5,DataLink Layer 5a-1
Summary of MAC protocols
?What do you do with a shared media?
? Channel Partitioning,by time,frequency or code
? Time Division,Code Division,Frequency Division
? Random partitioning (dynamic),
? ALOHA,S-ALOHA,CSMA,CSMA/CD
? carrier sensing,easy in some technoligies (wire),hard
in others (wireless)
? CSMA/CD used in Ethernet
? Taking Turns
? polling from a central cite,token passing
5,DataLink Layer 5a-2
LAN technologies
Data link layer so far:
? services,error detection/correction,multiple
access
Next,LAN technologies
? addressing
? Ethernet
? hubs,bridges,switches
? 802.11
? PPP
? ATM
5,DataLink Layer 5a-3
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-4
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
5,DataLink Layer 5a-5
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
? can move LAN card from one LAN to another
? IP hierarchical address NOT portable
? depends on network to which one attaches
5,DataLink Layer 5a-6
Recall earlier routing discussion
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
Starting at A,given IP
datagram addressed to B:
? look up net,address of B,find B
on same net,as A
? link layer send datagram to B
inside link-layer frame
B’s MAC
addr
A’s MAC
addr
A’s IP
addr
B’s IP
addr IP payload
datagram
frame
frame source,
dest address
datagram source,
dest address
5,DataLink Layer 5a-7
ARP,Address Resolution Protocol
? 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>
< ………………………….,>
? TTL (Time To Live),time
after which address
mapping will be forgotten
(typically 20 min)
Question,how to determine
MAC address of B
given B’s IP address?
5,DataLink Layer 5a-8
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
? 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)
? soft state,information that times out (goes
away) unless refreshed
5,DataLink Layer 5a-9
Routing to another LAN
walkthrough,routing from A to B via R
A
R
B
5,DataLink Layer 5a-10
? A creates IP packet with source A,destination B
? A uses ARP to get R’s physical layer address for 111.111.111.110
? A creates Ethernet frame with R's physical address as dest,
Ethernet frame contains A-to-B IP datagram
? A’s data link layer sends Ethernet frame
? R’s data link layer receives Ethernet frame
? R removes IP datagram from Ethernet frame,sees its
destined to B
? R uses ARP to get B’s physical layer address
? R creates frame containing A-to-B IP datagram sends to B
A
R B
5,DataLink Layer 5a-11
Ethernet
,dominant” LAN technology,
? cheap $20 for 100Mbs!
? first wildey used LAN technology
? Simpler,cheaper than token LANs and ATM
? Kept up with speed race,10,100,1000 Mbps
Metcalfe’s Etheret
sketch
5,DataLink Layer 5a-12
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-13
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-14
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-15
Ethernet’s CSMA/CD (more)
Jam Signal,make sure all other transmitters are
aware of collision; 48 bits;
Exponential Backoff:
? Goal,adapt retransmission attemtps to estimated
current load
? heavy load,random wait will be longer
? 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-16
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-17
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-18
10BaseT and 100BaseT (more)
? Max distance from node to Hub is 100 meters
? Hub can disconnect,jabbering adapter
? Hub can gather monitoring information,statistics
for display to LAN administrators
5,DataLink Layer 5a-19
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-20
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:
? 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
5,DataLink Layer 5a-21
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
? set to indicate destination up,frame copied OK from ring
? DLC-level ACKing
5,DataLink Layer 5a-22
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-23
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-24
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:
? simple,inexpensive device
? Multi-tier provides graceful degradation,portions
of the LAN continue to operate if one hub
malfunctions
? extends maximum distance between node pairs
(100m per Hub)
5,DataLink Layer 5a-25
Hub limitations
? single collision domain results in no increase in max
throughput
? 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-26
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-27
Bridges (more)
?Bridge advantages:
? Isolates collision domains resulting in higher
total max throughput,and does not limit the
number of nodes nor geographical coverage
? 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-28
Bridges,frame filtering,forwarding
?bridges filter packets
? same-LAN -segment frames not forwarded onto
other LAN segments
?forwarding,
? how to know which LAN segment on which to
forward frame?
? looks like a routing problem (more shortly!)
5,DataLink Layer 5a-29
Backbone Bridge
5,DataLink Layer 5a-30
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-31
Bridge Filtering
? bridges learn which hosts can be reached through
which interfaces,maintain filtering tables
?when frame received,bridge,learns” location of
sender,incoming LAN segment
? records sender location in filtering table
? filtering table entry,
? (Node LAN Address,Bridge Interface,Time Stamp)
? stale entries in Filtering Table dropped (TTL can be
60 minutes)
5,DataLink Layer 5a-32
Bridge Filtering
? filtering procedure:
if destination is on LAN on which frame was received
then drop the frame
else { lookup filtering table
if entry found for destination
then forward the frame on interface indicated;
else flood; /* forward on all but the interface
on which the frame
arrived*/
}
5,DataLink Layer 5a-33
Bridge Learning,example
Suppose C sends frame to D and D replies back with
frame to C
? C sends frame,bridge has no info about D,so
floods to both LANs
? bridge notes that C is on port 1
? frame ignored on upper LAN
? frame received by D
5,DataLink Layer 5a-34
Bridge Learning,example
? D generates reply to C,sends
? bridge sees frame from D
? bridge notes that D is on interface 2
? bridge knows C on interface 1,so selectively
forwards frame out via interface 1
5,DataLink Layer 5a-35
Bridges Spanning Tree
? for increased reliability,desirable to have
redundant,alternate paths from source to dest
? with multiple simultaneous paths,cycles result -
bridges may multiply and forward frame forever
? solution,organize bridges in a spanning tree by
disabling subset of interfaces
Disabled
5,DataLink Layer 5a-36
WWF Bridges vs,Routers
? both store-and-forward devices
? routers,network layer devices (examine network layer
headers)
? bridges are Link Layer devices
? routers maintain routing tables,implement routing
algorithms
? bridges maintain filtering tables,implement
filtering,learning and spanning tree algorithms
5,DataLink Layer 5a-37
Routers vs,Bridges
Bridges + and -
+ Bridge operation is simpler requiring less
processing bandwidth
- Topologies are restricted with bridges,a spanning
tree must be built to avoid cycles
- Bridges do not offer protection from broadcast
storms (endless broadcasting by a host will be
forwarded by a bridge)
5,DataLink Layer 5a-38
Routers vs,Bridges
Routers + and -
+ arbitrary topologies can be supported,cycling is
limited by TTL counters (and good routing protocols)
+ provide firewall protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher processing bandwidth
? bridges do well in small (few hundred hosts) while
routers used in large networks (thousands of hosts)
5,DataLink Layer 5a-39
Ethernet Switches
? layer 2 (frame) forwarding,
filtering using LAN
addresses
? Switching,A-to-B and A’-
to-B’ simultaneously,no
collisions
? large number of interfaces
? often,individual hosts,
star-connected into switch
? Ethernet,but no
collisions!
5,DataLink Layer 5a-40
Ethernet Switches
?cut-through switching,frame forwarded
from input to output port without awaiting
for assembly of entire frame
?slight reduction in latency
?combinations of shared/dedicated,
10/100/1000 Mbps interfaces
5,DataLink Layer 5a-41
Ethernet Switches (more)
Dedicated
Shared
5,DataLink Layer 5a-42
Chapter 5,Summary
? principles behind data link layer services:
? error detection,correction
? sharing a broadcast channel,multiple access
? link layer addressing,ARP
? various link layer technologies
? Ethernet
? hubs,bridges,switches
? journey down the protocol stack now OVER!
? Next stops,security,network management