1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 16

3. Understand different ways to move through network (forwarding)
Where we are now …
Understand different ways to move through network (forwarding)
Read signs at each switch (datagram)
Follow a known path (virtual circuit)
Carry instructions (source routing)
Bridge approach to extending LAN concept

4. Next Where we are now … Example of a real network (ATM)
How switches are built and contention within switches

5. ATM (Asynchronous Transfer Mode)
Defined by ATM Forum (formed in Oct. 1991)
Telephone industry (link providers to build networks)
Data network industry
High speed switching technology: right thing at right place at right time ? ? ?

6. ATM (Asynchronous Transfer Mode)
Common in WANs, can also be used in LANs
Competing technology with Ethernet, but areas of application only partially overlap
Connection-oriented packet-switched network
Virtual-circuit routing
Typically implemented on SONET (other physical layers possible)

7. ATM (Asynchronous Transfer Mode)
Signaling (connection setup) Protocol: Q.2931
Discovering routes and allocating resources at switches
ATM address format
E.164 and NSAP (Network Service Access Point)
Different from MAC addresses

8. ATM Signaling Connection setup called signaling (standard Q.2931)
Route discovery, resource resv, QoS, ...
Send through network
Request setup circuit
Send setup frame on setup circuit
Establish locally
No intermediate switch involvement
Requires pre-established virtual path

9. Fixed length (53 bytes) frames are called cells
Cell Switching (ATM)
Fixed length (53 bytes) frames are called cells
5-byte (header + 1-byte CRC-8) + 48-byte payload
Standard defines 3 layers (5 sublayers)
Layers interface to physical media and to higher layers (e.g., encapsulating variable-length frames)

10. 2-level connection hierarchy
Cell Switching (ATM)
2-level connection hierarchy
Virtual circuits
Virtual paths
Bundles of virtual circuits
Travel along common route
Reduces forwarding information

11. Why Hierarchical Connections ?
Simpler ...
Setup
New virtual circuits follow virtual path route
Forwarding
Virtual path identifier (VPI) used between switches (smaller forwarding table)
VCI used for last hop (to host)
Rerouting around failures
Change virtual path once vs. 64k virtual circuits

12. Variable-length Frames …
Small data is sent in a minimum-sized packet
No need for extraneous padding
Large file is sent by breaking it into many maximum-sized packets
Low overhead (header to data bytes ratio), increasing bandwidth efficiency
Minimize total number of packets sent
Minimize per-packet processing

13. Drawbacks of Fixed-length Frames
No optimally good fixed length
Higher overhead for large frames
Must be fragmented
One header per fragment (less efficient)
Low utilization for small frames
Must be padded

14. Then Why Fixed-length Frames ?
Require simpler hardware …
Facilitates the impl of hardware switches
Helpful building fast, highly scalable switches
Easier to build hardware doing simple job: to process known-length frame
Parallelism in processing stages: lots of switches doing same thing in parallel
Analogy: processor instruction pipeline with variable-length stages

15. Better behavior of non-preemptive queues
Why Short Frames ?
Better behavior of non-preemptive queues
Reduced granularity of preemption
High-priority frame may wait for max-size frame
Long frame (4kB) admits long wait
Short frame limits wait
Limits end-to-end jitter, or variance in latency

16. Shorter queues Why Short Frames ?
Switches typically store and forward packets
Cannot send until full packet arrives
Short frames (fragmentation) allows first part to be sent while remainder arrives

17. Queuing Behavior Examples
Consider 4kB vs. 53B frames, 100 Mbps link
Preemption: high-priority frame arrives just as switch starts sending low-priority frame
4kB: wait for 4096 x 8 / 100 = microseconds
53B: wait for 53 x 8 / 100 = 4.24 us

18. Queuing Behavior Examples
Shorter queues: two chunks (or frames) arrive simultaneously at time 0
4kB: link is idle until all data arrive at time us; 8 kB left to send
53B: link nearly fully utilized (waits 4.24us); at time microseconds, roughly 4kB left to send

19. Why 53-byte Frames ? Telephone community wish: carry voice effectively
Demands ATM to improves latency for audio data
Voice encoded at 64kbps: 8-bit smpl at 8KHz
Need full cell’s worth of samples before sending cell
1Kbyte cells -> 125ms per cell (human detectable)
53 byte cells implies 6 ms of data

20. Compromise: 48 bytes = 32(Europe) + 64(US) / 2
Why 53-byte Frames ?
Smaller latency implies no need for echo cancellers
Audio reconstruction
Expect low rate of cell loss; can interpolate loss (6 ms)
Compromise: 48 bytes = 32(Europe) + 64(US) / 2

21. ATM Cell Format User-Network Interface (UNI)
Host-to-switch format
GFC: Generic Flow Control (still being defined)
VCI/VPI: Virtual Circuit/Path Identifier
Type: management, congestion control, AAL5 (later)
CLP: Cell Loss Priority
HEC: Header Error Check (CRC-8)
Network-Network Interface (NNI)
Switch-to-switch format
GFC becomes part of VPI field 4 8 16 3 1 8
384 (48 bytes)
GFC
VPI
VCI
Type
CLP
HEC(CRC-8)
payload

22. Segmentation and Reassembly
Variable-length packets passed to ATM are often larger than 48 bytes
May not fit in the ATM payload
Fragmentation is required at the source
High-level message into low-level packets

23. Segmentation and Reassembly
Destination reassembles the fragments
Transforms fragments back into the message
ATM name this procedure as Segmentation and Reassembly (SAR)

24. Segmentation and Reassembly
ATM Adaptation Layer (AAL)
Application to ATM cell mapping
AAL header contains information for reassembly
AAL1, AAL2 for applications needing guaranteed rate
AAL3/4 designed for variable-length packet data
AAL5 is an alternative standard for packet data
AAL
AAL … … A TM A TM

25. ATM Layers ATM Adaptation Layer (AAL) ATM Layer Physical layer
Convergence Sublayer (CS) supports different application service models
Segmentation and Reassembly (SAR) supports variable-length frames
ATM Layer
Handles virtual circuits, cell header generation, flow control
Physical layer
Transmission Convergence (TC) handles error detection, framing
Physical medium dependent (PMD) sublayer handles encoding
AAL CS
SAR
ATM
PHY TC
PMD

26. AAL 3/4
Provides information to allow variable size packets to be sent in fixed-size ATM cells
Convergence Sublayer Protocol Data Unit (CS-PDU)
CPI: Common Part Indicator (version field)
Btag/Etag:beginning and ending tags (same)
BAsize: hint on reassembly buffer space to allocate
Length: size of whole PDU
Segmented into cells: header/trailer + 44-byte data 8 8 16
< 64 KB
0-24 8 8 16
CPI
Btag
BAsize
Pad
Etag
Length
payload

27. ATM Cell Format for AAL 3/440 2 4 10
352 (44 bytes) 6 16
ATM header
type
seq
MID
length
CRC-10
payload
Type (is-start? and is-end? bits)
BOM (10): beginning of message
COM (00): continuation of message
EOM (01): end of message
SSM (11): single-segment message
SEQ: sequence number (for cell loss/reordering)
MID: multiplexing ID (mux onto virtual circuits)
Length: number of bytes of PDU in this cell

28. Encapsulation and Segmentation for AAL3/4
4 bytes
< 64 KB
4-7 bytes
CS-PDU header
CS-PDU trailer
User data
44 bytes
44 bytes
44 bytes
<44 bytes
AAL header
AAL trailer
ATM header
Cell payload
Padding

29. AAL 3/4 Summary Many factors limit the utilization to 83%
Only 44 bytes of data in 53 bytes of cell
CS-PDU encapsulation further reduces the efficiency (header-to-payload ratio)
Partial filling of last cell also impacts the effective utilization
Lot of overhead to perform simple function of segmentation and reassembly …

30. AAL 5 AAL 5 designed as replacement for AAL3/4
No PDU information in ATM header
Only uses 1 bit of ATM header to mark end of PDU
Does not require additional per-cell headers/trailers
No additional layer of multiplexing in a single VC
CS-PDU contains data with only 8-byte trailer
Uses stronger error correct at PDU level (CRC-32)
Protection against lost, corrupt and misordered cells is provided by CS-PDU

31. AAL 5 CS-PDU CS-PDU Format Cell Format
Pad so trailer always falls at the end of ATM cell
Length: size of PDU (data only)
CRC-32 (detects missing or misordered cells)
Cell Format
End-of-PDU bit in Type field of ATM header
< 64 KB
0 - 47 2 2 32
pad
reserved
length
CRC-32
data

32. Encapsulation and Segmentation for AAL 5
Padding
CS-PDU
User data
trailer
48 bytes
48 bytes
48 bytes
ATM header
Cell payload