Switch Table entry as described above for (SA,X)
  • If destination DA is not in Switch Table, forward it on all outbound interfaces except X
  • If destination DA is in Switch Table and its interface is X, drop it (DA is on the same segment as SA so the frame does not need to be forwarded)
  • If destination DA is in Switch Table and its interface is Y where Y!=X, switch and forward it to interface Y.
• A network administrator may be faced with a choice of using switch versus router
  • It is permissible to go either way, can have hierarchy of IP routers w/i organization
  • Consider processing speed of 2 layer (link, physical) versus 3 layer processing
  • Consider ease of administration (switch as self-learning)
  • If network is relatively small, use switches. If larger, consider combination
• Sometimes the term bridge is used interchangeably with switch. Strictly speaking, a bridge is a 2-layer (link, physical) device used to connect two dissimilar LAN segments and thus has only two interfaces. A switch can do everything a bridge does plus can handle more than two segments.