Transcript Physical Layer, Data Link Layer, MAC Protocols, ARP

Ch. NAL& reyaL kniL :2
Chapter 2
Datalink Layer
& LAN Protocols
1
Ch. NAL& reyaL kniL :2
Physical Media
 physical link:
transmitted data bit
propagates across link
 guided media:

signals propagate in
solid media: copper,
fiber
 unguided media:
 signals propagate
freely, e.g., radio
Twisted Pair (TP)
 two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
ethernet
Category 5 TP:
100Mbps ethernet
2
Ch. NAL& reyaL kniL :2
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
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
Ethernet
3
Ch. NAL& reyaL kniL :2
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)
4
Ch. NAL& reyaL kniL :2
Physical link types:
 Point to point link
 Shared medium link
- also called:



Broadcast link
Multi-access link
LAN
Point to point link
 Two stations only
Shared medium link:
 Many stations on same
medium segment
 Intermittent transmission:
only when needed
 Qn: WHY?
 Collisions occur
 unless protocol makes
 Continuous transmission
special arrangements for
co-ordination of
 Needed to keep bit clock
transmission
synchronization
 Sends filler when no data  Bit synchronization done
 Full duplex
per frame
5
Ch. NAL& reyaL kniL :2
The Data Link Layer
Our goals:
Overview:
 understand principles
 link layer services
behind data link layer
services:



error detection,
correction
sharing a broadcast
channel: multiple access
link layer addressing
 error detection, correction
 multiple access protocols and
LANs
 link layer addressing
 specific link layer technologies:

Ethernet
 instantiation and
implementation of various
link layer technologies
6
Ch. NAL& reyaL kniL :2
Link Layer: setting the context
7
Ch. NAL& reyaL kniL :2
Recap: The Hourglass Architecture of the Internet
Telnet Email
TCP
FTP WWW
UDP
IP
Ethernet Wireless FDDI
8 8
Ch. NAL& reyaL kniL :2
Link Layer: setting the context
 two physically connected devices:
 host-router, router-router, host-host
 unit of data: frame
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
network
link
physical
Hl Hn Ht M
frame
adapter card
9
Ch. NAL& reyaL kniL :2
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
10
Ch. NAL& reyaL kniL :2
Link Layer Services
 Framing, link access:



encapsulate datagram into frame, adding header, trailer
implement channel access if shared medium,
‘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 both link-level and end-end reliability?
11
Ch. NAL& reyaL kniL :2
Link Layer Services (more)
 (OPTIONAL) Flow Control:

pacing between sender and receivers
 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
12
Ch. NAL& reyaL kniL :2
Adaptors Communicating
datagram
sending
node
frame
adapter
 link layer
implemented in
“adaptor” (aka NIC)

Ethernet card,
modem, 802.11 card
 adapter is semi-
autonomous,
implementing link &
physical layers
receiving
node
link layer protocol
frame
adapter
 sending side:
 encapsulates datagram in a
frame, delimits frame
 adds error checking bits, rdt
param’s, flow ctrl, etc.
 receiving side
 recognizes frame start /end
 checks errors, rdt, flow ctrl, ..
 extracts datagram, passes to L3
13
Ch. NAL& reyaL kniL :2
Link Layer: Implementation
 implemented in “adapter”
e.g., PCMCIA card, Ethernet card
 typically includes: RAM, DSP chips, host bus
interface, and link interface

M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
adapter card
network
link
physical
Hl Hn Ht M
frame
14
Ch. NAL& reyaL kniL :2
Error Detection
EDC= 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
Checksum
Generator
=?
EDC”
Checksum Generator
15
Ch. NAL& reyaL kniL :2
Parity Checking
Single Bit Parity:
Detect all single bit errors
Two dimensional Bit Parity:
Correct all single bit errors,
Detect all X bit errors X=?
Parity bit=1 iff
Number of 1’s even
0
0
16
Ch. NAL& reyaL kniL :2
Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment (note: used at transport layer only)
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?
17
Ch. NAL& reyaL kniL :2
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
18
Ch. NAL& reyaL kniL :2
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R
R = remainder[
D.2r
G
]
19
Ch. NAL& reyaL kniL :2
Example 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 implementation
Using shift and XOR registers
http://en.wikipedia.org/wiki/CRC-32#Implementation
20
Ch. NAL& reyaL kniL :2
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)
21
Ch. NAL& reyaL kniL :2
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
22
Ch. NAL& reyaL kniL :2
Multiple Access protocols
 claim: humans use multiple access protocols
all the time
 class can "guess" multiple access protocols
multiaccess protocol
 multiaccess protocol
 multiaccess protocol
 multiaccess protocol

1:
2:
3:
4:
23
Ch. NAL& reyaL kniL :2
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
24
Ch. NAL& reyaL kniL :2
MAC Protocols: Measures
 Channel Rate = R bps
 Efficient:
 Single
user: Throughput R
 Fairness
N
users
 Min. user throughput R/N
 Decentralized

Fault tolerance
 Simple
25
Ch. NAL& reyaL kniL :2
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
26
Ch. NAL& reyaL kniL :2
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
frequency bands
bands 2,5,6 idle
27
Ch. NAL& reyaL kniL :2
TDMA & FDMA: Performance
 Channel Rate = R bps
 Single user
 Throughput
R/N
 Fairness
 Each
user gets the same allocation
 Depends on maximum number of users
 Decentralized

Requires resource division
 Simple
28
Ch. NAL& reyaL kniL :2
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
29
Ch. NAL& reyaL kniL :2
Slotted Aloha [Norm Abramson]
 time is divided into equal size slots (= pkt trans. time)
 node with new arriving pkt: transmit at beginning of
next slot
 if collision: retransmit pkt in future slots with
probability p, until successful.
Success (S), Collision (C), Empty (E) slots
30
Ch. NAL& reyaL kniL :2
Slotted Aloha efficiency
Q: 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: channel
use for useful
transmissions 37%
of time!
31
Ch. NAL& reyaL kniL :2
Goodput vs. Offered Load
Slotted Aloha
0.5
1.0
1.5
2.0
G = offered load = Np
 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
32
Ch. NAL& reyaL kniL :2
Maximum Efficiency vs. n
0.4
1/e = 0.37
maximum efficiency
0.35
0.3
0.25
0.2
At best: channel
use for useful
transmissions 37%
of time!
0.15
0.1
0.05
0
2
7
12
17
n
33
Ch. NAL& reyaL kniL :2
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]
34
Ch. NAL& reyaL kniL :2
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
0.4
0.3
Slotted Aloha
0.2
0.1
protocol constrains
effective channel
throughput!
Pure Aloha
0.5
1.0
1.5
2.0
G = offered load = Np
35
Ch. NAL& reyaL kniL :2
Aloha: Performance
 Channel Rate = R bps
 Single user
 Throughput
R
 Fairness
 Multiple
users
 Combined throughput only 0.37*R
 Decentralized

Slotted Aloha needs slot synchronization
 Simple
36
Ch. NAL& reyaL kniL :2
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!
37
Ch. NAL& reyaL kniL :2
CSMA collisions
spatial layout of nodes along Ethernet
collisions can occur:
propagation delay means
two nodes may not yet
hear each other’s
transmission
collision:
entire packet transmission
time wasted
note:
role of distance and
propagation delay in
determining collision prob.
38
Ch. NAL& reyaL kniL :2
CSMA/CD: Collision Detection
spatial layout of nodes along Ethernet
spatial layout of nodes along Ethernet
C
D
A
t0
t0
time
B
time
A
B
C
B detects
collision,
aborts
D
D detects
collision,
aborts
instead of wasting the whole packet
transmission time, abort after detection.
CSMA
CSMA/CD
39
Ch. NAL& reyaL kniL :2
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
40
Ch. NAL& reyaL kniL :2
CSMA/CD collision detection
41
Ch. NAL& reyaL kniL :2
CDMA/CD
 Channel Rate = R bps
 Single user
 Throughput
 Fairness
R
 Multiple
users
 Depends on Detection Time
 Decentralized

Completely
 Simple
 Needs collision detection hardware
42
Ch. NAL& reyaL kniL :2
“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” protocols
look for best of both worlds!
43
Ch. NAL& reyaL kniL :2
“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)
44
Ch. NAL& reyaL kniL :2
Reservation-based protocols
Distributed 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
45
Ch. NAL& reyaL kniL :2
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
46
Ch. NAL& reyaL kniL :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

47
Ch. NAL& reyaL kniL :2
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
48
Ch. NAL& reyaL kniL :2
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
49
Ch. NAL& reyaL kniL :2
Comparison of IP address and MAC Address
 IP address is
 MAC address is flat
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

 MAC address: no need
for global uniqueness,
but in fact is globally
unique
 MAC address is
assigned to a device

portable
NOT portable
50
LAN Addresses and ARP
Each adaptor on LAN has unique MAC address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
Broadcast address =
FF-FF-FF-FF-FF-FF
= adaptor card (NIC)
LAN
(wired or
wireless)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5-51 Ch. :2
kniL
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
237.196.7.78
237.196.7.23
1A-2F-BB-76-09-AD
237.196.7.14
LAN
71-65-F7-2B-08-53
237.196.7.88
 Each IP node (Host, Router)
on a LAN has an ARP table
 ARP Table: IPMAC addr
mapping for LAN nodes
 ARP protocol: used to get
new entries in ARP table when
needed
 ARP message has following
parameters:

58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98


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
5-52 Ch. :2
kniL
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 and
Src_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
kniL
Ch. NAL& reyaL kniL :2
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
Src MAC
Dest MAC
Type
Source IP
Src MAC
Dest IP
Dest MAC
α
FF-…-FF
Query
a
α
b
?
ARP Message
2. B reads the message and sends ARP reply to A
Ethernet Header

reply sent as a unicast frame to A’s MAC address
Src MAC
Dest MAC
Type
Source IP
Src MAC
Dest IP
Dest MAC
β
α
Query
b
β
a
α
Ethernet Header
ARP Message
5-54
Ch. NAL& reyaL kniL :2
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 Etheret
sketch
55
Ch. NAL& reyaL kniL :2
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
56
Ch. NAL& reyaL kniL :2
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
57
Ch. NAL& reyaL kniL :2
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}
58
Ch. NAL& reyaL kniL :2
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 ten or more collisions, choose K from
{0,1,2,3,4,…,1023}
59
Ch. NAL& reyaL kniL :2
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%
60
Ch. NAL& reyaL kniL :2
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!
61
Ch. NAL& reyaL kniL :2
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
62
Ch. NAL& reyaL kniL :2
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
63
Ch. NAL& reyaL kniL :2
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
64
Ch. NAL& reyaL kniL :2
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
65
Ch. NAL& reyaL kniL :2
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
66
Ch. NAL& reyaL kniL :2
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
67