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CS4550:
Computer Networks II
high speed networks,
part 2 : Frame Relay, ATM
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
 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
IP frame format example
Flag Addr Ctl
Pro
IP
FCS Flag
X7e DLCI x03 xCC Datagram
•Based on HDLC
•Handles Multi-protocols from layer 3
•Layer 2 service
X7e
ATM : asynchronous 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
 many
technology; cells are small packets
view ATM as the “network of the
future”
ATM : 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
 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 assumptions
1. Will be designed as a hierarchy, as is current
telephone network
2. Connection-oriented service
3. 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
become
to be seen how pervasive ATM will
Evolution of transfer modes (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
 no
error protection/flow control on link by link
basis
-why?
 connection-oriented
-why?
 minimal header functionality
-why?
 small, fixed cell size
-why?
ATM layers

apps
AAL
ATM


physical

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
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 connections: VP,VC
Virtual path
transmission link
ATM network example
y
x
w
8
5
z
2
9
v
3
suppose X wants to pass data to y. what
must the user and the ATM network do?
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
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 example
u
x
w
y
8
5
z
2
9
v
3
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.
NNI
UNI
ATM cell format
0
7
3 4
GFC/VPI
VPI
VPI
VCI
VCI
VCI
pay.type
HEC
info field : 48 bytes
CL
P
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 parameters
HEC : header error control
ATM : 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
LLC
SNAP
DSAP SSAP CTL ID Type
X’AA X’AA X03 X’0 X’08
ATM
h data
h data
….
IP
Trailer
DATAGRAM
h data
quality 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 duration
2. average cell rate
3. allowable loss rate
4. cell delay
5. cell delay variation (jitter), or burstiness
quality 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 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
1. network resource mgt : allocate NW resources
optimally; minimize conflicts
2. 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”
ATM congestion control tools
 selective
cell discarding - more aggressive in
discarding cells
 explicit
forward congestion indication - notifies
other nodes that congestion occurred; a
warning