Ethernet, Hubs, Bridges, Switches, Other Technologies used at the

Ethernet, Hubs, Bridges, Switches, Other Technologies used at the
Link Layer: Implementation
❒ Typically, implemented in “adapter”
❍ e.g., PCMCIA card, Ethernet card
❍ typically includes: RAM, DSP chips, host bus
interface, and link interface
Ethernet, Hubs, Bridges,
Switches, Other Technologies
used at the Link Layer, ARP
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Last Modified:
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5: DataLink Layer
5: DataLink Layer
errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame
we learned how to do reliable delivery over an unreliable
seldom used on low bit error link (fiber, some twisted
wireless links: high error rates
• Q: why both link-level and end-end reliability?
5: DataLink Layer
pacing between sender and receivers
Error Detection:
❒ Error Correction:
receiver identifies and corrects bit error(s)
without resorting to retransmission
5: DataLink Layer
LAN technologies
“dominant” LAN technology:
❒ cheap $20 for 100Mbs!
❒ first widely used LAN technology
❒ Simpler, cheaper than token LANs and ATM
❒ Kept up with speed race: 10, 100, 1000 Mbps
Data link layer so far:
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❒ Flow Control:
❒ Reliable delivery between two physically connected
phys. link
Link Layer Services (more)
❒ 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, dest
• different from IP address!
data link
adapter card
Link Layer Services
services, error detection/correction, multiple
Next: LAN technologies
hubs, bridges, switches
❍ 802.11
Metcalfe’s Ethernet
5: DataLink Layer
5: DataLink Layer
Ethernet Frame Structure
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
❒ 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
❒ 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
❒ used to synchronize receiver, sender clock rates
5: DataLink Layer
5: DataLink Layer
Ethernet: uses CSMA/CD
Ethernet’s CSMA/CD (more)
A: sense channel, if idle
Jam Signal: make sure all other transmitters are
aware of collision; 48 bits;
Exponential Backoff:
❒ Goal: adapt retransmission attempts to estimated
current load
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}
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
else {wait until ongoing transmission is over and goto A}
5: DataLink Layer
5: DataLink Layer 5a-10
Ethernet Technologies: 10Base2
10BaseT and 100BaseT
❒ 10: 10Mbps; 2: under 200 meters max cable length
❒ thin coaxial cable in a bus topology
❒ 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
❒ 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-11
5: DataLink Layer 5a-12
10BaseT and 100BaseT (more)
Gbit Ethernet
❒ Max distance from node to Hub is 100 meters
❒ use standard Ethernet frame format
❒ Hub can disconnect “jabbering adapter”
❒ allows for point-to-point links and shared
❒ Hub can gather monitoring information, statistics
for display to LAN administrators
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-13
5: DataLink Layer 5a-14
Ethernet Limitations
❒ Physical Layer devices: essentially repeaters
Q: Why not just one big Ethernet?
❒ Limited amount of supportable traffic: on single
LAN, all stations must share bandwidth
❒ limited length: 802.3 specifies maximum cable
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
❒ large “collision domain” (can collide with many
❒ How can we get around some of these limitations?
5: DataLink Layer 5a-15
5: DataLink Layer 5a-16
Hubs (more)
Hub limitations
❒ Each connected LAN referred to as LAN segment
❒ single collision domain results in no increase in max
❒ 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
❍ extends maximum distance between node pairs
(100m per Hub)
5: DataLink Layer 5a-17
❍ multi-tier throughput same as single segment
❒ 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-18
Bridges (more)
❒ Link Layer devices: operate on Ethernet
❒ Bridge advantages:
❍ Isolates collision domains resulting in higher
total max throughput, and does not limit the
number of nodes nor geographical coverage
frames, examining frame header and
selectively forwarding frame based on its
❒ 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
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-19
Bridges: frame filtering, forwarding
5: DataLink Layer 5a-20
Backbone Bridge
❒ 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-21
Interconnection Without Backbone
5: DataLink Layer 5a-22
Bridge Filtering
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)
❒ bridges
❒ 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-23
5: DataLink Layer 5a-24
Bridge Filtering
Bridge Learning: example
❒ 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
which the frame
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
5: DataLink Layer 5a-25
bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
5: DataLink Layer 5a-26
Bridges Spanning Tree
Bridge Learning: example
❒ 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
❒ 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-27
Spanning Tree Algorithm
5: DataLink Layer 5a-28
Ethernet Switches
❒ Sophisticated bridges
❍ Switches usually switch in
hardware, bridges in
❍ large number of interfaces
❒ Like bridges, layer 2
(frame) forwarding,
filtering using LAN
❒ Can support combinations
of shared/dedicated,
10/100/1000 Mbps
5: DataLink Layer 5a-29
5: DataLink Layer 5a-30
Common Topology
❒ Switching: A-to-B and A’-to-B’ simultaneously, no
❒ cut-through switching: frame forwarded from
input to output port without awaiting for assembly
of entire frame
❍ slight reduction in latency
❒ Store and forward switching: entire frame
received before transmission out an output port
❒ Fragment-free switching: compromise, before
send out the output port receive enough of the
packet to do some error checking (ex. detect and
drop partial frames)
5: DataLink Layer 5a-31
Bridges vs. Switches vs. Routers
❒ Switches = sophisticated multi-port bridges
❒ All store-and-forward devices
❍ routers: Layer 3 (network layer) devices
❍ Bridges/switches are Layer 2 (Link Layer) devices
❒ routers maintain routing tables, implement routing
❒ Bridges/switches maintain filtering tables,
implement filtering, learning and spanning tree
5: DataLink Layer 5a-32
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-33
5: DataLink Layer 5a-34
Routers vs. Bridges
Routers + and + arbitrary topologies can be supported, cycling is
❒ Layer 3 Devices (Network Layer)
❍ Router
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-35
❒ Layer 2 Devices (Link Layer)
❍ Bridge
❍ Switch
❒ Layer 1 Devices (Physical Layer)
❍ Repeaters
❍ Hubs
5: DataLink Layer 5a-36
IEEE 802.11 Wireless LAN
❒ wireless LANs: untethered
Ad Hoc Networks
❒ Ad hoc network: IEEE 802.11 stations can
(often mobile) networking
❒ IEEE 802.11 standard:
dynamically form network without AP
MAC protocol
unlicensed frequency
spectrum: 900Mhz, 2.4Ghz
❒ Basic Service Set (BSS)
(a.k.a. “cell”) contains:
❍ wireless hosts
❍ access point (AP): base
❒ BSS’s combined to form
distribution system (DS)
❒ Applications:
❍ “laptop”
meeting in conference room, car
of “personal” devices
❍ battlefield
(Mobile Ad hoc Networks)
working group
❍ interconnection
5: DataLink Layer 5a-37
IEEE 802.11 MAC Protocol:
802.11 CSMA: sender
- if sense channel idle for
DISF sec.
then transmit entire frame
(no collision detection)
-if sense channel busy
then binary backoff
802.11 CSMA receiver:
if received OK
return ACK after SIFS
5: DataLink Layer 5a-38
IEEE 802.11 MAC Protocol
802.11 CSMA Protocol:
❒ NAV: Network
❒ 802.11 frame has
transmission time field
❒ others (hearing data)
defer access for NAV
time units
5: DataLink Layer 5a-39
5: DataLink Layer 5a-40
Collision Avoidance: RTS-CTS
Hidden Terminal effect
❒ hidden terminals: A, C cannot hear each other
obstacles, signal attenuation
❍ collisions at B
❒ goal: avoid collisions at B
❒ CSMA/CA: CSMA with Collision Avoidance
5: DataLink Layer 5a-41
❒ CSMA/CA: explicit
channel reservation
❍ sender: send short
RTS: request to send
❍ receiver: reply with
short CTS: clear to
❒ CTS reserves channel for
sender, notifying
(possibly hidden) stations
❒ avoid hidden station
5: DataLink Layer 5a-42
Collision Avoidance: RTS-CTS
❒ RTS and CTS short:
Token Passing: IEEE802.5 standard
❒ 4 Mbps
❒ max token holding time: 10 ms, limiting frame length
❍ collisions
less likely,
of shorter duration
❍ end result similar to
collision detection
❒ IEEE 802.11 alows:
❍ polling from AP
❒ SD, ED mark start, end of packet
5: DataLink Layer 5a-43
Token Passing: IEEE802.5 standard
❒ 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-44
Point to Point Data Link Control
❒ one sender, one receiver, one link: easier
❒ FC: frame control used for monitoring and
❒ 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
than broadcast link:
❍ no Media Access Control
❍ no need for explicit MAC addressing
❍ e.g., dialup link, ISDN line
❒ popular point-to-point DLC protocols:
❍ PPP (point-to-point protocol)
❍ HDLC: High level data link control
❒ limited number of stations: 802.5 have token
passing delays at each station
5: DataLink Layer 5a-45
5: DataLink Layer 5a-46
PPP Design Requirements
[RFC 1557]
PPP non-requirements
❒ packet framing: encapsulation of network-layer
❒ no error correction/recovery
datagram in data link frame
❍ carry network layer data of any network layer
protocol (not just IP) at same time
❍ ability to demultiplex upwards
bit transparency: must carry any bit pattern in the
data field
error detection (no correction)
connection livenes: detect, signal link failure to
network layer
network layer address negotiation: endpoint can
learn/configure each other’s network address
5: DataLink Layer 5a-47
❒ no flow control
❒ out of order delivery OK
❒ no need to support multipoint links (e.g.,
Error recovery, flow control, data re-ordering
all relegated to higher layers!|
5: DataLink Layer 5a-48
PPP Data Frame
PPP Data Frame
❒ Flag: delimiter (framing)
❒ info: upper layer data being carried
❒ Address: does nothing (only one option)
❒ check: cyclic redundancy check for error
❒ Control: does nothing; in the future
possible multiple control fields
❒ Protocol: upper layer protocol to which
frame delivered (eg, PPP-LCP, IP, IPCP, etc)
5: DataLink Layer 5a-49
Byte Stuffing
5: DataLink Layer 5a-50
Byte Stuffing
❒ “data transparency” requirement: data field must
be allowed to include flag pattern <01111110>
❍ Q: is received <01111110> data or flag?
flag byte
in data
to send
❒ Sender: adds (“stuffs”) extra < 01111110> byte
after each < 01111110> data byte
❒ Receiver:
two 01111110 bytes in a row: discard first byte,
continue data reception
❍ single 01111110: flag byte
flag byte pattern plus
stuffed byte in
transmitted data
5: DataLink Layer 5a-51
5: DataLink Layer 5a-52
IP over Other Wide Area
Network Technologies
PPP Data Control Protocol
Before exchanging networklayer data, data link peers
❒ configure PPP link (max.
frame length,
❒ learn/configure network
layer information
❍ for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP
❒ Frame Relay
❒ X-25
5: DataLink Layer 5a-53
5: DataLink Layer 5a-54
ATM architecture
ATM: network or link layer?
Vision: end-to-end
transport: “ATM from
desktop to desktop”
❍ ATM is a network
Reality: used to connect
IP backbone routers
❍ “IP over ATM”
❍ ATM as switched
link layer,
connecting IP
❒ Adaptation layer (AAL): only at edge of ATM network
data segmentation/reassembly
roughly analogous to Internet transport layer
❒ ATM layer: “network” layer
❍ Virutal circuits, routing, cell switching
❒ physical layer
5: DataLink Layer 5a-55
ATM Layer: ATM cell
5: DataLink Layer 5a-56
ATM cell header
❒ VCI: virtual channel ID
❒ 5-byte ATM cell header
❍ will
❒ 48-byte payload
Why?: small payload -> short cell-creation delay
for digitized voice
❍ halfway between 32 and 64 (compromise!)
Cell header
Cell format
change from link to link thru net
❒ PT: Payload type (e.g. RM cell versus data
❒ CLP: Cell Loss Priority bit
= 1 implies low priority cell, can be
discarded if congestion
❒ HEC: Header Error Checksum
❍ cyclic redundancy check
5: DataLink Layer 5a-57
Classic IP only
❒ 3 “networks” (e.g., LAN
❒ MAC (802.3) and IP
Datagram Journey in IP-overATM Network
IP over ATM
❒ replace “network” (e.g.,
LAN segment) with ATM
❒ IP addresses -> ATM
addressesjust like IP
addresses to 802.3 MAC
5: DataLink Layer 5a-58
❒ at Source Host:
IP layer finds mapping between IP, ATM dest address
(using ARP)
❍ passes datagram to AAL5
❍ AAL5 encapsulates data, segments to cells, passes to
ATM layer
❒ ATM network: moves cell along VC to destination (uses
existing one or establishes another)
❒ at Destination Host:
❍ AAL5 reassembles cells into original datagram
❍ if CRC OK, datgram is passed to IP
5: DataLink Layer 5a-59
5: DataLink Layer 5a-60
X.25 and Frame Relay
Like ATM:
❒ X.25 builds VC between source and
destination for each user connection
❒ wide area network technologies
❒ Per-hop control along path
❒ virtual circuit oriented
❍ error
❒ origins in telephony world
❒ can be used to carry IP datagrams and can
thus be viewed as Link Layers by IP
protocol just like ATM
control (with retransmissions) on
each hop
❍ per-hop flow control using credits
• congestion arising at intermediate
node propagates to previous node on
• back to source via back pressure
5: DataLink Layer 5a-61
5: DataLink Layer 5a-62
IP versus X.25
Frame Relay
❒ X.25: reliable in-sequence end-end
❒ Designed in late ‘80s, widely deployed in
delivery from end-to-end
❍ “intelligence
in the network”
❒ IP: unreliable, out-of-sequence end-
end delivery
❍ “intelligence
the ‘90s
❒ Frame relay service:
❍ no
error control
congestion control
❍ end-to-end
in the endpoints”
❒ 2000: IP wins
❍ gigabit routers: limited processing
5: DataLink Layer 5a-63
Frame Relay (more)
5: DataLink Layer 5a-64
Frame Relay (more)
❒ Designed to interconnect corporate customer LANs
typically permanent VC’s: “pipe” carrying aggregate
traffic between two routers
❍ switched VC’s: as in ATM
❒ corporate customer leases FR service from public
Frame Relay network (eg, Sprint, ATT)
flags address
5: DataLink Layer 5a-65
❒ Flag bits, 01111110, delimit frame
❒ Address = address and congestion control
10 bit VC ID field
3 congestion control bits
• FECN: forward explicit congestion
notification (frame experienced congestion
on path)
• BECN: congestion on reverse path
• DE: discard eligibility
5: DataLink Layer 5a-66
Frame Relay -VC Rate Control
LAN Addresses
Each adapter on LAN has unique LAN address
❒ Committed Information Rate (CIR)
defined, “guaranteed” for each VC
negotiated at VC set up time
❍ customer pays based on CIR
❒ DE bit: Discard Eligibility bit
Edge FR switch measures traffic rate for each
VC; marks DE bit
❍ DE = 0: high priority, rate compliant frame;
deliver at “all costs”
❍ DE = 1: low priority, eligible for discard when
5: DataLink Layer 5a-67
5: DataLink Layer 5a-68
LAN Addresses vs IP
LAN Address vs IP Addresses
32-bit IP address (128 bit IPv6):
❒ MAC address allocation administered by IEEE
❒ manufacturer buys portion of MAC address space
network-layer address
(to assure uniqueness)
❒ used to get datagram to destination network
❒ Analogy:
(recall IP network definition)
(a) MAC address: like Social Security Number
(b) IP address: like postal address
❒ MAC flat address => portability
LAN (or MAC or physical) address:
❒ used to get datagram from one interface to
another physically-connected interface (same
❒ 48 bit MAC address (for most LANs)
burned in the adapter ROM
5: DataLink Layer 5a-69
❒ look up net. address of B, find B
on same net. as A
❒ link layer send datagram to B
inside link-layer frame
frame source,
dest address
datagram source,
dest address
A’s IP
B’s IP
5: DataLink Layer 5a-70
How can we determine the
MAC address of B
given B’s IP address?
Recall earlier routing discussion
Starting at A, given IP
datagram addressed to B:
can move LAN card from one LAN to another
❒ IP hierarchical address NOT portable
❍ depends on network to which one attaches
IP payload
5: DataLink Layer 5a-71
5: DataLink Layer 5a-72
ARP: Address Resolution Protocol
ARP 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)
❒ A knows B's IP address, wants to learn physical
address of B
❒ A broadcasts ARP query pkt, containing B's IP
❍ 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-73
5: DataLink Layer 5a-74
Hands-on: arp
ARP in ATM Nets
❒ arp ipaddress
❍ Return the MAC address associated with the
given IP address
❒ ATM network needs destination ATM address
just like Ethernet needs destination Ethernet
❒ IP/ATM address translation done by ATM ARP
(Address Resolution Protocol)
❍ ARP server in ATM network performs
broadcast of ATM ARP translation request to
all connected ATM devices
❍ hosts can register their ATM addresses with
server to avoid lookup
❒ arp –a
❍ List the contents of the local ARP cache
❒ arp –s hostname macAddress
❍ Used by the system administrator to add a
specific entry to the local ARP cache
5: DataLink Layer 5a-75
5: DataLink Layer 5a-76
❒ A creates IP packet with source A, destination B
Routing to another LAN
❒ A uses ARP to get R’s physical layer address for
❒ A creates Ethernet frame with R's physical address as dest,
Ethernet frame contains A-to-B IP datagram
walkthrough: routing from A to B via R
❒ 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
❒ In routing table at source Host, find router
❒ In ARP table at source, find MAC address E6-E900-17-BB-4B, etc
5: DataLink Layer
5: DataLink Layer 5a-78
principles behind data link layer services:
❍ error detection, correction
❍ sharing a broadcast channel: multiple access
❍ link layer addressing, ARP
❒ various link layer technologies
❍ Ethernethubs, bridges, switches
❍ IEEE 802.11 LANs
❍ ATM, X.25, Frame Relay
❒ journey down the protocol stack now OVER!
❍ Next stops: security, network management(?)
5: DataLink Layer 5a-79
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