Download Data Modeling - Computer Science at Hiram College

Data Link and Physical Layers
CPSC 363 Computer Networks
Ellen Walker
Hiram College
(Includes figures from Computer Networking by Kurose & Ross, © Addison Wesley
2002)
Internet Layers
•
•
•
•
Application
Transport
Network
Data Link
– Delivers data from one node (host or router) to
another over a physical link
• Physical
Data Link Layer
“link”
• Node:
– a host or router
• Link:
– wired or wireless
connection between
nodes
• Frame:
– Layer 2 packet
Link Layer Protocols
•
•
•
•
Ethernet
802.11 wireless LAN
Token Ring
PPP
• A single datagram can travel across all of
these in its path from one host to another!
Travel Analogy
• Plan a trip
– Car to Akron Airport
– Plane to Munich Airport
– Subway to Munich center city
• Each mode of transportation is a link layer
protocol
• The routing protocol from the network layer is
like the travel agent who set up the whole trip.
Network Interface Card (NIC)
• Specialized hardware, sits on the host bus
• Connects between CPU and physical network
link
– Implements link layer protocols in hardware
– Looks like an I/O device
– “Where software meets hardware”
• One NIC per connection
– Switches have many
– Desktops, laptops often have 2 (wired, wireless)
Services of the link layer
• Framing
– Encapsulate the message (again!) within a link-layer frame
(new headers…)
– “MAC” address in frames is different from IP address
• Link Access
– Coordinate transmission of frames
• Reliable delivery (optional)
– Acknowledgement & retransmission (same as TCP)
• Flow control
More Services
• Error detection
– Drop error frames now; don’t wait for Transport
Layer
• Error correction
– Even better! Not only detect the error, but correct
it. This will require more “check bits”
• Half- or full-duplex
– Nodes at both ends can transmit at the same time
(full duplex) or must take turns (half-duplex)
Adapters
• Link level protocols implemented by adapters
– Network Interface Cards (inside the box)
– PCMCIA cards for laptops
• Adapters contain
– Physical connection to the physical link
(termination if needed)
– Antenna for wireless
– Processor chip(s) that implements protocols
– Memory chip(s) for buffering
Error Detection & Correction
• Add extra bits (EDC) to the message (D) before
sending
– EDC bits depend on the message bits, e.g. checksums
• At the other end, check to make sure received EDC
bits (EDC’) are correct for received message bits (D’)
– If not, report or correct the error
• Correction needs more EDC bits than detection
• Both detection and correction are limited. For each
algorithm, there is some number N where N “lucky”
bit flips will not be seen as an error. (Bigger EDC ->
bigger N)
Parity Check for Error Detection
• Add 1 extra bit to each d bits so that the total
number of 1’s in (d+1) bits is odd (or even)
– E.g.
1110 1101 1 (8 bits + 1, odd parity)
• Problem: even # of bit flips won’t be
recognized
– This can approach 50% in practice!
• Generalization: 2D Parity check
– Make a rectangle; 1 bit for each row, 1 for each
column
2D Parity can correct an error
• A one-bit error will cause 2 parity bits to be wrong
– Row of the error
– Column of the error
• Therefore, to correct the error, flip the bit at the row,
column intersection
• 2-bit errors can be detected but not correct
• More power takes more bits
– Detect a 1-bit error: 1 bit
– Correct a 1-bit error: R+C+1 bits, where R=#rows, C=
#columns
– For a 16-bit (4x4) 2d scheme, you need 9 parity bits.
Advantage of Error Correction
• Save time
– Don’t have to wait for packet to be retransmitted
• Save bandwidth
– No extra NAK packets or retransmitted packets
• Avoid loss
– Fewer packets sit in buffers that might become full
Checksumming
• Group data bits, add them up, take 1’s
complement to get EDC bits.
• If data + EDC != 1111111111111111 , error!
• Recall: used for UDP, TCP (entire packet); IP
(header only)
• Can detect any pattern without even # flips in
same column
• Simple and fast, not as powerful as CRC
(next slide)
Cyclic Redundancy Check
• AKA “polynomial codes” - each bit string is
viewed as a polynomial
– Coefficients are the 0 and 1 values
– Operations on bit string interpreted as polynomial
arithmetic
Cyclic Redundancy Check
• G is a fixed r+1 bit pattern, the generator
– Agreed to by sender & receiver
• CRC = (D*2r) % G (shift left r bits)
• If ((D’*2r – CRC’) % G) != 0, there has been
an error
• Compute and check by “long division”
• CRC can detect “burst error” of r bits or less,
any odd # flips
Computing a CRC
• Data = 110101, G = 1001, r = 3
• Long division without carries or borrows (XOR for –)
1001|110101000 <-- append 3 0’s to data
1001
1000
1001
1100
1001
1010
1001
011
<-- 3 CRC bits
Using CRC to verify correct
• Received data: 110101, CRC 011, G 1001
1001|110101011 <-- append CRC to data
1001
1000
1001
1101
1001
1001
1001
0 <-- remainder is 0, valid
Using CRC to detect error
• Received data: 100101, CRC 011, G 1001
1001|100101011 <-- append CRC to data
1001
001011
1001
10 <-- remainder not 0
Summary: error detection &
correction methods
• Parity bit
– Detects odd # flips, cost = 1 bit per N
• 2D Parity
– Corrects one flip, R+C+1 bits (per N = RC)
• Checksum
– Detects odd # flips per column, uses k bits, where k is the
number of columns
• CRC
– Detects “burst error” of r+1 bits or less, uses r bits, where r is
the size of the generator
Multiple Access Protocols
• Many sending and receiving nodes
• One broadcast channel
• Problem: how to effectively share the
broadcast channel
• Example: you and 25 friends you haven’t
seen in a while, all trying to talk at once…
Human Multiple Access protocols
•
•
•
•
Take turns to talk
Raise your hand if you have something to say
Don’t interrupt someone who is talking
Give everyone a chance
Computer Multiple Access Protocols
• Channel partitioning protocols
– Share bandwidth according to time slots,
frequencies, or code division
• Random access protocols
– Always transmit at full bandwidth, if there is a
conflict, retransmit the frame (after a random
delay)
• Taking-turns protocols
– Master node or “token” determines whose turn it is
to broadcast
Channel Partitioning Protocols
(review)
• Frequency Division Multiplexing (FDM)
– Each message travels in a unique Frequency
Band (like an FM radio station)
• Time Division Multiplexing (TDM)
– Time is divided into Frames, and Frames are
divided into Slots. Each message gets one slot.
Freq
Time
Code Division Multiplexing
• Each sender uses a different code, receiver
knows sender’s code to reconstruct message
from sum of all broadcasts
– Code is a sequence of +1,-1 that change faster
than data bits; get multiplied by (1,-1) data bits
– Receiver gets sum of what all senders send
• Analogy: cocktail party, but everyone is
speaking a different language (and you only
understand one of them)
CDM Example (Senders) [5.12]
CDM Example (Receiver 1) [5.12]
Random Access Protocols
• Sender sends a message…
• Sender listens for a collision
– If what the sender receives isn’t its own message, there
must be a collision
• If there is a collision, all messages must be resent
– If resent immediately, there will be another collision
– Therefore, each host waits a random amount of time before
resending
– If the collision rate gets too high, then it gets driven to 100%
by messages being resent over and over and over …
Slotted ALOHA (assumptions)
• All frames the same size, L
• Time divided into slots of L/R (one-frame
time)
• Nodes transmit only at beginning of slots
(synchronized)
• If 2 or more transmit, collision detected before
end of frame
Slotted ALOHA (algorithm)
• If node has frame to send, wait until next slot
and send it
• If no collision, node is done (can prepare to
send next frame)
• Otherwise, retransmit with probability p in
each subsequent slot until successful
Slotted ALOHA: Evaluation
• If only one node needs to transmit, it can use
the full channel
• Protocol is decentralized; each node makes
its own (re)transmission decisions
– But, nodes are synchronized
• Protocol is extremely simple
• But, maximum efficiency at p= 0.37, so on
average, only 37% of bandwidth is available
in the long run (p. 437)
Pure ALOHA
• No slots; nodes immediately transmit as soon
as frame received from Network layer
• If collision
– With probability p, immediately retransmit
– Else, wait 1 frame time, then with probability p,
retransmit… (etc)
• Maximum efficiency is half of slotted ALOHA
(tradeoff with synchronization)
CSMA / CD
• Carrier Sense
– Wait for “quiet” before sending a message
• Multiple Access
– All share a broadcast channel (wired or wireless)
• Collision Detection
– When a message is sent, check for collision, and if
so, wait a random time and resend
Why both Carrier Sensing and
Collision Detection?
• Messages take time
to propagate.
– Dark blue message
from B has not yet
arrived at D by t1, so
D broadcasts
– Before message is
over, they collide
(stripes)
Taking Turns
• Polling
– One master node repeatedly asks (polls) each
node in turn, asking whether it wants to send
– Also called “round robin”
• Token passing
– A special frame called a “token” circulates around
the network. In order to broadcast, a node must
hold the token
– When the message is done (or if no message to
send), pass the token to the next node
Local Area Networks
• Concentrated in a physical area, e.g.
company site, campus
• LAN provides access to the Internet through a
router (LAN = AS)
• Typical setup (Ethernet)
– Host to router across broadcast channel, 1 “link”
– CSMA / CD protocol
– 10Mbps, 100Mbps, 1Gbps or 10Gbps
Local Addresses
• Address belongs to network card (not the
computer)
– Media Access Control (MAC) address, also called
physical or hardware address
– Unique value assigned at network card
manufacture, e.g. 00:03:93:51:5e:34
– IEEE manages the address space; each
manufacturer has a range
• Address is permanent; must be mapped to
Internet address (e.g. mobile laptop)
Addressing on LAN
• Broadcast medium; every message received
by every host
• Hosts selectively ignore messages
– Contains my MAC address: pay attention
– Contains another MAC address: drop
– Broadcast address ff:ff:ff:ff:ff:ff:ff: pay attention
• Example broadcast message: “Who is IP
address 1.2.3.4?”
Address Resolution Protocol
• ARP module keeps a table of MAC and IP addresses
of hosts on the LAN
• To find a new mapping
– ARP broadcasts ARP packet (sending IP and LAN
addresses, receiving IP address)
– Host with matching address responds with its own ARP
packet (its own IP and LAN addresses as sender)
– ARP module receives the packet and updates its table.
• Each line in the table has limited lifetime (TTL) - if a
node is disconnected, its address mapping will
eventually disappear.
What about Routers?
• Router responds to any ARP request for an external
(out of the LAN) address.
– It can tell by the high order bits (class or CIDR)
• Off-network packet is encapsulated as frame and
sent to router
• Router collects frames, uses network layer routing
table to determine outgoing interface
• Packets re-encapsulated as frames for the other
LAN, using a different ARP table and hardware
address.
Dynamic Host Configuration Protocol
(DHCP)
• Dynamically assigns IP address to hosts
– Each address has a limited lifetime (lease); must
be renewed after that
• Client/Server protocol
– Client is new host attaching to network, needs IP
address and other configuration information (e.g.
CIDR bits)
– Server allocates (and tracks) IP addresses
Four Steps to IP via DHCP
1. DHCP Discover message (UDP to port 67)
•
•
Sent as broadcast (via IP, then via LAN)
Ignored by all but DHCP server(s)
2. DHCP server offer message(s) (UDP)
•
•
Still a broadcast, as client is not yet configured
Contains transaction ID, IP address, lease time
3. DHCP request message
•
Echoes parameters, back to chosen server
4. DHCP ACK
•
From server back to (now-configured) client
Ethernet
• Different rates (10Mbps to 10Gbps)
• Different physical setups
– 1 long coax cable with connections to hosts
– Long optical fiber
– Stars of twisted pair (CAT 5) connections with hubs in the
center
• Unreliable and connectionless transmission
– If an error is detected, the packet is dropped…
– …but the sender doesn’t know it!
– It’s up to higher level protocols (TCP) to arrange for
retransmission
Ethernet Frame Structure
• Data field: 46 to 1500 bytes (MTU is 1500)
• Src & Dest (MAC) address: 6 bytes each
• Type field: 2 bytes
– IP vs. AppleTalk vs. Novell IPX…
• CRC: 4 bytes
– For error detection
• Preamble: 8 bytes
– 10101010 (x7) 10101011 , for synchronization
Manchester Encoding
• Every bit has a transition (falling for 1, rising for 0)
• Allows for self-synchronization (transition in middle of
bit)
• Used in many Ethernet technologies, e.g. 10BaseT
(Physical layer!)
Ethernet-Specific CSMA/CD
• Before sending, put frame in a buffer
• Wait until channel is idle, then begin transmitting
frame
• While transmitting, monitor for signal energy from
other nodes (collision)
– If none, the frame is done
– If collision detected,
• stop transmitting and transmit 48-bit jam signal (abort)
• Wait a random amount of time and retransmit
• Each time a retransmission fails wait approximately twice as
long the next time (exponential backoff)
Exponential Backoff
• After each collision, choose K at random,
from {0 to 2m-1} where m is the number of
prior collisions
– Up to double, each time
– Value for m maxes out at 10 (never more than
1023)
• Wait K*512 bit times, then retransmit
• Distance limit chosen so that host with the
shortest retransmit time after a collision won’t
have another collision
Efficiency of Ethernet
• Efficiency is measured as fraction of time
during which frames are transmitted without
collisions when the network is “loaded”
• Efficiency increases as propagation time
decreases
• Efficiency increases as transmission time
increases
– Once a frame grabs the channel, it keeps it going
at full rate for a longer time.
LAN Topology
• Bus (10Base2) - max 185m between nodes
• Star (10BaseT, 100BaseT) - max 100m to
hub
HUB
Multi-segment LANs
• Connect multiple “segments” of LAN with hub, bridge and/or
switch
– Avoid distance limitations
– Mix standards
• Example: “Backbone” + sub-LANs
Gigabit and 10 Gigabit Ethernet
• Backward compatible with 10BaseT and 100BaseT
• Point-to-Point (switches) or shared broadcast (hubs)
– CSMA/CD for shared; distance severely restricted (512 bit
times?)
• Often used as backbone (next slide) for
interconnecting slower (10baseT and 100baseT)
LANs
• Runs on optical fiber, or (1G, now) CAT 5 cable
Hubs and Switches
• Hub
– Many interfaces; every input bit is broadcast on all interfaces
– One collision domain
• Switch (“layer 2 switch”)
– Each “side” of a switch is an isolated collision domain
– Forward and filter frames based on packet addresses (like router,
but at LAN level)
– May include extra features (such as full duplex, cut-through, more
interfaces)
Interconnecting with Hubs
• Total length extended
• One big collision domain
• Cannot interconnect 10BaseT with 100BaseT
hub
hub
hub
hub
Ethernet Limits
• Maximum bandwidth in collision domain is
fixed (e.g. 10Mbps for 10BaseT)
• Cannot connect 10BaseT to 100BaseT with
hub (or to Gigabit Ethernet)
• Restriction on
– Maximum hosts per collision domain
– Maximum distance between hosts per collision
domain
– Maximum # tiers in multi-tier design
Switch Routing
• Like Routers, bridges need to choose an
outgoing interface for each message, based
on address (in this case LAN, not IP)
• Because hosts come and go (or move),
switches must be self-learning
– No preconfiguration by administrator
– No specialized routing protocol packets!
– Messages themselves help to update the tables
Switch Operation
• Routing table is initially empty
• For every frame, store source LAN address,
incoming interface, and current time in table
• If destination LAN address isn’t in table,
forward to all interfaces, but if it is in the table,
forward to only the correct interface
• If an address in the table is too old, delete it.
• Spanning tree: make sure no bridge receives
the same host’s packet on multiple interfaces!
Switch vs. Router
• Switch is level-2 (Link), router is level-3
(Network)
• Advantages of switch over router:
– Plug-and-play (do not need to be configured)
– Faster (less processing per packet)
• Advantages of router over switch:
– Packets can take more direct (lower cost) paths;
not limited to spanning tree
– Provide firewall protection against broadcast
storms; isolation of traffic
Switches: A compromise
• Plug and play like bridges
• Generally more interfaces than bridges
– High performance design
– Mix of 10, 100, and Gb interfaces on one switch
– Not unreasonable to connect host directly to
switch, rather than using the bus medium
• If no waiting packets, “cut through” switching start before prior packet is complete