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CS4550:CS4550:
Computer Networks IIComputer Networks II
high speed networks, high speed networks, part 2 : Frame Relay, ATM part 2 : Frame Relay, ATM
frame relay frame relay
background : X.25 VC packet switched network
FR also a virtual circuit packet switched network, but with much less overhead
cheaper alternative to leased lines (T1); similar service, more economical
based on permanent VCs a standard for high speed service, not a
particular implementation
frame relay frame relay
operates at layer 2/3; IP can operate over FR, treating it as a layer 2 protocol
access speeds from 56K to a few megabits packet size 8000 bytes, optimized for data can be viewed as intermediate between
X.25 (traditional PS) and ATM (fast PS) has some similarities to both ATM and X.25 probably (?) to be overshadowed by ATM or
other higher speed networks
frame relay frame relay IP frame format example IP frame format example
Flag Addr Ctl Pro IP FCS Flag
X7e DLCI x03 xCC Datagram X7e•Based on HDLC
•Handles Multi-protocols from layer 3
•Layer 2 service
ATM : ATM : asynchronous transfer modeasynchronous transfer mode
new standard for optical fiber networks; meant for both voice & other data
layer : 2,3; intended to operate above SONET or equivalent.
cell-based technology; cells are small packets
many view ATM as the “network of the future”
ATM : asynchronous transfer modeATM : asynchronous transfer mode
specs developed by 2 orgs : ITU-T : a committee of the ITU
ATM forum : an international nonprofit industry consortium
the “ultimate” high-speed packet switching network
can be viewed as the culmination of circuit and packet switched tech., having the advantages of each, and without the problems
ATM : basic ideas ATM : basic ideas
virtual-circuit, packet switched network all data transmitted in small, fixed size
packets called “cells” (53 bytes) designed for optical fiber transmission :
assumes high speed, low error rate of fiber.
intended to carry all types of digital data : voice, computer data, video, etc.
may operate over SONET physical medium or others, such as FDDI.
ATM basic design assumptionsATM basic design assumptions
1. Will be designed as a hierarchy, as is current telephone network
2. Connection-oriented service3. Most physical networks will be fiber -
with low error rates, high data rates
4. Must support very low cost attachments -- e.g., tel calls
remains to be seen how pervasive ATM will become
Evolution of transfer modes Evolution of transfer modes (switching techniques) (switching techniques)
1. circuit switching : tel system
2. multirate circuit switching
3. fast circuit switching
4. packet switching : X.25, LANs, Internet
5. fast packet switching : frame relay, ATM
ATM basic principles ATM basic principles
no error protection/flow control on link by link basis
-why? connection-oriented
-why? minimal header functionality
-why? small, fixed cell size
-why?
ATM layersATM layers Physical layer: transmission
medium, signal encoding. optical fiber based.
ATM layer: switching, data transmission& logical connections. Heart of ATM.
ATM Adaptation layer: interface between ATM and upper layers.
applications: ATM customers
physical
ATM
AAL
apps
ATM : connections ATM : connections
ATM a “virtual circuit packet switched network” : connections made before data transmitted, & all packets in a connection follow same route.
virtual channel(VC) : a connection. Has a pair of endpoints, and the route (VP).
virtual path(VP) : set of VCs which share the same endpoints.
connections have “QOS parameters” - not allowed unless net can meet them.
ATM network example ATM network example
5
2
3
9
8x y
w
v
z
suppose X wants to pass data to y. whatmust the user and the ATM network do?
ATM network example ATM network example
the internal nodes (circles) represent ATM switches
the end users represented by squares
the communication links are SONET fiber optic links
ATM network example ATM network example
1. establish connection : user sends a request for a connection with several parameters: destination, bandwidth needed, quality of service. ATM net then determines whether the request can be accepted.
2. data transfer : user “talks.” ATM monitors, to ensure user does not violate contract.
3. terminate connection : when users are done, ATM must remove this connection from the network tables
ATM network exampleATM network example
5
2
3
9
8x y
w
v
z
connection established : now all packets on connection (x,y) will pass through 5,9,8; the route (5,9,8) is a “virtual path;” the VC includes x,y.
u
ATM cell formatATM cell format
GFC/VPI VPI
VPI VCI
VCI
VCI pay.type CLP
HEC
info field : 48 bytes
0 3 74NNIUNI
ATM cell format ATM cell format UNI : user-network interface NNI : network-network interface
GFC : generic flow control (only at UNI)VPI : virtual path id (routing) VCI : virtual channel id (routing) CLP : cell loss protection (0,1) et to 1 for high priority
Pay-Type: payload type parametersHEC : header error control
ATM : AAL, ATM adoption layerATM : AAL, ATM adoption layer
AAL interface between ATM layer and user layers (e.g., IP, PCM voice, etc.)
some AAL services: -handle transmission errors-segmentation and reassembly-lost /misinserted cells -flow control and timing control
ATM : AAL IP wrapping ATM : AAL IP wrapping
LLC SNAP
DSAP SSAP CTL ID Type IP Trailer
X’AA X’AA X03 X’0 X’08 DATAGRAM
ATM h data h data …. h data
quality of service parametersquality of service parameters
with each connection, there is an associated QOS, implied or stated. These are qualities such as: 1. peak cell rate, and peak duration2. average cell rate3. allowable loss rate 4. cell delay5. cell delay variation (jitter), or burstiness
quality of service parametersquality of service parameters
differing traffic types have different needs, for example:
-voice: small bandwidth, tolerates some losses, but needs small delay and jitter-data : tolerates more delay/jitter, but cannot tolerate losses; varying bandwidth needs
-video needs high bandwidth but may tolerate losses and different delays than above
challenge of ATM is to balance these widely varying needs, and make max use of net without overloading
ATM : traffic and congestionATM : traffic and congestion
traffic& congestion control attempt to keep the number of packets in the network within the capacity of the network to meet its requirements
more critical for ATM because of its real- time, QOS guarantees
traffic control - attempts to prevent congestion from happening
congestion control - attempts to reduce or control congestion
ATM traffic control tools ATM traffic control tools
1. network resource mgt : allocate NW resources optimally; minimize conflicts2. CAC : connection admission control - NW should not accept traffic it can’t handle 3. UPC : usage parameter control - monitor traffic flows, control cheating 4. priority control - using CLP field, discard low priority cells first 5. fast resource mgt - “further study”
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