Advanced Computer Networks: Lecture 9 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
Lecture No. 9
Sequence Number Space
• SeqNum field is finite; sequence
numbers wrap around
• Sequence number space must be
larger than number of outstanding
frames (SWS)
3
Sequence Number Space
• SWS <= MaxSeqNum-1 is not sufficient
– Suppose 3bit SeqNum field (0..7);
SWS=RWS=7
– Sender transmits frames 0..6; which arrive
successfully (receiver window advances)
– ACKs are lost; sender retransmits 0..6
– Receiver expecting 7, 0..5, but receives
second incarnation of 0..5 assuming them as
4
8th to 13th frame
Required Sequence Number Space ?
• Assume SWS=RWS (simplest, and
typical)
– Sender transmits full SWS
– Two extreme cases at receiver
• None received (waiting for 0…SWS1)
• All received (waiting for SWS…
2*SWS1)
5
Required Sequence Number Space ?
• All possible packets must have unique
SeqNum
• SWS < (MaxSeqNum+1)/2 or
SWS+RWS < MaxSeqNum+1 is the
correct rule
• Intuitively, SeqNum “slides” between
two halves of sequence number space
6
What Next ?
• Arbitrating access to a shared
medium
• After that: network adapters and
example protocols
7
Shared Access Networks
Outline
•
•
•
•
Bus (Ethernet)
Token ring (FDDI)
Wireless (802.11)
Network Adapter
8
Shared Access Media
• Arbitrating access to a shared
media
– Multiple hosts on a single link
9
Multiple Access Media
• Multiple senders on some media
– Buses (Ethernet, including links in
switched form)
– Radio, satellite
– Token rings
…
10
Multiple Access Media
• Need method to moderate access
– Fair arbitration
– Good performance
11
Shared Media
• Communication needs vary
– Over time
– Between hosts
• Network is not fully utilized
12
Shared Media
• Recall methods for multiplexing
– Frequencydivision multiplexing (FDM,
separate bands)
– Timedivision multiplexing (TDM,
synchronous time slots)
– Statistical TDM (STDM, time slots on demand)
• STDM most appropriate with stated
assumptions
13
Shared Media: Problems
• Problem: demands can conflict, e. g. ,
two hosts send simultaneously
– STDM does not address this problem
centralized
– Solution is a medium access control
(MAC) algorithm
14
Shared Media: Solutions
• Three solutions (out of many)
– Carrier sense multiple access with
collision detection (CSMA / CD)
• Send only if medium is idle
• Stop sending immediately if collision
detected
– Token ring/FDDI pass a token around a
ring; only token holder sends
– Radio / wireless (IEEE 802.11)
15
Ethernet
16
History of Ethernet
• Developed by Xerox PARC in mid1970s
• Roots in Aloha packetradio network
• Standardized by Xerox / DEC / Intel in
1978
• Similar to IEEE 802.3 standard
• IEEE 802.3u standard defines Fast
Ethernet (100 Mbps)
• New switched Ethernet now popular
17
Ethernet Topologies
• Bus— all nodes connected to a wire
. . .
• Star— all nodes connected to a central repeater
• Combinations thereof
18
Ethernet Adaptor
•
•
•
•
Segment of up to 500 m
Nodes tap into segments
Taps must be 2.5m apart
Transceiver performs
carrier sensing
• Transceiver transmits
and receive signals
• Protocol is implemented
in the adaptor
Transceiver
Ethernet cable
Adaptor
Host
19
Ethernet – Alternative Technologies
• Can be constructed from a thinner cable (10Base2)
rather than 50ohm coax cable (10Base5)
• Newer technology uses 10BaseT (twisted pair)
– Several pointtopoint segments coming out of a
multiway repeater called “hub”
Hub
Hub
20
Ethernet Components
10Base5 (ThickNet)
Controller (Ethernet Card)
Vampire Tap
Bus Topology
Transceiver
21
Ethernet Components
10Base2 (ThinNet)
Controller (Ethernet Card)
Transceiver
BNC Tjunction
Bus Topology
22
Ethernet Components
10BaseT (Twisted Pair)
Controller (Ethernet Card)
Hub
Star Topology
23
Ethernet – Multiple Segments
• Repeaters forward the broadcast signal on all out
going segments (10Base5)
• Maximum of 4 repeaters (2500m), 1024 hosts
…
Repeater
…
Host
…
24
Ethernet Packet Frame
• Preamble allows the receiver to synchronize with
signal
• Frame must contain at least 46 bytes to detect
collision
• 802.3 standard substitutes length with type field
– Type field (demux key) is the first thing in data portion
– A device can accept both frames: type > 1500
64
48
48
16
Preamble
Dest
addr
Src
addr
Type
32
Body
CRC
25
