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Ch.2: Link Layer &LAN
Chapter 2 Datalink Layer
& LAN Protocols
1
Ch.2: Link Layer &LAN
2
Physical Media
physical link: transmitted data bit propagates across link
guided media: signals propagate in
solid media: e.g copper, fiber
unguided media: signals propagate
freely, in air or vacuume.g., radio
Twisted Pair (TP) two insulated copper
wires Category 3: traditional
phone wires, 10 Mbps ethernet
Category 5 TP: 100Mbps ethernet
Ch.2: Link Layer &LAN
3
Physical Media: coax, fiber
Coaxial cable: wire (signal carrier)
within a wire (shield) baseband: single
channel on cable broadband: multiple
channel on cable
bidirectional common use in
10Mbs Ethernet
Fiber optic cable: glass fiber carrying
light pulses high-speed operation:
100Mbps Ethernet high-speed point-to-
point transmission (eg, 40 Gps)
very low error rate
Ch.2: Link Layer &LAN
4
Physical media: Wireless
signal carried in electromagnetic spectrum
no physical “wire” bidirectional propagation
environment effects: reflection obstruction by objects interference
Wireless link types: microwave
e.g. up to 45 Mbps channels
LAN (e.g., 802.11b/g) 11/54 Mbps
wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps 3G ~ 2.4 Mbps
satellite up to 50Mbps channel
• multiple smaller channels
270 Msec end-end delay geosynchronous versus
LEOS (low earth orbit)
Ch.2: Link Layer &LAN
5
Physical link types: Point to point link Shared medium link
- also called: Broadcast link Multi-access link LAN
Shared medium link: Many stations on same
medium segment Intermittent
transmission: only when needed
Qn: WHY? Collisions occur
unless protocol makes special arrangements for co-ordination of transmission
Bit synchronization done per frame
Point to point link Two stations only Continuous
transmission Needed to keep bit
clock synchronization Sends filler when no
data
Full duplex
Ch.2: Link Layer &LAN
6
The Data Link LayerOur goals: understand principles
behind data link layer services: error detection,
correction sharing a broadcast
channel: multiple access
link layer addressing instantiation and
implementation of various link layer technologies
Overview: link layer services error detection, correction multiple access protocols
and LANs link layer addressing specific link layer
technologies: Ethernet
Ch.2: Link Layer &LAN
7
Link Layer: setting the context
Ch.2: Link Layer &LAN
8 8
Recap: The Hourglass Architecture of the Internet
IP
Ethernet FDDIWireless
TCP UDP
Telnet Email FTP WWW
L2
L3
L4
L5
Ch.2: Link Layer &LAN
9
Link Layer: setting the context acts between two physically connected devices:
host-router, router-router, host-host
unit of data: frame
applicationtransportnetwork
linkphysical
networklink
physical
M
M
M
M
Ht
HtHn
HtHnHl MHtHnHl
framephys. link
data linkprotocol
adapter card
Ch.2: Link Layer &LAN
10
Link layer: Context
Data-link layer has responsibility of transferring datagram from one node to another node over a link
Datagram transferred by different link protocols over different links, e.g., Ethernet on first link, frame relay on
intermediate links 802.11 on last link
transportation analogy
trip from New Haven to San Francisco taxi: home to union
station train: union station
to JFK plane: JFK to San
Francisco airport shuttle: airport to
hotel
Ch.2: Link Layer &LAN
11
Link Layer Services Framing, link access:
encapsulate datagram into frame, adding header, trailer
(IF SHARED LINK) link access: implement channel access, ‘physical addresses’ used in frame headers to identify
source, destination • different from IP address!
(OPTIONAL) Reliable data delivery: seldom used on low bit-error link
• E.g., fiber, twisted pair wireless links: high error rates
• Qn: why use both link-level and end-end reliability?
Ch.2: Link Layer &LAN
12
Link Layer Services (more)
(OPTIONAL) Flow Control: pacing between sender and receivers
(OPTIONAL) Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors:
• signals sender for retransmission or drops frame(depending on protocol)
(OPTIONAL) Error Correction: receiver identifies and corrects bit error(s)
without resorting to retransmission Qn: Why congestion ctrl not listed here?
Ch.2: Link Layer &LAN
13
Adaptors Communicating
link layer implemented in “adaptor” (aka NIC) Ethernet/ PCMCIA card,
modem, 802.11 card
adaptor is semi-autonomous, implementing link & physical layers
sending side: encapsulates datagram in a
frame, delimits frame adds error checking bits,
Optionally: rdt param’s, etc.
receiving side recognizes frame start /end checks errors, Optionally:
check rdt, send Ack+flow ctrl info, ..
extracts datagram, passes to L3
sendingnode
frame
receivingnode
datagram
frame
adapter adapter
link layer protocol
Ch.2: Link Layer &LAN
14
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields
• Error detection not 100% reliable! Qn: why?• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction
ChecksumGenerator
Checksum Generator
EDC”=?
Ch.2: Link Layer &LAN
15
Parity Checking
Single Bit Parity:Detect all single bit errors
0 0
Parity bit=1 iffNumber of 1’s even
Two dimensional Bit Parity:Correct all single bit errors, Detect all X bit errors X=?
Ch.2: Link Layer &LAN
16
Internet checksum
Sender: treat segment contents
as sequence of 16-bit integers
checksum: addition (1’s complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver: compute checksum of received
segment check if computed checksum
equals checksum field value: NO - error detected YES - no error detected.
But maybe errors nonetheless?
Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
Ch.2: Link Layer &LAN
17
Checksumming: Cyclic Redundancy Check choose a (r+1) bit pattern (generator), G
G is fixed, known to Sender & Receiver Sender: Wants to send data bits D Finds r CRC bits, R, such that
(D || R) is exactly divisible by G (viewed as modulo 2 polynomials (*)) Sends D and R Receiver: divides (D || R) by G.
If remainder ≠ 0 : error detected! can detect all burst errors less than r+1 bits
widely used in practice (Ethernet, ATM, HDLC)
(*) This means that addition and subtraction use bitwise XOR
Ch.2: Link Layer &LAN
18
CRC ExampleWant:
D.2r XOR R = nGequivalently:
D.2r = nG XOR R equivalently: if we divide D.2r by
G, want remainder R
R = remainder[ ]D.2r
G
Ch.2: Link Layer &LAN
19
Examples of G(x)
16 bits CRC: CRC-16: x16+x15+x2+1,
CRC-CCITT: x16+x12+x5+1 both can catch
• all single or double bit errors• all odd number of bit errors• all burst errors of length 16
or less• >99.99% of the 17 or 18 bits
burst errors
CRC-CCITT hardware implementationUsing shift and XOR registers
http://en.wikipedia.org/wiki/CRC-32#Implementation
Ch.2: Link Layer &LAN
20
Multiple Access Links and Protocols
Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP,
HDLC) broadcast (shared wire or medium; e.g,
Ethernet, Token Ring, WiFi, WaveLAN, etc.)
switched (e.g., switched Ethernet, ATM etc)
Ch.2: Link Layer &LAN
21
Multiple Access protocols single shared communication channel two or more simultaneous transmissions by nodes:
interference only one node can send successfully at a time
multiple access protocol: distributed algorithm that determines how stations share
channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:
• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance
Ch.2: Link Layer &LAN
22
Multiple Access protocols
claim: humans use multiple access protocols all the time
class can "guess" multiple access protocols multiaccess protocol 1: multiaccess protocol 2: multiaccess protocol 3: multiaccess protocol 4:
Ch.2: Link Layer &LAN
23
MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency)
allocate piece to node for exclusive use
Random Access allow collisions “recover” from collisions
“Taking turns” tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
Ch.2: Link Layer &LAN
24
MAC Protocols: Measures
Channel Rate = R bps Efficient:
Single user: Throughput R Fairness
N usersMin. user throughput R/N
Decentralized Fault tolerance
Simple
Ch.2: Link Layer &LAN
25
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.
Ch.2: Link Layer &LAN
26
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.
frequ
ency
bands time
Ch.2: Link Layer &LAN
27
TDMA & FDMA: Performance
Channel Rate = R bps Single user
Throughput R/N Fairness
Each user gets the same allocationDepends on maximum number of users
Decentralized Requires resource division
Simple
Ch.2: Link Layer &LAN
28
Random Access protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes
two or more transmitting nodes -> “collision”, random access MAC protocol specifies:
how to detect collisions how to recover from collisions (e.g., via delayed
retransmissions)
Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA and CSMA/CD
Ch.2: Link Layer &LAN
29
Slotted Aloha [Norm Abramson]
time is divided into equal size slots (= pkt trans. time)
node w. new pkt: transmit at beginning of next slot a satellite acts as Access Point, and sends Ack to
sender if successful. It also sends sync signal to all stations
if collision: retransmit pkt in future slots with probability p, until successful.
Success (S), Collision (C), Empty (E) slots
Ch.2: Link Layer &LAN
30
Slotted Aloha efficiencyQ: what is max fraction slots successful?A: Suppose N stations have packets to send
each transmits in slot with probability p prob. successful transmission S is:
by single node: S= p (1-p)(N-1)
by any of N nodes S = Prob (only one of the nodes transmits)
= N p (1-p)(N-1)
… choosing optimum p =1/N
as N -> infinity ...
S≈ 1/e = .37 as N -> infinity
At best: channeluse for useful transmissions 37%of time!
Ch.2: Link Layer &LAN
31
Goodput vs. Offered LoadS =
thro
ughput
=
“goodput”
(
succ
ess
rate
)
G = offered load = Np0.5 1.0 1.5 2.0
Slotted Aloha
when pN < 1, as p (or N) increases probability of empty slots reduces probability of collision is still low, thus goodput increases
when pN > 1, as p (or N) increases, probability of empty slots does not reduce much, but probability of collision increases, thus goodput decreases
goodput is optimal when pN = 1
Ch.2: Link Layer &LAN
32
Maximum Efficiency vs. n
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
2 7 12 17 n
ma
xim
um
eff
icie
nc
y1/e = 0.37
At best: channeluse for useful transmissions 37%of time!
Ch.2: Link Layer &LAN
33
Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization pkt needs transmission:
send without awaiting for beginning of slot
collision probability increases: pkt sent at t0 collide with other pkts sent in [t0-1,
t0+1]
Ch.2: Link Layer &LAN
34
Pure Aloha (cont.)P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,t0+1]
= p . (1-p)N-1 . (1-p)N-1
P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1
… choosing optimum p=1/(2N-1)
as N -> infty ... S≈ 1/(2e) = .18
S =
thro
ughput
=
“goodput”
(
succ
ess
rate
)
G = offered load = Np0.5 1.0 1.5 2.0
0.1
0.2
0.3
0.4
Pure Aloha
Slotted Alohaprotocol constrainseffective channelthroughput!
Ch.2: Link Layer &LAN
35
Aloha: Performance
Channel Rate = R bps Single user
Throughput R Fairness
Multiple usersCombined throughput only 0.37*R
Decentralized Slotted Aloha needs slot synchronization
Simple
Ch.2: Link Layer &LAN
36
CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission
Persistent CSMA: retry immediately with probability p when channel becomes idle
Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!
Ch.2: Link Layer &LAN
37
CSMA collisions
collisions can occur:propagation delay means two nodes may not yethear each other’s transmissioncollision:entire packet transmission time wasted
spatial layout of nodes along Ethernet
note:role of distance and propagation delay in determining collision prob.
Ch.2: Link Layer &LAN
38
spatial layout of nodes along EthernetA B C D
tim
e
t0
spatial layout of nodes along EthernetA B C D
tim
e
t0
B detectscollision, aborts
D detectscollision,aborts
CSMA/CD: Collision Detection
instead of wasting the whole packettransmission time, abort after detection.
CSMA CSMA/CD
Ch.2: Link Layer &LAN
39
CSMA/CD (Collision Detection)CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time colliding transmissions aborted, reducing
channel wastage persistent or non-persistent retransmission
collision detection: easy in wired LANs: measure signal strengths,
compare transmitted, received signals in wireless LAN:
• receiver closed when transmitting• the interfering station may not be heard by contender
human analogy: the polite conversationalist
Ch.2: Link Layer &LAN
40
CSMA/CD collision detection
Ch.2: Link Layer &LAN
41
CDMA/CD
Channel Rate = R bps Single user
Throughput R Fairness
Multiple usersDepends on Detection Time
Decentralized Completely
Simple Needs collision detection hardware
Ch.2: Link Layer &LAN
42
“Taking Turns” MAC protocols
channel partitioning MAC protocols: share channel efficiently at high load inefficient at low load: delay in channel
access, 1/N bandwidth allocated even if only 1 active node!
Random access MAC protocols efficient at low load: single node can fully
utilize channel high load: collision overhead
“taking turns” protocolslook for best of both worlds!
Ch.2: Link Layer &LAN
43
“Taking Turns” MAC protocols
Polling: master node
“invites” slave nodes to transmit in turn
Request to Send, Clear to Send msgs
concerns: polling overhead latency single point of
failure (master)
Token passing: control token passed
from one node to next sequentially.
token message concerns:
token overhead latency single point of failure
(token)
Ch.2: Link Layer &LAN
44
Reservation-based protocolsDistributed Polling: time divided into slots begins with N short dedicated reservation slots
reservation slot time equals to channel end-end propagation delay Qn: WHY?
station with message to send posts reservation reservation seen by all stations
after reservation slots, message transmissions ordered by
known priority
Ch.2: Link Layer &LAN
45
Summary of MAC protocols
What do you do with a shared media? Channel Partitioning: by time, frequency or
code• Time Division, Frequency Division, Code Division
Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD• carrier sensing: easy in some technologies (wire),
hard in others (wireless)• CSMA/CD used in Ethernet
Taking Turns• polling from a central cite, token passing• Popular in cellular 3G/4G networks where
base station is the master
Ch.2: Link Layer &LAN
46
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
Ch.2: Link Layer &LAN
47
LAN Addresses
32-bit IP address: network-layer address used to get datagram to destination network
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 at production time
Ch.2: Link Layer &LAN
48
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy: (a) MAC address: like ID number תעודת זהות
(b) IP address: like postal address כתובת מגורים 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
ARP protocol translates IP address to MAC address
Ch.2: Link Layer &LAN
49
Comparison of IP address and MAC Address IP address is
hierarchical for routing scalability
IP address needs to be globally unique (if no NAT)
IP address depends on IP network to which an interface is attached NOT portable
MAC address is flat
MAC address: no need for global uniqueness, but in fact is globally unique
MAC address is assigned to a device portable
5-50 Ch.2: Link
Layer &LA
N
LAN Addresses and ARPEach adaptor on LAN has unique MAC address
Broadcast address =FF-FF-FF-FF-FF-FF
= adaptor card (NIC)
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5-51 Ch.2: Link
Layer &LA
N
ARP: Address Resolution Protocol
Each IP node (Host, Router) on a LAN has an ARP table
ARP Table: IPMAC addr mapping for LAN nodes
ARP protocol: used to get new entries in ARP table when needed
ARP message has following parameters: Source IP addr + MAC addr. Dest. IP addr + MAC addr. TTL (Time To Live): time after
which address mapping will be discarded (typically 20 min)
ARP Messages: Query, Reply
Question: how to determineMAC address of Bknowing B’s IP address?
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237.196.7.23
237.196.7.78
237.196.7.14
237.196.7.88
5-52 Ch.2: Link
Layer &LA
N
ARP protocol usage node A wants to send datagram
to B, but doesn’t find B’s MAC address in its ARP table.
A broadcasts an ARP query containing B's IP address and asking for B’s MAC address frame dest MAC address =
FF-FF-FF-FF-FF-FF all nodes on LAN receive query only B answers (ARP reply)
reply sent to A’s MAC address only
other nodes ignore query the reply shows B's MAC
address see messages in next slide
A caches the (IP,MAC) address pair in its ARP table until TTL expires (timeout) soft state: info deleted
unless refreshed Qn1: Which other node can
update its ARP table? Qn2: What happens if the
ARP query has dest IP = src IP ?
Qn3: What happens if A sends query with My_IP = IP address of C andSrc_MAC=My_MAC= MAC of A ?
ARP is “plug-and-play”: i.e. nodes create their ARP
tables without action of network administrator
5-53
Ch.2: Link Layer &LAN
ARP Messages A (a, α ) knows B’s IP addr. (b) & wants to know B’s MAC
addr (β) 1. A sends ARP Query Message for B’s MAC address:
message sent as broadcast frame on Ethernet
2. B reads the message and sends ARP reply to A reply sent as a unicast frame to A’s MAC address
Src MAC Dest MAC Type Source IP Src MAC Dest IP Dest MAC
α FF-…-FF Query a α b ?
Src MAC Dest MAC Type Source IP Src MAC Dest IP Dest MAC
β α Query b β a α
ARP MessageEthernet Header
ARP MessageEthernet Header
Ch.2: Link Layer &LAN
54
Ethernet“dominant” LAN technology: cheap $5-10 for 10/100/1000 Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 1, 10, 100, 1000 Mbps
Metcalfe’s Etheretsketch
Ch.2: Link Layer &LAN
55
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
Ch.2: Link Layer &LAN
56
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
Ch.2: Link Layer &LAN
57
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}
Ch.2: Link Layer &LAN
58
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are aware of collision; 48 bits;
Exponential Backoff: Goal: adapt retransmission attempts 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 n-th collision: choose K from {0,1,…, 2n-1} after 10 collisions, choose K from {0,1 … ,
1023} after 16 collisions, give up
Ch.2: Link Layer &LAN
59
Exponential Backoff (simplified)
N users Interval of size 2n
Prob Node/slot is 1/2n
Prob of success N(1/2n)(1 – 1/2n)N-1
Average slot success N(1 – 1/2n)N-1
Intervals size: 1, 2, 4, 8, 16 … Fraction (out of N) of success:
2n = N/8 -> 0.03 % 2n = N/4 -> 2% 2n = N/2 -> 15% 2n = N -> 37 % 2n = 2N -> 60%
DataLink Layer
5-60
Ethernet length limitations Ethernet defines:
MAX. allowed distance between stations on LAN
MIN. allowed frame size
The rule is:Ttrans(frame) > 2 * Tprop(Max)
Ensures that all collisions will be detected by sender
1 st bit arrives at B(no transmission allowed from this
time)
Max.propagation time
propagation time
back
After this time the transmitter is sure that no collision occurred for
this frame
Ch.2: Link Layer &LAN
61
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!
Ch.2: Link Layer &LAN
62
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” (multi-port repeater) Hub acts as a multi-legged (broadcast)
repeater Effectively same as a single segment
Ch.2: Link Layer &LAN
63
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
Ch.2: Link Layer &LAN
64
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
Wide area networks
Ch.2: Link Layer &LAN
65
Token Rings (IEEE 802.5) A ring topology is a single
unidirectional loop connecting a series of stations in sequence
Each bit is stored and forwarded by each station’s network interface
Ch.2: Link Layer &LAN
66
Token Ring: IEEE802.5 standard 4 Mbps (also 16 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
Ch.2: Link Layer &LAN
67
Token Ring: 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