4377, COSC Spring 2001 - Chapter 5b
4/4/01
LAN technologies
Data link layer so far:
r
Next: LAN technologies
r
services, error detection/correction, multiple access
addressing r Ethernet r hubs, bridges, switches r 802.11 r PPP r ATM
5: DataLink Layer 5a-1
LAN Addresses and ARP
32-bit IP address:
network-layer address used to get datagram to destination network (recall IP network definition)
LAN (or MAC or physical) address:
used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM
5: DataLink Layer 5a-2
1
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
5: DataLink Layer
5a-3
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability
r
IP hierarchical address NOT portable
r
can move LAN card from one LAN to another depends on network to which one attaches
5: DataLink Layer
5a-4
2
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Recall earlier routing discussion
Starting at A, given IP datagram addressed to B:
look up net. address of B, find B
A 223.1.1.1
223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 223.1.3.2
on same net. as A
link layer send datagram to B
B
inside link-layer frame
frame source, dest address
E
datagram source, dest address A' IP s addr frame B' IP s addr datagram
B' MAC A' MAC s s addr addr
IP payload
5: DataLink Layer
5a-5
ARP: Address Resolution Protocol
Question: how to determine MAC address of B given B' IP address? s Each IP node (Host, Router) on LAN has ARP module, table ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL> < ... ... ... ... ... ... ... ... ... ... .. >
r
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
5: DataLink Layer
5a-6
3
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
ARP protocol
A knows B's IP address, wants to learn physical address of B A broadcasts ARP query pkt, containing B's IP address r all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) physical layer address A caches (saves) IP-to-physical address pairs until information becomes old (times out) r soft state: information that times out (goes away) unless refreshed
5: DataLink Layer 5a-7
Routing to another LAN
walkthrough: routing from A to B via R
A
R
B
5: DataLink Layer
5a-8
4
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
A creates IP packet with source A, destination B A uses ARP to get R' physical layer address for 111.111.111.110 s A creates Ethernet frame with R's physical address as dest,
Ethernet frame contains A-to-B IP datagram A' data link layer sends Ethernet frame s
R' data link layer receives Ethernet frame s
R removes IP datagram from Ethernet frame, sees its
destined to B R uses ARP to get B' physical layer address s R creates frame containing A-to-B IP datagram sends to B
A R
B
5: DataLink Layer 5a-9
Ethernet
"dominant" LAN technology: cheap $20 for 100Mbs! first wildly used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe' Etheret s sketch
5: DataLink Layer 5a-10
5
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates
5: DataLink Layer 5a-11
Ethernet Frame Structure (more)
Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped
5: DataLink Layer 5a-12
6
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Baseband Manchester Encoding
Baseband here means that no carrier is modulated; instead bits are encoded using Manchester encoding and transmitted directly by modified voltage of a DC signal Manchester encoding ensures that a voltage transition occurs in each bit time which helps with receiver and sender clock synchronization
5: DataLink Layer 5a-13
More Baseband Encoding
1
Unipolar NRZ Polar NRZ NRZ-Inverted (Differential Encoding) Bipolar Encoding Manchester Encoding Differential Manchester Encoding
5: DataLink Layer 5a-14
0
1
0
1
1
1
0
0
7
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
4B/5B Encoding
Manchester only has 50%
efficiency.
Insert extra bits into the
bit stream to break up long sequence of 0s and 1s. 4-bit data are encoded in a 5-bit code. No more than one leading 0 and no more than two trailing 0s. Use NRZI (non-return to zero inverted) to transmit. 4B/5B: 80% efficiency. Used in 100Mbit Ethernet.
4-Bit DataS ymbol 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1110 1111
5-BitCode 11110 01001 10100 10101 01010 01011 01110 01111 10010 10011 10110 10111 11010 11100 11101
5: DataLink Layer 5a-15
Ethernet: uses CSMA/CD
A: sense channel, if idle
then { transmit and monitor the channel;
If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero}
}
else {wait until ongoing transmission is over and goto A}
5: DataLink Layer 5a-16
8
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Ethernet' CSMA/CD (more) s
Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: Goal: adapt retransmission attemtps to estimated current load
r
first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3}... after ten or more collisions, choose K from {0,1,2,3,4,... ,1023}
5: DataLink Layer 5a-17
heavy load: random wait will be longer
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!
5: DataLink Layer 5a-18
9
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
10BaseT and 100BaseT
10/100 Mbps rate; latter called "fast ethernet" T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus "star topology" CSMA/CD implemented at hub
5: DataLink Layer 5a-19
10BaseT and 100BaseT (more)
Max distance from node to Hub is 100 meters Hub can disconnect "jabbering Hub adapter can gather monitoring information, statistics for display to LAN administrators
5: DataLink Layer 5a-20
10
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
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
5: DataLink Layer 5a-21
Token Passing: IEEE802.5 standard
4 Mbps max token holding time: 10 ms, limiting frame length
SD, ED mark start, end of packet AC: access control byte:
r
token bit: value 0 means token can be seized, value 1 means data follows FC priority bits: priority of packet reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free
r r
5: DataLink Layer 5a-22
11
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Token Passing: IEEE802.5 standard
FC: frame control used for monitoring and maintenance source, destination address: 48 bit physical address, as in Ethernet data: packet from network layer checksum: CRC FS: frame status: set by dest., read by sender
r r
set to indicate destination up, frame copied OK from ring DLC-level ACKing
5: DataLink Layer 5a-23
Interconnecting LANs
Q: Why not just one big LAN?
Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: 802.3 specifies maximum cable length large "collision domain" (can collide with many stations) limited number of stations: 802.5 have token passing delays at each station
5: DataLink Layer 5a-24
12
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Hubs
Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top
5: DataLink Layer 5a-25
Hubs (more)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: r simple, inexpensive device r Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions r extends maximum distance between node pairs (100m per Hub)
5: DataLink Layer 5a-26
13
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Hub limitations
single collision domain results in no increase in max throughput r multi-tier throughput same as single segment throughput individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)
5: DataLink Layer 5a-27
Bridges
Link Layer devices: operate on Ethernet
frames, examining frame header and selectively forwarding frame based on its destination Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit
5: DataLink Layer 5a-28
14
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Bridges (more)
Bridge advantages: r Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage
r
Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters
5: DataLink Layer 5a-29
r
Bridges: frame filtering, forwarding
bridges filter packets r same-LAN -segment frames not forwarded onto other LAN segments forwarding: r how to know which LAN segment on which to forward frame? r looks like a routing problem (more shortly!)
5: DataLink Layer 5a-30
15
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Backbone Bridge
5: DataLink Layer 5a-31
Interconnection Without Backbone
Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment
5: DataLink Layer 5a-32
16
COSC 4377, Spring 2001 - Chapter 5b
4/4/01
Bridge Filtering
bridges l...