1. Hubs, Bridges and Switches
Used for extending LANs in terms of geographical coverage, number of nodes, administration capabilities, etc.
Differ in regards to:
collision domain isolation
layer at which they operate
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2. Hubs
Physical Layer devices: essentially repeaters 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 a backbone hub at its top
Each connected LAN is referred to as a LAN segment
Hubs do not isolate collision domains: a node may collide with any node residing at any segment in the LAN
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3. Hubs (Cont.) Hub Advantages: + Simple, inexpensive device
+ Multi-tier provides graceful degradation: portions of the LAN continue to operate if one of the hubs malfunction
+Extends maximum distance between node pairs (100m per Hub)
Hub Limitations:
- Single collision domain results in no increase in max throughput; the multi-tier throughput same as the the single segment throughput
- Individual LAN restrictions pose limits on the number of nodes in the same collision domain (thus, per Hub); and on the total allowed geographical coverage
- Cannot connect different Ethernet types (eg 10BaseT and 100baseT)
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4. Bridges
Link Layer devices: they operate on Ethernet frames, examining the frame header and selectively forwarding a frame base on its destination
Bridge isolates collision domains since it buffers frames
When a frame is to be forwarded on a segment, the bridge uses CSMA/CD to access the segment and transmit
Bridge advantages:
+ Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage
+ Can connect different type Ethernet since it is a store and forward device
+ Transparent: no need for any change to hosts LAN adapters
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5. Backbone Bridge
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6. Interconnection Without Backbone
Not recommended for two reasons:
- Single point of failure at Computer Science hub
- All traffic between EE and SE must path over CS segment
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7. Bridge Filtering
Bridges learn which hosts can be reached through which interfaces and maintain filtering tables
A filtering table entry:
(Node LAN Address, Bridge Interface, Time Stamp)
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 on which the frame arrived*/ }
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8. Bridge Learning
When a frame is received, the bridge “learns” from the source address and updates its filtering table (Node LAN Address, Bridge Interface, Time Stamp)
Stale entries in the Filtering Table are dropped (TTL can be 60 minutes)
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9. Bridges Spanning Tree
For increased reliability, it is desirable to have redundant, alternate paths from a source to a destination
With multiple simultaneous paths however, cycles result on which bridges may multiply and forward a frame forever
Solution is organizing the set of bridges in a spanning tree by disabling a subset of the interfaces in the bridges:
Disabled
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10. Bridges Vs. Routers
Both are store-and-forward devices, but Routers are Network Layer devices (examine network layer headers) and Bridges are Link Layer devices
Routers maintain routing tables and implement routing algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms
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11. Routers Vs. Bridges (Cont)
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)
Routers + and -
+ Arbitrary topologies can be supported, cycling is limited by TTL counters
+ 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 are required in large networks (thousands of hosts)
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12. Ethernet Switches
A switch is a device that incorporates bridge functions as well as point-to-point ‘dedicated connections’
A host attached to a switch via a dedicated point-to-point connection; will always sense the medium as idle; no collisions ever!
Ethernet Switches provide a combinations of shared/dedicated, 10/100/1000 Mbps connections
Some E-net switches support cut-through switching: frame forwarded immediately to destination without awaiting for assembly of the entire frame in the switch buffer; slight reduction in latency
Ethernet switches vary in size, with the largest ones incorporating a high bandwidth interconnection network
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13. Ethernet Switches (Cont)
Dedicated
Shared
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14. IEEE Wireless LAN
Wireless LANs are becoming popular for mobile Internet access
Applications: nomadic Internet access, portable computing, ad hoc networking (multihopping)
IEEE standards defines MAC protocol; unlicensed frequency spectrum bands: 900Mhz, 2.4Ghz
Basic Service Sets + Access Points => Distribution System
Like a bridged LAN (flat MAC address)
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15. AD Hoc Networks
IEEE stations can dynamically form a group without AP
Ad Hoc Network: no preexisting infrastructure
Applications: “laptop” meeting in conference room, car, airport; interconnection of “personal” devices (see bluetooth.com); battelfield; pervasive computing (smart spaces)
IETF MANET (Mobile Ad hoc Networks) working group
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16. IEEE 802.11 MAC Protocol CSMA Protocol:
sense channel idle for DISF sec (Distributed Inter Frame Space)
transmit frame (no Collision Detection)
receiver returns ACK after SIFS (Short Inter Frame Space)
if channel sensed busy => binary backoff
NAV: Network Allocation Vector (min time of deferral)
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17. Hidden Terminal effect
CSMA inefficient in presence of hidden terminals
Hidden terminals: A and B cannot hear eachother because of obstacles or signal attenuation; so, their packets collide at B
Solution? CSMA/CA
CA = Collision Avoidance
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18. Collision Avoidance
CTS “freezes” stations within range of receiver (but possibly hidden
from transmitter); this prevents collisions by hidden station during data
RTS and CTS are very short: collisions during data phase are thus
very unlikely (the end result is similar to Collision Detection)
Note: IEEE allows CSMA, CSMA/CA and “polling” from AP
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19. Point to Point protocol (PPP)
Point to point, wired data link easier to manage than broadcast link: no Media Access Control
Several Data Link Protocols: PPP, HDLC, SDLC, Alternating Bit protocol, etc
PPP (Point to Point Protocol) is very popular: used in dial up connection between residential Host and ISP; on SONET/SDH connections, etc
PPP is extremely simple (the simplest in the Data Link protocol family) and very streamlined
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20. PPP requirements Pkt framing: encapsulation of packets
bit transparency: must carry any bit pattern in the data field
error detection (no correction)
multiple network layer protocols
connection liveness
Network Layer Address negotiation: Hosts/nodes across the link must learn/configure each other’s network address
PPP non-requirements
error correction/recovery
flow control
sequencing
multipoint links (eg, polling)
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21. 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 to which frame must be delivered (eg, PPP-LCP, IP, IP-CP, etc)
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22. Byte Stuffing
For “data transparency”, the data field must be allowed to include the pattern < > ; ie, this must not be interpreted as a flag
to alert the receiver, the transmitter “stuffs” an extra < > byte after each < > data byte
the receiver discards each followed by another , and continues data reception
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23. PPP Data Control Protocol
PPP-LCP establishes/releases the PPP connection; negotiates options
Starts in DEAD state
Options: max frame length; authentication protocol
Once PPP link established, IP-CP (Contr Prot) moves in (on top of PPP) to configure IP network addresses etc.
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24. ATM
ATM (Asynchronous Transfer Mode) is the switching and transport technology of the B-ISDN (Broadband ISDN) architecture (1980)
Goals: high speed access to business and residential users (155Mbps to 622 Mbps); integrated services support (voice, data, video, image)
Focus on bandwidth allocation facilities (in contrast to IP best effort)
ATM main role today: “switched” link layer for IP-over-ATM
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25. ATM VCs
ATM is a virtual circuit transport: cells (53 bytes) are carried on VCs
in IP over ATM: Permanent VCs (PVCs) between IP routers;
scalability problem: N(N-1) VCs all IP router pairs
Switched VCs (SVCs) used for short lived connections
Pro’s of ATM VC approach:
can guarantee performance for connections mapped to each VC (bandwidth, delay, delay jitter, hot standby etc)
Con’s of ATM VC approach:
inefficient support of datagram traffic: PVC solution (one PVC between each host pair) does not scale; SVC introduces excessive latency on short lived connections; also high SVC processing O/H
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26. ATM Address Mapping
Router interface (to ATM link) has two addresses: IP and ATM addr
To route an IP packet through the ATM network, the IP node:
(a) inspects own routing tables to find next IP router address
(b) then, using ATM ARP table, finds ATM addr of next router
(c) passes packet (with ATM address) to ATM layer
At this point ATM layer takes over:
(1) it determines the interface and VC on which to send out the packet
(2) if no VC exists (to that ATM addr) one is set up
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27. ATM Physical Layer Two Physical sublayers:
(a) Physical Medium Dependent (PMD) sublayer
(a.1) SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM);
self healing rings etc; several speeds: OC1 = Mbps; OC3 = Mbps; OC12 = Mbps
(a.2) TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps
(a.3) unstructured: just cells (busy/idle)
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28. ATM Physical Layer (cont)
(b) Transmission Convergence Sublayer (TCS): it adapts PMD sublayer to ATM transport layer
TCS Functions:
- header checksum generation: 8 bits CRC; it protects a 4-byte header; can correct all single errors.
- cell delineation
- with “unstructured” PMD sublayer, transmission of idle cells when no data cells are available in the tx queue
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29. ATM Layer
ATM layer in charge of transporting cells across the ATM network
ATM layer defines the structure of the ATM cell header (5bytes);
payload = 48 bytes; total cell length = 3 bytes
VCI (virtual channel ID): translated from link to link;
PT (Payload type): indicate the type of paylod (eg mngt cells)
|CLP (Cell Loss Priority) bit: CLP = 1 implies that the cell is low priority cells, can be discarded if router is congested
HEC (Header Error Checksum ) byte.
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