Transcript Ethernet

10BaseT, 100BaseT, hub
 T= Twisted pair (copper wire)
 Nodes connected to a hub, 100m max distance
 Hub: physical layer repeaters
 repeat received bits on one interface to all other interfaces; no
buffering
 Transmission by one node may collide with any node residing
at any segment connected to the same hub
twisted pair
hub
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Interconnecting using hubs
 Can use a backbone hub to interconnect LAN segments
 Extends max distance between nodes
 Create a single large collision domain
 Can’t interconnect 10BaseT & 100BaseT
hub
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Ethernet Switch
 Link layer device: stores and forwards Ethernet frames
 forwards frame based on MAC dest address
 uses CSMA/CD to access segment
 Transparent: hosts are unaware of presence of switches
 plug-and-play: switches do not need to be configured
switch
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3
2
hub
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Building a forwarding table by self learning
 When receive data frame: associate sender address with
incoming interface
 record sender/interface pair in a forwarding table
Each entry: MAC Address, Interface, Time Stamp
 stale entries in table dropped (TTL can be 60 min)

 Data forwarding algorithm: when receive a frame
if entry found for destination
then{
if destination on interface from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood (forward to all but the interface the frame came from)
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Switch example
Suppose C sends a data frame  to D
address
A
B
E
G
C
D
switch
1
3
2
hub
hub
hub
A
interface
1
1
2
3
1
2
I
B
C
F
D
E
G
H
 Switch receives  from C
 Add to forwarding table: C is on interface 1
 D is not in table: forwards  to interfaces 2 and 3
 frame received by D
When D replies back with a frame  to C
 Add to forwarding table: D is on interface 2
 Forward  to C
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More on Switch
 Traffic isolation:
 same-LAN-segment frames (usually) not forwarded onto other
LAN segments
 segments become separate collision domains
switch
collision domain
hub
Collision
domain
hub
collision
domain
hub
 cut-through switching: frame forwarded from input to
output port without first collecting entire frame
 can combine 10/100/1000 Mbps interfaces
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Switches vs. Routers
 both are store-and-forward devices
 routers: network layer devices (examine network layer
headers)
 Switches: link layer devices
 routers maintain routing tables, implement routing
algorithms
 switches maintain switch tables, implement selflearning algorithms
Switch
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Switches: advantages and limitations
 Transparent: no need for any change to hosts
 Isolates collision domains
 resulting
in higher total max throughput
 Can connect different types of Ethernet
 because
it is a store and forward device
 Constrained topology: tree only
 all
inter-segment traffic concentrated on a single tree
 (all multicast traffic forwarded to all LAN’s)
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Routers: advantages and limitations
 Support arbitrary topologies
 Efficient support for multicast routing
 And
can prevent broadcast storms
 Require IP address configuration (not plug and
play)
 More complex data processing than switches
 bridges do well in small setting (few hundred
hosts), routers are used in large networks
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Point to Point Data Link Control
 One sender, one receiver, one link
 e.g.,
dialup link, ISDN line
 easier than broadcast link:
 no
Media Access Control
 no need for explicit MAC addressing
 popular point-to-point DLC protocols:
 PPP (point-to-point
protocol)
 HDLC: High level data link control (Data link used to
be considered “high layer” in protocol stack!
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PPP Design Requirements [RFC 1661, 1662]
 packet framing: encapsulation of network-layer
datagram in data link frame
carry data of any network layer protocol (not just IP)
 ability to de-multiplex upwards

 bit transparency: must carry any bit pattern in data field
 error detection
 connection liveness: detect, signal link failure to network
layer
 network layer address negotiation: endpoint can
learn/configure each other’s network address
Non-requirements
 no error correction/recovery
 no flow control
 out of order delivery OK 11
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PPP Data Frame
 Flag: delimiter (framing)
 Address: does nothing (only one option)
 Control: does nothing; in the future possible multiple
control fields
 Protocol: upper layer protocol to which frame delivered
(eg, PPP-LCP, IP, IPCP, etc)
 info: upper layer data being carried
 check: cyclic redundancy check for error detection
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Byte Stuffing
 “data transparency” requirement: data field must be
allowed to include flag pattern <01111110>
 Q: is received <01111110> data or flag?
 Define the
Control Escape octet as 01111101
 Sender: adds (“stuffs”) < 01111101> byte after each <
01111110> data byte
 Receiver:
< 01111101> followed by <01111110>: discard first byte,
continue data reception
 single 01111110: flag byte

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Byte Stuffing
flag byte
pattern
in data
to send
flag byte pattern plus
stuffed byte in
transmitted data
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PPP Data Control Protocol
Before exchanging networklayer data, data link peers
must
 configure PPP link (max.
frame length, authentication)
 learn/configure network
layer information
 for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP address
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