由于接到一个ATM类型的case,原来没有接触过ATM网络,所以这几天学习了一下关于ATM的基础知识。分享如下,以备以后参考:
1. ATM的层次模型
ATM层和OSI模型不完全一致,它主要处理物理层和数据链路层问题,其协议分层如下图5所示,其中简要地列出了各层的中英对照和相应的功能。
ATM可用速率分若干等级:
a)1.544Mbps/2.048Mbps;
b)25Mbps;
c)45Mbps;
d)155Mbps;
e)622Mbps,
它们可以分别容纳不同QoS和速率要求的业务。顺便指出NMS的ArTeMux系列ATM产品可以为UTP(Unshielded-twist-pair),MMF(Multi Mode Optical Fiber)和SMF(Single Mode Optical Fiber)提供25Mbps和155Mbps两种速率(但25Mbps是北美标准,不适合中国)。
2. ATM的信元
ATM信元实际上就是分组,为区分X.25分组(Packet)改称为信元(Cell)。ATM信元具有固定的长度,总长为53字节。其中前5个字节为信头(Header),包含各种控制信息,包括信元目的(逻辑)地址、纠错码、业务控制和维护信息等;后面48字节为信息字段(Payload),又称净荷,其中包含了业务的数据,它们将被透明地传输。信元结构如下图6所示。在UNI接口和NNI接口上ATM的信元头结构是不同的。
GFC(Generic Flow Control):一般流量控制。如图7所示,GFC仅在UNI信头存在,因为ATM只在端设备与用户设备处进行流控制,以减少网络过载的可能性。
VPI(Virtual Path Identifier):虚通道标识符。在ATM中,若干虚通路(VC)组成一个虚通道(VP),并以VP作为网络管理单位,相当于X.25中的逻辑信道群号(LCGN)。
VCI(Virtual Connection Identifier):虚通路标识符。类似于X.25中逻辑信道号(LCN),用于标志一个VPI群中的唯一呼叫,在呼叫建立时分配,呼叫结束时释放。在ATM中的呼叫由VPI和VCI共同决定,且唯一确定。
PTI(Payload Type Identifier):净荷类型。用于指示信息字段的信息是用户信息还是网络信息。
CLP(Cell Loss Priority):信元抛弃优先级。当CLP为"1"时,表示当网络拥塞时可以抛弃该信元;相反,不能抛弃CLP为"0"的信元。
HEC(Header Error Control):信头差错控制。为了提高处理效率(同时传输线路条件允许如此),ATM仅进行信头 差错控制,以防VPI/VCI差错,即呼叫间"串话"。
3. ATM的信令接口:
UNI和NNI ATM有两种类型的接口:一种是用户-网络接口(UNI:User-Network Interface),一种是网络-网络接口(NNI:Network-Network Interface)。如下图7所示。
其中的UNI接口完成终端用户系统与专有ATM交换机和公众ATM网络之间及专用ATM交换机与公众ATM网络之间的消息互通。而NNI是专为公众或专有ATM网络之间的消息互通。通过这两种接口,一方面可以维持现有的话音、视频、数据等本地网络不变而轻松地在UNI下得以综合,另一方面可以利用NNI高速地传输多种类型的信息。
4. ATM的地址
在ATM中,地址是呼叫建立过程中通过UNI信令确定的,然后据此寻找合适的路由,并建立VC(虚连接)。虚连接由一系列VPI/VCI路由构成,并用VPI/VCI标识。ATM中使用20字节的地址结构,如下所示:
ATM地址有三种格式:
- DCC格式:按国家分配的地址;
- ICD格式:按国际组织分配的地址;
- E.164格式:传统电话编号方式。
它们的区别由交换机MAC地址中的地址前缀指明。
其中适配器的MAC地址,又称ESI(End System Identifier)是48比特的MAC地址(与现有LAN-MAC地址兼容)。选择字(SEL)在NNI接口中没有意义,仅在UNI接口处解释,在NMS的ArTeMux中用于识别终端系统中的终端接入点(地址空间大小为256)。
5. ATM的地址注册
在呼叫建立过程中终端系统和ATM交换机之间(UNI)通过过渡性本地管理接口(ILMI:Interim Local Management Interface)协议完成地址注册。注册过程如下图所示。
(1) ATM终端系统通知ATM交换机自己的MAC地址ESI;
(2) 如果正常,ATM发回包含ATM地址的其余部分(包括ATM交换机MAC)的响应包。
6. ATM的虚连接(Virtual Connection)
ATM和STM都是面向连接的传输模式。但ATM和STM不同,其连接是"临时"的,不像STM中那样,用户在呼叫期间独占物理通道(的一部分),而是逻辑上的"虚连接",故称"虚电路"。用户间的信元传输必须在虚电路建立之后,才能进行;信元按序发送,并按序到达目的终端;且各虚电路拥有自己(在呼叫建立期间协商好)的业务性能参数。
(1) 虚电路(Virtual Circuit)概念
虚通路(VC):两个终端接入点的逻辑连接。
虚通道(VP):一组虚通路集合。
VCI和VPI分别用于标识为一个呼叫链路所分配的虚通路和虚通道。前面所讲的ATM地址仅在UNI接口处用于识别用户终端及用户终端内的逻辑设备;在ATM交换和传输控制过程中都以VPI+VCI作为参考值(参见ISDN及七号信令部分中的呼叫参考值)。VC和VP的关系如图9所示。
值得指出的是,一个呼叫链路由端-端VCI/VPI和ATM交换机的路由表唯一确定,一个呼叫链路上的信元的VPI+VCI在ATM交换过程中会被改变(由路由表决定)。
(2) ATM交换
ATM中的两种交换:虚通道交换(VCS:Virtual Channel Switching)和虚通路交换(VPS:Virtual Path Switching)。前者指同一个VP内或不同VP内的VC之间的交换;后者指一个VP内所有信元同时被映射到另一个VP内的过程。见下图10。
(3) ATM的两种虚连接
永久虚连接(PVC:Permanent Virtual Connection):是一种静态虚连接,必须手工配置。其优点是,不必每次呼叫都进行虚连接配置,所以连接快(小于30us),仅由系统响应时间决定。其缺点是,这些PVC必须手工配置,不能进行大量PVC配置。
动态虚连接(SVC:Switched Virtual Connection),又称交换虚连接:是一种动态的虚连接,由终端用户或终端应用发起连接请求,系统临时建立。连接时间由ATM网络决定,在系统拥塞时可能失败。但SVC比PVC具有更高的QoS适应性和带宽利用率。
二者都必须能进行点-点连接和点-多点连接。
==============================================================================
在设备上show的信息如下,仅供学习理论参考
BGL14.1-O.13-7200-4(config-if)#do show int ATM2/0.1
ATM2/0.1 is up, line protocol is up
Hardware is ENHANCED ATM PA, address is c8f9.f9e1.f038 (bia c8f9.f9e1.f038)
Internet address is 10.100.252.10/30
MTU 4470 bytes, BW 149760 Kbit/sec, DLY 80 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ATM
Keepalive not supported
12015 packets input, 1297620 bytes
12015 packets output, 1297620 bytes
36045 cells input, 36045 cells output
11686 OAM cells input, 11729 OAM cells output
AAL5 CRC errors : 0
AAL5 SAR Timeouts : 0
AAL5 Oversized SDUs : 0
Last clearing of "show interface" counters never
BGL14.1-O.13-7200-4(config-if)#do show int ATM2/0
ATM2/0 is up, line protocol is up
Hardware is ENHANCED ATM PA, address is c8f9.f9e1.f038 (bia c8f9.f9e1.f038)
MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit/sec, DLY 80 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ATM, loopback not set
Keepalive not supported
Encapsulation(s): AAL5 AAL0
4095 maximum active VCs, 1 current VCCs
VC Auto Creation Disabled.
VC idle disconnect time: 300 seconds
21 carrier transitions
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: Per VC Queueing
5 minute input rate 46000 bits/sec, 55 packets/sec
5 minute output rate 46000 bits/sec, 55 packets/sec
35305 packets input, 3157964 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
35347 packets output, 3160092 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
BGL14.1-O.13-7200-4#show controllers atm2/0
Interface ATM2/0 is up
Hardware is ENHANCED ATM PA - OC3 (155000Kbps)
Framer is PMC PM5346 S/UNI-155-LITE, SAR is LSI ATMIZER II
Firmware rev: G163, Framer rev: 0, ATMIZER II rev: 3
idb=0x06ACE1AC, ds=0x076569E0, vc=0x076B87A0
slot 2, unit 2, subunit 0, fci_type 0x0056, ticks 29791
4000 rx buffers: size=512, encap=64, trailer=28, magic=4
Curr Stats:
VCC count: current=1, peak=1
SAR crashes: Rx SAR=0, Tx SAR=0
rx_cell_lost=0, rx_no_buffer=0, rx_crc_10=0, rx_no_mem=0
rx_cell_len=0, rx_no_vcd=1, rx_cell_throttle=0, tx_aci_err=0
Rx Free Ring status:
base=0xE427C040, size=4096, write=280
Rx Compl Ring status:
base=0x388F5680, size=4096, read=663
Tx Ring status:
base=0xE47039C0, size=8192, write=1092
Tx Compl Ring status:
base=0x388FD6C0, size=8192, read=4642
BFD Cache status:
base=0x076B2720, size=6144, read=6143
Rx Cache status:
base=0x06ACFAE0, size=16, write=7
Tx Shadow status:
base=0x076AA6A0, size=8192, read=1087, write=1092
Control data:
rx_max_spins=3, max_tx_count=17, tx_count=5
rx_threshold=2667, rx_count=7, tx_threshold=4608
tx bfd write indx=0x10DF, rx_pool_info=0x04908900
Control data base address:
rx_buf_base = 0x386A37E0 rx_p_base = 0x07663FC0
rx_pak = 0x05584128 cmd = 0x06ACF4A0
framer = 0x02161B00 framer_cb = 0x05D634E0
framer_base = 0xE4100000 pci_pa_stats = 0x38905700
device_base[0] = 0xE4000000 device_base[1] = 0xE4400000
ssram_base[0] = 0xE4200000 ssram_base[1] = 0xE4600000
sdram_base[0] = 0xE4300000 sdram_base[1] = 0xE4700000
pa_cmd_buf[0] = 0xE427FC00 pa_cmd_buf[1] = 0xE467FC00
vcd_base[0] = 0xE4200000 vcd_base[1] = 0xE4618000
chip_dump[0] = 0x3890572C chip_dump[1] = 0x3890582C
sar_buf_base[0] = 0xE4328000 sar_buf_base[1] = 0xE470C000
bfd_base[0] = 0xE4264000 bfd_base[1] = 0xE4600000
acd_base[0] = 0xE4222080 acd_base[1] = 0xE4638240
Bad particle count = 0
SAR AutoRecovery : Disabled
SAR AR rx stuck count=0 tx stuck count=0
rx_crashes =0 tx_crashes =0
Framer Information:
Framing mode: SDH (STM-1)
No alarm detected
Facility statistics: current interval elapsed 91 seconds
sbip lbip lfebe pbip pfebe hcse
------------------------------------------------------------
30 0 0 0 38 20 0
33 0 0 0 1674 906 1
sbip: Section BIP8
lbip: Line BIP8/24
lfebe: Line FEBE
pbip: Path BIP8
pfebe: Path FEBE
hcse: Rx Cell HCS Error
BGL14.1-O.13-7200-4#
==========================================================================
BGL14-1-R-06-ASR1002-X-2#show int ATM0/2/0
ATM0/2/0 is up, line protocol is up
Hardware is SPA-3XOC3-ATM-V2, address is 4001.7a65.fa20 (bia 4001.7a65.fa20)
MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit/sec, DLY 80 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ATM, loopback not set
Keepalive not supported
Encapsulation(s): AAL5 AAL0
8191 maximum active VCs, 1 current VCCs
ATM Dynamic Bandwidth Disabled.
VC Auto Creation Disabled.
VC idle disconnect time: 300 seconds
4 carrier transitions
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 07:32:05
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 66000 bits/sec, 79 packets/sec
5 minute output rate 65000 bits/sec, 78 packets/sec
122903 packets input, 12662956 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
4 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
122903 packets output, 12662956 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
BGL14-1-R-06-ASR1002-X-2#show con
BGL14-1-R-06-ASR1002-X-2#show controllers ATM0/2/0
Interface ATM0/2/0 (SPA-3XOC3-ATM-V2[0/2]) is up
Framing mode: SONET OC3 STS-3c
SONET Subblock:
SECTION
LOF = 0 LOS = 1 BIP(B1) = 0
LINE
AIS = 0 RDI = 1 FEBE = 22 BIP(B2) = 0
PATH
AIS = 0 RDI = 1 FEBE = 3 BIP(B3) = 0
LOP = 1 NEWPTR = 0 PSE = 0 NSE = 0
Active Defects: None
Active Alarms: None
Alarm reporting enabled for: SF SLOS SLOF B1-TCA B2-TCA PLOP B3-TCA
ATM framing errors:
HCS (correctable): 0
HCS (uncorrectable): 0
APS
not configured
COAPS = 0 PSBF = 0
State: PSBF_state = False
Rx(K1/K2): 00/00 Tx(K1/K2): 00/00
Rx Synchronization Status S1 = 00
S1S0 = 02, C2 = 13
PATH TRACE BUFFER : STABLE
BER thresholds: SF = 10e-3 SD = 10e-6
TCA thresholds: B1 = 10e-6 B2 = 10e-6 B3 = 10e-6
Clock source: line
BGL14-1-R-06-ASR1002-X-2#
BGL14-1-R-06-ASR1002-X-2#
BGL14-1-R-06-ASR1002-X-2#show int ATM0/2/0
ATM0/2/0 is up, line protocol is up
Hardware is SPA-3XOC3-ATM-V2, address is 4001.7a65.fa20 (bia 4001.7a65.fa20)
MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit/sec, DLY 80 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ATM, loopback not set
Keepalive not supported
Encapsulation(s): AAL5 AAL0
8191 maximum active VCs, 1 current VCCs
ATM Dynamic Bandwidth Disabled.
VC Auto Creation Disabled.
VC idle disconnect time: 300 seconds
4 carrier transitions
Last input 00:00:01, output 00:00:01, output hang never
Last clearing of "show interface" counters 07:39:18
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
123077 packets input, 12672004 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
4 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
123077 packets output, 12672004 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
BGL14-1-R-06-ASR1002-X-2#show int ATM0/2/0.1
ATM0/2/0.1 is up, line protocol is up
Hardware is SPA-3XOC3-ATM-V2, address is 4001.7a65.fa20 (bia 4001.7a65.fa20)
Description: to-zonghang-router-01
Internet address is 10.100.252.9/30
MTU 4470 bytes, BW 149760 Kbit/sec, DLY 80 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ATM
Keepalive not supported
123224 packets input, 12679648 bytes
112000 packets output, 12096000 bytes
11223 OAM cells input, 11223 OAM cells output
AAL5 CRC errors : 0
AAL5 SAR Timeouts : 0
AAL5 Oversized SDUs : 0
Last clearing of "show interface" counters 07:45:22
BGL14-1-R-06-ASR1002-X-2#