Fundamentals of Ethernet LANS Flashcards
Ethernet LAN switch
A device which provides many physical ports into which cables can be connected
Ethernet Products
- 10 Mbps | Ethernet | 10Base T | 802.3 | Copper 100m
- 100 Mbps | Fast Ethernet | 100Base T | 802.3u | Copper 100m
- 1000 Mbps | Gigabit Ethernet | 1000Base LX | 802.3z | Fiber 5000m
- 1000 Mbps | Gigabit Ethernet | 1000Base T | 802.3ab | Copper 100m
- 10 Gbps | 10 Gig Ethernet | 10GBase T | 802.3an | Copper 100m
What is an Ethernet LAN
It is a combination of user devices, LAN switches, and different kinds of cabling. Each link can use different types of cables, at different speeds
Components of Transmitting Data using Twisted Pair (UTP - Unshielded Twisted Pairing)
- Creating an electrical circuit - A electrical circuit requires a complete loop, so the two nodes, using circuitry on their Ethernet ports, connect the wires in one PAIR to complete a loop, allowing electricity to flow
- To send data, the two devices follow some rules called an encoding scheme
Problem with TP
- Value of twisting - when electrical current passes over any wire, it creates electromagnetic interference (EMI) that interferes with the electrical signals in nearby wires (Crosstalk), Twisting helps cancel most of the EMI
- Issue of wiretapping
- Distance restriction
Components Of A UTP Ethernet Link
Ethernet link refers to any physical cable between two Ethernet nodes
- UTP cable, holds some copper wires grouped as twisted pair, color coded
- RJ 45 connectors, has 8 physical location into which 8 wires in the cable can be inserted called pin positions
- NIC card with ports
Straight Through Cable Pinout (10Base T & 100Base T) (Setup) (Host & Switch)
2 pairs of wires in a UTP cable, One for each direction
As a rule, Ethernet NIC transmitters (Host) use the pair connected to pins 1 and 2; the NIC receivers
use a pair of wires at pin positions 3 and 6.
LAN switches, knowing those facts about what
Ethernet NICs do, do the opposite: Their receivers use the wire pair at pins 1 and 2, and their
transmitters use the wire pair at pins 3 and 6
Pinout config: 1-1, 2-2, 3-3, 4-4
Crossover Cable
Pinout config: 1-3 2-6 3-1 6-2
10Base T & 100Base T Pin Pairs
Transmit on Pins 1,2
PC NICs
Router
Wireless AP
Transmits on Pins 3,6
Hubs
Switches
Rule of cabling pinout logic
Crossover cable: if the endpoints transmit on the same pin pair
Straight through cable: If the endpoints transmit on different pin pairs
UTP cabling pinouts for 1000Base T
1000BaseT requires 4 wire pairs
Straight through config:
1-1, 2-2, 3-3, 6-6, 4-4, 5-5, 7-7, 8-8 (Order is important)
Crossover config:
1-3, 2-6, 4-5, 7-8, 3-1, 6-2, 5-4, 8-7
Fiber Cabling
Uses glass as medium which light passes, varying that light over time to encode 0s and 1s
Made out of 5 layers
- Outer Jacket
- Strengthener
- Buffer
(Light Transmission Components)
- Cladding, reflects light internally to travel through the core
- Core
Optical transmitter, shines a light into the core, light passes through it
Disadvantages of fiber
- Brittle, Breaks easily
2. Hard, difficult to bend
Types of Fiber Cabling
Multimode Fiber, Single Mode Fiber
Multimode Fiber
Characterized by the fact that the cable allows for multiple angles of light waves entering the core, Uses a larger core
Single mode Fiber
Uses a smaller diameter core, around 1/5 the diameter of common multimode cables, Uses a laser based transmitter which sends a light at a single angle
Multimode Vs Single
MM - Improves maximum distance over UTP, Less expensive
SM - 10GB Ethernet over Fiber allow for distances up to 400m, allows distances into tens of kilometer, Slightly more expensive hardware
UTP Vs Fiber
UTP, work poorly in some electrically noisy environment, can be affected by electromagnetic interference, emit faint signal outside the cable
Fiber, Improves distance, faster, does not create faint signal emission (network more secure)
Criteria | UTP | MM | SM
Cost of Cabling Low Medium Medium
Cost of Switch Port Low Medium High
Max Distance 100m 500m 40km
Interference | Some | None | None
Copying Emission | Some | None | None
Ethernet Data Link Protocol Frame Format
Ordered
- Preamble | 7 Bytes | Synchronization
- Start Frame Delimiter | 1 Bytes | Signifies next byte begins the Destination MAC address field
- Destination MAC Field | 6 Bytes | Address of intended recipient
- Source MAC Field | 6 Bytes | Address of sender
- Type | 2 Bytes | Defines the type of protocol listed inside the frame. IPv4 Or IPv6
- Data & Pads | 46-1500 Bytes | Holds data from a higher layer
- Frame Check Seq | 4 Bytes | Error detection capabilities
Ethernet Address (MAC address)
6 byte long, usually represented as 12 digit hexadecimal numbers.
Eg. 0000.0C12.3456
Most MAC addresses represent a single NIC, so these addresses are often called a UNICAST Ethernet address
What is Unicast addresses?
The term unicast is simply a formal way to refer to the fact that address represents one interface to the Ethernet LAN
Sending to a single device
How does MAC address establishes uniqueness?
MAC address 6 bytes
3 Bytes - Unique 3 byte code given by IEEE, Organizationally Unique Identifier (OUI)
3 Bytes - Given by Manufacturer
MAC Address
OUI Vendor Size in bits 24 Bits 24Bits Size in hex 6 Hex 6 Hex Example 00602F 3A07BC
Universal address, Group address
Universal address emphasizes the fact that the address assigned to a NIC by a manufacturer should be unique
Group addresses identify more than one LAN interface card. A frame sent to a group address might be delivered to a small set of devices on the LAN
( Broadcast Address, Multicast Address )
Broadcast address, Multicast address
Broadcast address: Frames sent to this address should be delivered to ALL devices on the Ethernet LAN, Has a value of FFFF.FFFF.FFFF
Multicast address: Frames sent to a multicast Ethernet address will be copied and forwarded to a SUBSET of the devices on the LAN
Frame Check Seq
An error detection mechanism which finds out whether a frame’s bits changed while crossing over an Ethernet link. If results obtained are the same, the frame did not change, else discard
Uses cyclic redundancy check (CRC)
Error detection = Error recovery?
Error detection does not mean error recovery. Ethernet defines that the errored frame should be discarded but Ethernet does not attempt to recover the lost frame
Other protocol notably, TCP recover the lost data by noticing that it is lost and sending data again
Data transfer in Modern Ethernet LAN
- PC1 builds and sends the original Ethernet frame, using its own MAC address as the source address and PC2’s MAC address as the destination address
- Switch SW1 receives and forward the Ethernet frame out its interface to SW2
- SW2 receives and forwards the Ethernet frame out its interface to PC2
- PC2 receives the frame, recognizes the destination MAC address as its own and processes the frame
Duplexes
Half Duplex; the device must wait to send if it is currently receiving a frame, cannot send and receive at the same time
Full duplex; The device does not have to wait before sending, it can send and receive at the same time
LAN hubs
Forwards data using physical layer standards rather than data-link standards, Consider to be Layer 1 devices
When an electrical signal comes in one hub port, the hub REPEATS that signal out all other ports
No concept of Ethernet frames, addresses, making decisions based on addresses
Downside Of Hub
1. If two or more devices transmitted a signal at the same instant, the electrical signal collides and becomes garbled
Purpose
1. Repeats all received electrical signals, Floods each frame out all other ports except incoming port. Therefore, two or more transmission either way through a hub would cause a collision
Collision
Carrier sense multiple access with collision detection (CSMA/CD)
Steps:
1. A device with a frame to send listens on the line until the Ethernet is not busy
- When the Ethernet is not busy, the sender begins sending the frame
- When collision occurs, the transmitting nodes sends a jamming signal to all nodes
- All nodes then independently choose a random time to wait before trying again, to avoid unfortunate timing
- The next attempt starts again at Step 1
This type of design shares bandwidth. Enforces the rules only one device to successfully send a frame at any point in time
WAN Tech Layers
WAN tech define the physical standards and data link protocols used to communicate long distances
Types of WAN
Leased line WAN
Ethernet WAN
Leased line WANs
Physical Details:
- Delivers bits in both direction ( Full duplex )
- At predetermined speed
- Uses two pairs of wires, one pair for each direction of sending data
Leased on a monthly basis
Leased line terminology
Leased circuit - Electrical circuit between the two endpoints
Serial link - Bits flow serially and that routers use serial interfaces
Point to Point link - Topology stretches between two points and two points only
T1 - Specific type of leased line transmits data at 1.544Mbps
Private line - Data sent over the line cannot be copied by other telco customers, data is private
Leased line cabling
Telcos put their equipment in buildings called central offices
Telcos installs cables from the CO to most every other building in the city. The telco would then configure its switches to use some of the capacity on each cable to send data in both directions, creating the equivalent of a crossover cable between the two routers
Attributes of Leased line
Only provides layer 1 services. Promises to deliver bits between devices connected to the leased line. Does not define a data-link layer protocol to be used
Data link layer protocols for Leased Line (Control the delivery of data over physical layer)
High Level Data Link Control (HDLC)
Point to Point Protocol (PPP)
HDLC
Adopts a simple point to point topology
When one router sends and HDLC frame, the frame can go only one place: to the other end of the link
HDLC frame has address field but destination is implied
HDLC frame format
Flag - List a recognizable bit pattern so that the receiving nodes realize that a new frame is arriving
Address - Identifies the destination device
Control - Mostly used for purposes no longer in use today for links between routers
Type - Identifies the type of Layer 3 packet encapsulated inside the frame
FCS - Identifies a field used by the error detection process
Router using a WAN (Network Layer)
- PC1’s network layer logic tells it to send the packet to a nearby router ( R1 )
- Router R1’s network layer logic tells it to forward ( route ) the packet out the leased line to Router R2 next
- Router R2’s network layer logic tells it to forward the packet out the LAN link to PC2 next
Router using a WAN (Data Link Layer)
- To send the IP packet to Router R1 next, PC1 encapsulates the IP packet in an Ethernet (802.3) frame that has the destination MAC address of R1
- Router R1 de-encapsulates the IP packet from the Ethernet frame, encapsulates the packet into an HDLC frame using an HDLC header and trailer, and forwards the HDLC frame to Router R2 next
- Router R2 de-encapsulates the IP packet from the HDLC frame, encapsulates the packet into an Ethernet frame that has the destination MAC address of PC2, and forwards the Ethernet frame to PC2
Objectives of leased line and HDLC frames
Leased line - Provides the physical means to transmit bits in both direction
HDLC - Frames that provides a means for network layer packet to be encapsulated
Pros and Cons of leased line
Pros, relatively long life in the WAN marketplace. Simple for customer, widely available, high quality, and are private
Cons, compared to newer WAN tech, Higher cost, typically long lead times to get services installed, leased line LANs are slow,
