Title: CS3505: Loca Area Networks CSMA/CD : IEEE 802.3 and LAN Internetworks
1CS3505Loca Area Networks CSMA/CD IEEE
802.3andLAN Internetworks
2 CSMA/CD
- basic LAN architecture and protocol
- most widespread LAN estimate more that 50 of
all LANs on the Internet... implemented by many
different companies - developed by Robert Metcalfe, XEROX PARC, early
1970s..... led to founding of 3COM company,
(Santa Clara). later Metcalfe sold his company
for 400M)
3CSMA / CD basic protocol
- broadcast medium
- first version used bus
- when the MAC receives a packet to transmit
- 1. sense carrier (listen)
- if no signal detected
- then begin Xmitting message, continue
sensing - if collision detected
- then Xmit jam, stop Xmitting, wait, then
goto (1) - when end-of-packet Xmitted, END.
- else carrier is busy go to (1)
4CSMA / CD basic protocol
- the wait random time is precisely defined
binary exponential backoff - physical encoding digital signals, manchester
encoding. Broadcast medium. - because collisions are detected, wasted time is
short
5CSMA / CD backoff algorithm
- binary exponential backoff (BACKOFF)
- 1 slot 51.2 ?s
- time following collision measured in slots
- a random slot between 1 and 1023 is chosen, and
the packet is retransmitted then
6CSMA / CD backoff algorithm
- example suppose 2 stations collide. What
happens? - what is Prob another collision?
- why measure time in 51.2 ?s slots?
7CSMA / CD frame format
- length 64 to 1518 bytes
- preamble 7 bytes SOF 1 byte
- DA,SA 2/6 bytes length 2 bytes
- data 0-1500 pad 0-46 FCS 4
-
8CSMA / CD topologies, media , etc.
- transmission media
- coaxial cable (decreasing)
- twisted pair (Cat 5, widely used)
- fiber (less common but increasing)
- topologies
- bus - original design used for many years
- star with dumb or smart hub now usual
- data rates
- 10 Mbps
- 100 Mbps
- Gbps where needed available
9CSMA / CD network components
- medium (coax, tp, fiber)
- transceivers
- drop cable station to coax
- NIC (MAC protocol logic)
- repeaters (needed to extend coax)
- test equipment
- hub (multiport repeater) for star configuration
- bridges (to connect to other LANs)
10CSMA / CD physical layout
- coax. cable, physical(and logical) bus CSMA/CD
-
11CSMA / CD physical layout
- star/hub configuration. physical star, logical
bus - 2-twisted pair connections hub is a repeater
12CSMA / CD some specifics
- prop speed 0.77 c on coax, 0.59 c on t.p.
- at most 4 repeaters between 2 stations so at
most 5 cable segments - 500 m /segment max, or 2000 with repeaters
(coax) - drop cables 25 m max
- max 100 stations per segment on coax
- at least 2.5 m between adjacent receivers on coax
- at most 1024 stations per ethernet
13CSMA / CD standard s
- IEEE 802.3 several physical configurations
- 10BASE5 baseband coaxial cable original
- 10BASE2 thin coaxial cable cheaper alternative
- 10BASE-T twisted pair, hub configuration
- 10BROAD36 uses broadband coax (TV cable)
- 10BASE-F fiber
- 100BASE-X fiber OR twisted pair
- NOTE all use the same frame format and basic MAC
protocol
14CSMA / CD practical considerations
- you have 3 PCs in your house what will it take
(equipment/dollars) to connect them together ? - design or plan a network for a small business
with 30-40 machines (PCs, Macs, etc), which is
located in a single building. (eqmt, dollars)
15CS3505Bridges / LAN internetworks
16Bridges connecting LANs together
- why do we need to connect LANs
- what is a bridge?
- types of bridges
- routing in LAN internetworks
- comparison bridges, routers, repeaters
- connecting similar LANs
- connecting dissimilar LANs
17why LANs need to be connected
- 1. connect 2 existing LANs (CS, math)
- 2. LAN too big split it, but stay connected
- -- too many stations or traffic for one LAN
- 3. connect geographically separate LANs.
- -- 2 offices in different towns
- 4. reduce collisions
- --increase efficiency
- 5. security
- --help restrict traffic to one LAN
-
18bridge what is it?
- low level switch that connects two or more
LANs. low level gt MAC layer - transparent there is no change in the LANs or
in the protocols of the networks - able to do simple routing
- retains the simplicity and flexibility of LANs
- faster than software switches (routers)
- reasonable cost cheaper than routers
19bridge or switch? terminology
- 1st bridges sold in 1984, were 2-ports
- early 1990s, multiport bridges appeared were
called switches by marketing vendors
technically no difference between a switch
(layer 2) and a bridge - multiple LANs connected by high port density
bridges commonly called switched LANs -
actually an internet of LANs - switching hub, LAN switch - other terms for a
multiport bridge
20bridges LAN connectors types
- local bridge
- remote (2 half bridges)
- same LAN, different LAN
- two port, multiport
- hub (not a bridge)
- repeater (not a bridge)
- router(not a bridge)
21bridge basic function
- suppose a bridge B connects networks X Y.
- Then B
- 1. reads all packets on X and Y, noting the
destination, source addresses (DA, SA) - 2. each packet on Y with DA on X is copied and
transmitted on X. - 3. each packet on X with DA on Y is copied,
transmitted on Y. - the bridge operates on X, Y using the MAC
protocol of those LANs.
22bridge connecting 2 ethernets
23notes on bridges
- all stations have unique MAC addresses
- bridge must know which LAN station is on
- multi-port bridges - similar extends to multiple
LANs - no change or adjustment in NIC needed bridge
completely transparent - bridge operates on each LAN using the MAC
protocol - remote (half bridge) - may use another protocol
between the 2 half bridges, while using MAC on
each LAN
24LAN internet
25half - bridge, connecting 2 LANs
- 2 halves communicate through some other protocol,
e.g., PPP, HDLC.
26bridges - routing
- how do bridges know which packets to forward,
and in which direction? - 2 basic techniques
- 1. fixed routing - the information is loaded
manually into the bridge (typing it in, etc.).
This info is then stored in a routing table. - 2. dynamic routing learning bridges -
- the bridge learns where the stations are by
watching the traffic on its ports
27bridges - routing
- for fixed routing, many topologies possible
- dynamic routing - the internet must be configured
as a tree this simplifies routing - tree LANs and bridges are the nodes, and the
links between them are the edges, and - LANs can be connected only to bridges, not
(directly) to other LANs - if a cycle exists, the bridges will detect it and
remove one from the active network, so that a
tree structure is maintained
28bridges - dynamic routing
- bridge has a routing table, 3 fields
- dest.address, next port , time
- when bridge receives a packet DA,SA on port X
- 1. if SA found in table, reset timer,
- else add SA, port, time to table.
- 2. if DA found in table send packet out on
next port indicated else send packet on all
ports except X.
29bridge dynamic routing
- timer typical value 300 seconds (why have the
timer? is this a good default value?) - given the tree structure, bridges will learn a
stations direction (explain how?) - MAC addresses could be divided into (network,
station) parts. If so, tables can be made
smaller, but same algorithm used.
30bridges, routers, repeaters, hubs
- repeaters simply connect 1 cable to another,
repeat the bits. No routing decisions or
filtering. - hubs serve to extend the ethernet. No routing
or filtering of messages. - bridges - connect LANs together at the MAC layer
filter and rout messages at the MAC layer. - routers -. Layer 3/3.5 (internet). Software, IP
protocol.. Usually more expensive. Discussed in
CS4550 ....