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. CS 118 Project # 2 Dear Professors,
Please announce the following project information in class:
1) Students should work in group of at most two (one is fine). Students
from different lecture sections are now allowed to form groups.
2) Each group must sign up ASAP with the TA (Jack Wei) to get trace
assignment. him with names, SID#, and s when signing up.
3) Make sure you go to this Friday's discussion and also check out the web
on Friday for finalized requirements of project 2.
Thank you,
Jack
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25. 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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26. 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 between all IP router pairs
Switched VCs (SVCs) used for short lived connections
Pro’s of ATM VC approach:
can guarantee QoS performance to a connection mapped to a 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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27. 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, the 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) a SVC is set up
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28. 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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29. 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 transmit queue
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30. ATM Layer
ATM layer in charge of transporting cells across the ATM network
ATM layer protocol defines ATM cell header format (5bytes);
payload = 48 bytes; total cell length = 53 bytes
VCI (virtual channel ID): translated from link to link;
PT (Payload type): indicates the type of payload (eg mngt cell)
|CLP (Cell Loss Priority) bit: CLP = 1 implies that the cell is low priority cell, can be discarded if router is congested
HEC (Header Error Checksum ) byte
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31. ATM Adaptation Layer
ATM Adaptation Layer (AAL): “adapts” the ATM layer to the upper layers (IP or native ATM applications)
AAL is present only in end systems, not in switches
The AAL layer has its header/trailer fields, carried in the ATM cell
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32. AAL (cont)
Different versions of AAL layers, depending on the service to be supported by the ATM transport:
AAL1: for CBR (Constant Bit Rate) services such as circuit emulation
AAL2: for VBR (Variable Bit Rate) services such as MPEG video
AAL5: for data (eg, IP datagrams)
Two sublayers in AAL:
(Common Part) Convergence Sublayer: encapsulates IP payload
Segmentation/Reassembly Sublayer: segments/reassembles the CPCS (often quite large, up to 65K bytes) into 48 byte ATM segments
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33. AAL5 - Simple And Efficient AL (SEAL)
AAL5: low overhead AAL used to carry IP datagrams
SAR header and trailer eliminated; CRC (4 bytes) moved to CPCS
PAD ensures payload multiple of 48bytes (LENGTH = PAD bytes)
At destination, cells are reassembled based on VCI number; AAL indicate bit delineates the CPCS-PDU; if CRC fails, PDU is dropped, else, passed to Convergence Sublayer and then IP
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34. Datagram journey in IP-over-ATM network
At Source Host:
(1) IP layer finds the mapping between IP and ATM exit address (using ARP); then, passes the datagram to AAL5
(2) AAL5 encapsulates datg and it segments to cells; then, down to ATM
In the network, the ATM layer moves cells from switch to switch, along a pre-established VC
At Destination Host, AAL5 reassembles cells into original datg;
if CRC OK, datgr is passd up the IP protocol.
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35. ARP in ATM Nets ATM can route cells only if it has the ATM address
Thus, IP must translate exit IP address to ATM address
The IP/ATM addr translation is done by ARP (Addr Recogn Protocol)
Generally, ATM ARP table does not store all ATM addresses: it must discover some of them
Two techniques:
(1) Broadcast the ARP request to all destinations:
(1.a) the ARP Request msg is broadcast to all ATM destinations using a special broadcast VC;
(1.b) the ATM destination which can match the IP address returns (via unicast VC) the IP/ATM address map;
Note: broadcast overhead prohibitive for large ATM nets
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36. ARP in ATM Nets (cont) (2) ARP Server:
(2.a) source IP router forwards ARP request to server on dedicated VC (Note: all such VCs from routers to ARP have same ID)
(2.b) ARP server responds to source router with IP/ATM translation
Note: Hosts must register themselves with the ARP server
Comments: more scaleable than ABR Broadcast approach (no broadcast storm). However, it requires an ARP server, which may be swamped with requests
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37. X.25 and Frame Relay
Wide Area Network technologies (like ATM); also, both Virtual Circuit oriented , like ATM
X.25 was born in mid ‘70s, with the support of theTelecom Carriers, in response to the ARPANET datagram technology (religious war..)
Frame relay emerged from ISDN technology (in late ‘80s)
Both X.25 and Frame Relay can be used to carry IP datagrams; thus, they are viewed as Link Layers by the IP protocol layer (and are thus covered in this chapter)
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38. X.25
X.25 builds a VC between source and destination for each user connection
along the path, error control (with retransmissions) on each hop using LAP-B, a variant of the HDLC protocol
also, on each VC, hop by hop flow control using credits; congestion arising at an intermediate node propagates to source via backpressure
as a result, packets are delivered reliably and in sequence to destination; per flow credit control guarantees fair sharing
putting “intelligence into the network” made sense in mid 70s (dumb terminals without TCP)
today, TCP and practically error free fibers favor pushing the “intelligence to the edges”; moreover, gigabit routers cannot afford the X.25 processing O/H
as a result, X.25 is becoming rapidly extinct
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39. Frame Relay Designed in late ‘80s and widely deployed in the ‘90s
FR VCs have no error control
Flow (rate) control is end to end; much less processing O/H than hop by hop credit based flow control
Designed to interconnect corporate customer LANs
Each VC is like a “pipe” carrying aggregate traffic between two routers
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40. Frame Relay (cont)
Corporate customer leases FR service from a public Frame Relay network (eg, Sprint or ATT)
Alternative, large customer may build Private Frame Relay network.
Frame Relay implements mostly permanent VCs (aggregate flows)
10 bit VC ID field in the Frame header
If IP runs on top of FR, the VC ID corresponding to destination IP address is looked up in the local VC table
FR switch simply discards frames with bad CRC (TCP retransmits..)
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41. Frame Relay -VC Rate Control
CIR = Committed Information Rate, defined for each VC and negotiated at VC set up time; customer pays based on CIR
DE bit = Discard Eligibility bit in Frame header
DE bit = 0: high priority, rate compliant frame; the network will
try to deliver it at “all costs”
DE bit = 1: low priority, “marked” frame; the network discards it
when a link becomes congested (ie, threshold exceeded)
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42. Frame Relay - CIR & Frame Marking
Access Rate: rate R of the access link between source router (customer) and edge FR switch (provider); 64Kbps < R < 1,544Kbps
Typically, many VCs (one per destination router) multiplexed on the same access trunk; each VC has own CIR
Edge FR switch measures traffic rate for each VC; it marks
(ie DE <= 1) frames which exceed CIR (these may be later dropped)
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43. Frame Relay - Rate Control
Frame Relay provider “almost” guarantees CIR rate (except for overbooking)
No delay guarantees, even for high priority traffic
Delay will in part depend on rate measurement interval Tc; the larger Tc, the burstier the traffic injected in the network, the higher the delays
Frame Relay provider must do careful traffic engineering before committing to CIR, so that it can back up such commitment and prevent overbooking
Frame Relay CIR is the first example of traffic rate dependent charging model for a packet switched network
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