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Csci 183/183W/232 Direct Link networks 1
Direct Link Networks• Encoding and Framing• Error Detection/Correction• Reliable Transmission will be covered
later (with TCP)• Media Access Control • (Wired) LAN Technologies• Readings
– Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer.
Csci 183/183W/232 Direct Link networks 2
Encoding
• Signals propagate over a physical medium– modulate electromagnetic waves (on amplitude,
frequency, and phase): the process of encoding source data onto a carrier signal with frequency f_c
– e.g., vary voltage (amplitude)
• Encode binary data onto signals– e.g., 0 as low signal and 1 as high signal– known as Non-Return to zero (NRZ)
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
Csci 183/183W/232 Direct Link networks 3
Problems: Consecutive 1s or 0s
• Low signal (0) may be interpreted as no signal
• High signal (1) leads to baseline wander– Receiver keeps an average of the signal it has seen
so far and uses this average to distinguish between low and high signals
• Unable to recover clock– Clocks at the sender and receiver use signal
transition to synchronize each other
Csci 183/183W/232 Direct Link networks 4
Alternative Encodings• Non-return to Zero Inverted (NRZI)
– make a transition from current signal to encode a one; stay at current signal to encode a zero
– solves the problem of consecutive ones
• Manchester– transmit XOR of the NRZ encoded data and the clock– only 50% efficient.
Csci 183/183W/232 Direct Link networks 5
Encodings (cont)
• 4B/5B– every 4 bits of data encoded in a 5-bit code– 5-bit codes selected to have no more than one
leading 0 and no more than two trailing 0s – thus, never get more than three consecutive 0s– resulting 5-bit codes are transmitted using NRZI – achieves 80% efficiency
Csci 183/183W/232 Direct Link networks 6
Encodings (cont)
Bits
NRZ
Clock
Manchester
NRZI
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
Csci 183/183W/232 Direct Link networks 7
Framing• Break sequence of bits into a frame –
why? determine where a frame starts and ends
• Typically implemented by network adaptor
Frames
BitsAdaptor Adaptor Node BNode A
Csci 183/183W/232 Direct Link networks 8
Approaches• Sentinel-based
– delineate frame with special pattern: 01111110– e.g., HDLC, SDLC, PPP
– problem: special pattern appears in the payload– solution: bit stuffing
• sender: insert 0 after five consecutive 1s• receiver: delete 0 that follows five consecutive 1s• What if the receiver sees a 1?
Header Body
8 16 16 8
CRCBeginningsequence
Endingsequence
Csci 183/183W/232 Direct Link networks 9
Approaches (cont)• Couter-based
– include payload length in header– e.g., DDCMP
– problem: count field corrupted– solution: catch when CRC fails– How many frames are affected when counter or
sentinel is corrupted?
SY
N
Header Body
8 8 4214 168
SY
N
Cla
ss CRCCount
Csci 183/183W/232 Direct Link networks 10
Approaches (cont)
• Clock-based– each frame is 125us long– e.g., SONET: Synchronous Optical Network
Overhead Payload
90 columns
9 rows
Special bit pattern
Csci 183/183W/232 Direct Link networks 11
Handling Errors
• Data can be corrupted during transmission– Bit values changed
• Frame includes additional information– Set by sender– Checked by receiver
• Error-detection vs error-correction– Both need redundant information– Detection: error exists or not.– Correction: repair if there was an error
• Statistical guarantee
Csci 183/183W/232 Direct Link networks 12
Error Detecting and Correcting Codes
• How many check/redundancy bits?– To detect single-bit error
• 1 bit: even/odd parity
– To correct a single-bit error in m-bit message• Need a minimum of r bits such that• (m + r + 1) 2r
• Example: 3-bit message needs 3-bit redundancy
• Correction or detection+retransmission?
Csci 183/183W/232 Direct Link networks 13
Error Detection Techniques
• Checksum– Treat data as sequence of integers– Compute and send arithmetic sum– Detects some multiple bit errors not all– Internet Checksum
• Cyclic Redundancy Check– Mathematical function of data– More complex to compute– Handles more errors
Csci 183/183W/232 Direct Link networks 14
Cyclic Redundancy Check
• Add k bits of redundant data to an n-bit message– want k << n– e.g., k = 32 and n = 12,000 (1500 bytes)
• Represent n-bit message as n-1 degree polynomial– e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1
• Let k be the degree of some divisor polynomial– e.g., C(x) = x3 + x2 + 1
Csci 183/183W/232 Direct Link networks 15
CRC (cont)• Transmit polynomial P(x) that is evenly
divisible by C(x) – shift left k bits, i.e., M(x)xk
– subtract remainder of M(x)xk / C(x) from M(x)xk
• Receiver polynomial P(x) + E(x)– E(x) = 0 implies no errors
• Divide (P(x) + E(x)) by C(x); remainder zero if:– E(x) was zero (no error), or– E(x) is exactly divisible by C(x)
Csci 183/183W/232 Direct Link networks 16
CRC Example
Generator 11011111100110011010000 Message1101
10011101
10001101
10111101
11001101
10001101
101 Remainder
Csci 183/183W/232 Direct Link networks 17
Choice of C(x)
• Want to ensure C(x) doesn’t divide E(x)• We can detect
– All single-bit errors if• C(x) has at least 2 terms
– All double-bit errors if • C(x) doesn’t divide xj + 1
– X15 + x14 + 1 doesn’t divide xj + 1 for any j below 32768
– Any odd number of errors if• C(x) contains the factor x+1
– Any “burst” error of length less than or equal to k bits
– Most burst errors of length greater than k bits
Csci 183/183W/232 Direct Link networks 18
Error Detection Summary
• To detect data corruption– Sender adds additional information to packet– Receiver checks
• Techniques– Parity bit– Checksum– Cyclic Redundancy Check
Csci 183/183W/232 Direct Link networks 19
Hamming Error Correcting Code
• Achieve the theoretical lower limit of the number of redundant bits
• Correcting single-bit errors• Method:
– Put information bits at positions that are not equal to powers of two– Compute the check bits at positions that are power of two
• A check bit at position 2^j, where j=0, 1, …, checks an information bit at position i if and only if the binary representation of i contains a 1 at position j.
• The value of an “even” check bit equals the xor of all checked information bits
• At the receiver side, all checked bits and the check bit are xored and a zero indicates error free; otherwise, the sum of the positions of the check bits with a non-zero value indicates the bit that is in error
• Example– Information bits: 1001000– Coded bits: 00110010000
Csci 183/183W/232 Direct Link networks 20
Error Recovery
• Reliable delivery over unreliable channel– How to recover from corrupted/lost packets
• Error detection and retransmission– With acknowledgements and timeouts– Also called Automatic Repeat Request (ARQ)– Retransmission incurs round trip delay
• Error correcting codes– Also called Forward Error Correction (FEC)– No sender retransmission required
Csci 183/183W/232 Direct Link networks 21
Summary
• Encoding • Framing• Error correction/detection codes
– Parity/Checksum/CRC
Csci 183/183W/232 Direct Link networks 22
Shared Media andLocal Area Networks
• Shared Media Access Problem– Media access control (MAC) and LAN– MAC addresses and network adaptors (NICs)– MAC protocols: random access vs. controlled access
• Ethernet• Token Ring and FDDI (read yourself)• 802.11 Wireless LAN (WiFi and WiMAX)
(next lecture)
Csci 183/183W/232 Direct Link networks 23
Multiple Access Links and LANs
Two types of “links”:• point-to-point, e.g.,
– PPP for dial-up access, or over optical fibers
• broadcast (shared wire or medium), e.g.– traditional Ethernet– 802.11 wireless LAN
Csci 183/183W/232 Direct Link networks 24
Typical LAN Structure
RAM
RAMROM
Ethernet Processor
• Transmission Medium
• Network Interface Card (NIC)
• Unique MAC “physical” address
Csci 183/183W/232 Direct Link networks 25
Adaptors Communicating
• link layer implemented in “adaptor” (aka NIC), with “transceiver” in it– Ethernet card, dial-up
modem, 802.11 wireless card• sending side:
– encapsulates packet in frame– adds error checking bits, flow
control, reliable data transmission, etc.
• receiving side– looks for errors, flow
control, reliable data transmission, etc
– extracts packet, passes to receiving node
• data link & physical layers are closely coupled!
sendingnode
frame
rcvingnode
network packet
frame
adapter adapter
link layer protocol
Csci 183/183W/232 Direct Link networks 26
MAC (Physical) Addresses• Addressing needed in shared media
– MAC (media access control) or physical addresses – To identify source and destination interfaces and get
frames delivered from one interface to another physically-connected interface (i.e., on same physical local area network!)
• 48 bit MAC address (for most LANs)– fixed for each adaptor, burned in the adapter ROM– MAC address allocation administered by IEEE
• 1st bit: 0 unicast, 1 multicast.• all 1’s : broadcast
• MAC flat address -> portability – can move LAN card from one LAN to another
Csci 183/183W/232 Direct Link networks 27
MAC (Physical, or LAN) Addresses
MAC addressing operations on a LAN:• each adaptor on the LAN “sees” all frames• accept a frame only if dest. (unicast) MAC
address matches its own MAC address• accept all broadcast (MAC= all 1’s) frames• accept all frames if set in “promiscuous” mode• can configure to accept certain multicast
addresses (first bit = 1)
Csci 183/183W/232 Direct Link networks 28
MAC Sub-layer
Data linklayer
802.3CSMA-CD
802.5Token Ring
802.2 Logical link control
Physicallayer
MAC
LLC
802.11Wireless
LAN
Network layer Network layer
Physicallayer
OSIIEEE 802
Various physical layers
OtherLANs
Csci 183/183W/232 Direct Link networks 29
Broadcast Links: Multiple Access
Single shared communication channel• Only one can send successfully at a time• Two or more simultaneous transmissions
– interference!• How to share a broadcast channel
– media access control uses same shared media
• Humans use multi-access protocols all the time
Csci 183/183W/232 Direct Link networks 30
Random Access• Stations contend for channels• Overlapping transmissions (collisions)
can occur– Carrier sensing?– Collision detection?
• Protocols– Aloha– Slotted Aloha– Carrier Sense Multiple Access: Ethernet
Csci 183/183W/232 Direct Link networks 31
Controlled Access
• Stations reserve or are allocated channel– No collisions– Allocation: static or dynamic
• Protocols– Static channel allocation
• Time division multiple access– Demand adaptive channel allocation
• Reservation protocols• Token passing (token bus, token ring)
Csci 183/183W/232 Direct Link networks 33
Pure (unslotted) Aloha
• Simpler, no synchronization• Just send: no waiting for beginning of slot
Csci 183/183W/232 Direct Link networks 34
Slotted Aloha• Time is divided into equal size slots• Nodes transmit at the beginning of a slot• If collision, retransmit later
Success (S), Collision (C), Empty (E) slots
Csci 183/183W/232 Direct Link networks 35
Performance of Aloha Protocols
G = offered load = Np0.5 1.0 1.5 2.0
0.1
0.2
0.3
0.4
Pure Aloha
Slotted Aloha
S =
th
rou
gh
pu
t =
“g
ood
pu
t”
(su
ccess
rate
)
Csci 183/183W/232 Direct Link networks 36
Carrier Sense Multiple Access
• Aloha is inefficient (and rude)– Doesn’t listen before talking
• CSMA: Listen before transmit– If channel idle, transmit entire packet– If busy, defer transmission
• How long should we wait?– Human analogy: don’t interrupt others
• Can carrier sense avoid collisions completely?
Csci 183/183W/232 Direct Link networks 37
Persistent and Non-persistent CSMA
• Non-persistent– If idle, transmit– If busy, wait random amount of time– If collision, wait random amount of time
• p-persistent– If idle, transmit with probability p– If busy, wait till it becomes idle– If collision, wait random amount of time
Csci 183/183W/232 Direct Link networks 38
CSMA/CD
CSMA with collision detection (CD)• Listen while talking• Stop transmitting when collision detected
– Compare transmitted and received signals– Save time and bandwidth– Improvement over persistent and nonpersistent
protocols
• Human analogy– Polite conversationalist
• Worst case time to detect a collision?
Csci 183/183W/232 Direct Link networks 42
Ethernet Overview• History
– developed by Xerox PARC in mid-1970s– roots in Aloha packet-radio network– standardized by Xerox, DEC, and Intel in 1978– similar to IEEE 802.3 standard
• CSMA/CD– carrier sense– multiple access– collision detection
• Frame Format
Destaddr
64 48 32
CRCPreamble Srcaddr
Type Body
1648
Csci 183/183W/232 Direct Link networks 43
Ethernet“Dominant” LAN technology: • cheap $20 for 100Mbs!• first widely used LAN technology• Simpler, cheaper than token ring LANs and ATM• Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe’s Ethernetsketch
Csci 183/183W/232 Direct Link networks 44
Ethernet Frame FormatSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame DIX frame format
IEEE 802.3 format
Destaddr
8 bytes 6 4
CRCPreamble Srcaddr Type Data
26 0-1500
Destaddr
8 bytes 6 4
CRCPreamble Srcaddr Length Data
26 0-1500
• Ethernet has a maximum frame size: data portion <=1500 bytes• It imposes a minimum frame size: 64 bytes (excluding preamble) If data portion <46 bytes, pad with “junk” to make it 46 bytes Q: Why minimum frame size in Ethernet?
Csci 183/183W/232 Direct Link networks 45
Fields in Ethernet Frame Format• Preamble:
– 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 (SoF: start-of-frame)
– used to synchronize receiver, sender clock rates, and identify beginning of a frame
• Addresses: 6 bytes– if adapter receives frame with matching destination address,
or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol (specified by TYPE field)
– otherwise, adapter discards frame
• Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk– 802.3: Length gives data size; “protocol type” included in
data
• CRC: checked at receiver, if error is detected, the frame is simply dropped
Csci 183/183W/232 Direct Link networks 46
IEEE 802.3 MAC: EthernetMAC Protocol:• CSMA/CD• Truncated binary exponential backoff
– for retransmission n: 0 < r < 2k * ASlotTime, where k=min(n,10)
– give up after 16 retransmissions
• Slot Time is the critical system parameter– upper bound on time to detect collision– upper bound on time to acquire channel– upper bound on length of frame segment generated
by collision– quantum for retransmission scheduling– max{round-trip propagation, MAC jam time}
Csci 183/183W/232 Direct Link networks 47
IEEE 802.3 Parameters
• 1 bit time = time to transmit one bit– 10 Mbps 1 bit time = 0.1 s
• Maximum network diameter 2.5km– Maximum 4 repeaters
• “Collision Domain” – Distance within which collision can occur and be detected – IEEE 802.3 specifies:
worst case collision detection time: 51.2 s
• Slot time– 51.2 s = 512 bits = 64 bytes
• Why minimum frame size?– 51.2 s => minimum # of bits can be transited at
10Mpbs is 512 bits => 64 bytes is required for collision detection
Csci 183/183W/232 Direct Link networks 48
Ethernet MAC Protocol: Basic Ideas
1-persistent CSMA/CD• Carrier sense: station listens to channel first
– Listen before talking
• If idle, station may initiate transmission– Talk if quiet
• Collision detection: continuously monitor channel– Listen while talking
• If collision, stop transmission– One talker at a time
• Exponential binary back-off algorithm
Csci 183/183W/232 Direct Link networks 50
Ethernet CSMA/CD Algorithm
1. Adaptor gets datagram and creates frame
2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits
3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! Signal to network layer “transmit OK”
4. If adapter detects another transmission while transmitting, aborts and sends jam signal
5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2
6. Quit after 16 attempts, signal to network layer “transmit error”
Csci 183/183W/232 Direct Link networks 51
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are aware of collision; 32 bits;
Bit time: .1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec
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 second collision: choose K from {0,1,2,3}…
• after ten collisions, choose K from {0,1,2,3,4,…,1023}
Csci 183/183W/232 Direct Link networks 52
CSMA/CD EfficiencyRelevant parameters
– cable length, signal speed, frame size, bandwidth
• tprop = max prop between 2 nodes in LAN• ttrans = time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0• Goes to 1 as ttrans goes to infinity• Much better than ALOHA, but still decentralized, simple, and cheap
transprop tt /51
1efficiency
Csci 183/183W/232 Direct Link networks 53
IEEE 802.3 Physical Layer
(a) transceivers(b)
10base5 10base2 10baseT 10baseFX
Medium Thick coax Thin coaxTwisted
pairOptical fiber
Max. Segment Length
500 m 200 m 100 m 2 km
Topology Bus Bus StarPoint-to-point link
IEEE 802.3 10 Mbps medium alternatives
Thick Coax: Stiff, hard to work with T connectors flaky
Hubs & Switches!
Csci 183/183W/232 Direct Link networks 54
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!• has become a legacy technology
Csci 183/183W/232 Direct Link networks 55
10BaseT • 10 Mbps rate• T stands for Twisted Pair• Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub
• Hubs are essentially physical-layer repeaters:– bits coming in one link go out all other links– no frame buffering– no CSMA/CD at hub: adapters detect collisions– provides net management functionality
hub
nodes
Csci 183/183W/232 Direct Link networks 56
Ethernet Hubs & Switches
(a)
Single collision domain(b)
High-Speed backplane or interconnection fabric
Twisted Pair CheapEasy to work withReliableStar-topology CSMA-CD
Twisted Pair CheapBridging increases scalabilitySeparate collision domainsFull duplex operation
Csci 183/183W/232 Direct Link networks 57
Evolution of EthernetFrom early 80’s 10Base Ethernet to 90’s
100Base (Fast) Ethernet to today’s Gigabit Ethernet to 10 Gigabit
Ethernet, ……
IEEE 802.3 Original Parameters• transmission Rate: 10 Mbps• Min Frame: 512 bits = 64 bytes• slot time: 512 bits/10 Mbps = 51.2 sec
– 51.2 sec x 2x105 km/sec =10.24 km– 5.12 km round trip distance
• max Length: 2500 meters + 4 repeaters
• For compatibility, desire to maintain same frame format!– Each x10 increase in bit rate, must be accompanied by x10
decrease in distance ?!
Csci 183/183W/232 Direct Link networks 58
100Base T (Fast) Ethernet: Issues• 1 bit time = time to transmit one bit
– 100 Mbps 1 bit time = 0.01 s• If we keep the same “collision domain”, i.e., worst case collision detection time kept at 51.2 s
Q: What will be the minimum frame size?– 51.2 s => minimum # of bits can be transited at
100Mpbs is 5120 bits => 640 bytes is required for collision detection
– This requires change of frame format and protocol!
• Or we can keep the same minimum frame size, but reduce “collision domain” or network diameter!• slot time from 51.2 s to 5.12 s!• maximum network diameter 100 m!
Csci 183/183W/232 Direct Link networks 59
Fast (100Mbps) Ethernet
100baseT4 100baseT 100baseFX
MediumTwisted pair category 3
UTP 4 pairsTwisted pair category 5
UTP two pairs
Optical fiber multimode
Two strands
Max. Segment Length
100 m 100 m 2 km
Topology Star Star Star
IEEE 802.3 100 Mbps Ethernet medium alternatives
To preserve compatibility with 10 Mbps Ethernet:• Same frame format, same interfaces, same protocols• Hub topology only with twisted pair & fiber• Bus topology & coaxial cable abandoned• Category 3 twisted pair (ordinary telephone grade) requires 4
pairs• Category 5 twisted pair requires 2 pairs (most popular)• Most prevalent LAN today
Csci 183/183W/232 Direct Link networks 60
Gigabit Ethernet
Gigabit Ethernet Physical Layer Specification(IEEE 802.3 1 Gigabit Ethernet medium alternatives)
1000baseSX 1000baseLX 1000baseCX 1000baseT
MediumOptical fiber multimode
Two strands
Optical fiber single modeTwo strands
Shielded copper cable
Twisted pair category 5
UTP
Max. Segment Length
550 m 5 km 25 m 100 m
Topology Star Star Star Star
Csci 183/183W/232 Direct Link networks 61
Gigabit Ethernet• use standard Ethernet frame format• allows for point-to-point links and shared broadcast
channels• in shared (half-duplex hub) mode, CSMA/CD is used
- slot time increases to 512 bytes- small frames need to be extended to 512 B- carrier extension:– frame bursting: allow stations to transmit burst of
short frames• Commonly used today: Gigabit switches!
– Full-Duplex at 1 Gbps for point-to-point links– Frame structure preserved but CSMA-CD essentially
abandoned– Extensive deployment in backbone of enterprise data
networks and in server farms
Csci 183/183W/232 Direct Link networks 62
Carrier Extension
RRRRRRRRRRRRRFrame
Carrier Extension
512 bytes
• For 10BaseT : 2.5 km max; slot time = 64 bytes• For 1000BaseT: 200 m max; slot time = 512 bytes• Carrier Extension : continue transmitting control
characters [R] to fill collision interval.• This permits minimum 64-byte frame to be
handled.• Control characters discarded at destination.• For small frames net throughput is only slightly
better than Fast Ethernet.
Csci 183/183W/232 Direct Link networks 63
Frame Bursting
512 bytes
ExtensionFrame Frame Frame Frame
Frame burst
• Source sends out burst of frames without relinquishing control of the network.
• Uses Ethernet Interframe gap filled with extension bits (96 bits)
• Maximum frame burst is 8192 bytes• Three times more throughput for small frames.
Csci 183/183W/232 Direct Link networks 64
10 Gigabit Ethernet IEEE 802.3 10 Gbps Ethernet medium alternatives
10GbaseSR 10GBaseLR 10GbaseEW 10GbaseLX4
Medium
Two optical fibersMultimode at 850 nm
64B66B code
Two optical fibers
Single-mode at 1310 nm
64B66B
Two optical fibers
Single-mode at 1550 nmSONET compatibility
Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band8B10B code
Max. Segment Length
300 m 10 km 40 km 300 m – 10 km
• Frame structure preserved• CSMA-CD protocol officially abandoned• LAN PHY for local network applications• WAN PHY for wide area interconnection using SONET OC-192c • Extensive deployment in metro networks anticipated
Csci 183/183W/232 Direct Link networks 65
Server
100 Mbps links
10 Mbps links
ServerServer
Server
100 Mbps links
10 Mbps links
Server
100 Mbps links
10 Mbps links
Server
Gigabit Ethernet links
Gigabit Ethernet links
Server farm
Department A Department B Department C
Hub Hub Hub
Ethernet switch
Ethernet switch
Ethernet switch
Switch/router Switch/router
Typical Ethernet Deployment
Csci 183/183W/232 Direct Link networks 66
Ethernet Summary
• 1-persistent CSMA/CD• 51.2 s to seize the channel• Collision not possible after 51.2 s• Minimum frame size of 64 bytes• Binary exponential backoff• Works better under light load• Delivery time non-deterministic
Csci 183/183W/232 Direct Link networks 67
Summary (continued)
• Address– Unique number assigned to station– Put in frame header– Recognized by hardware
• Address forms– Unicast– Broadcast– Multicast
Csci 183/183W/232 Direct Link networks 68
Summary (continued)
• Type information– Describes data in frame– Set by sender– Examined by receiver
• Frame format– Header contains address and type information– Payload contains data being sent